Automotive Terms and Definitions

ABS 
Abbreviation for Anti-Lock Braking System.

ABS Pressure Switch
Sensor that monitors hydraulic system pressure and controls pump motor in an ABS application.

AC
Alternating current.

Acceleration Sensor
Provides acceleration data to computer in electronic shock absorber system.

Accelerator Pump
Device to force a fuel stream into air horn as needed

Accessory Systems
Those that increase comfort or are a convenience for vehicle occupants, such as air conditioning or power windows, and a window defogger.

Accumulator
Air conditioning system component that removes moisture and stores extra refrigerant

AC Generator
Device that produces alternating current; an alternator.

Active Sensor
One that generates its own voltage signal in response to a change in a condition

Active Suspension System
A method of ride control using hydraulic rams, rather than conventional shock absorbers and springs

Actuator Device that performs an action or outputs a signal in response to a signal from a computer.

Adhesive
Substance used to chemically and mechanically bond two materials together

Adjustable Rocker Arm
Used with mechanical lifters; permits changing valve train clearance.

Air Blowgun
Tool used to blow off bits of dirt, solvent, or other debris from parts.

Air Chisel
A cutting tool powered by compressed air that is used to remove faulty exhaust system parts.

Air Cleaner Carbon Element
A canister of carbon granules that absorbs fuel vapors when engine is shut off. 

Air Compressor
A pump that forces air, under pressure, into a storage tank.

Air Cooling system 
The use of cooling fins and air movement to dissipate heat from the engine.

Airflow Sensor
Device to monitor flow of outside air into engine.

Air Horn
Part of the carburetor that channels air into the intake manifold.

Air Injection System
A method of reducing HC and CO emissions by forcing fresh air into the exhaust ports of the engine.

Air Jet Chamber
One that uses an injected stream of air for improved fuel mixing and combustion.

Air Pollution
Release of harmful substances into the air due to engine operation and similar causes.

Air Pump
Belt-driven pump that provides input for the air injection system.

Air Shocks
Shock absorbers that use air pressure, rather than springs, to maintain vehicle height.

Air Spring
Air-filled rubber cylinder that is lighter than the equivalent coil spring.

Air Tests
The use of air pressure to diagnose automatic transmission problems.

Air Tool
A tool powered by compressed air.

ALDL
Assembly line diagnostic link, also called the diagnostic connector.

Alignment Rack
Fixture onto which the vehicle is driven for alignment measurement and adjustment activities.

Alternate Engine
Engine types other than traditional internal combustion, four-stoke-cycle, piston engine.

Alternate Fuel
Fuels other than gasoline and diesel fuel.

Alternator
An AC generator.

Alternator Bearings
Needle or ball-type bearings used 10 provide a low-friction surface for a rotor.

Altitude Compensator
A system that changes the air-fuel ratio as the vehicle moves to higher or lower elevations.

AM
Amplitude modulation (type of radio broadcasting).

Ambient Temperature Switch
One that prevents air conditioner operation when outdoor temperatures are below a set point.

Amplifier
Electronic circuit that uses a small current to control a much larger current.

Amplify
To increase in strength or volume.

Anaerobic Sealer
A sealer that cures in the absence or air.

Analog
A signal that can vary continuously in value.

Analog Signal
One that continually changes strength. Analog voltmeter code Trouble code read by counting the number of needle deflections on a meter.

Antenna Trimmer
Adjustment screw on a radio, used to obtain best reception.

Antifreeze
Liquid used in a cooling system that is mixed with water and prevents the water from freezing. The solution serves as the engine coolant.

Antifreeze Strength
Measurement of concentration of coolant solution.

Antifriction Bearing
Bearing that uses balls or rollers to decrease friction.

Anti-Lock Brake Computer
ECM that accepts wheel sensor inputs and controls braking of the vehicle.

Anti-Lock Brakes
Computer-controlled brakes that will not "lock" and permit wheels to skid.

Anti-Rattle Clips
Metal components designed to keep brake pads from vibrating and rattling.

Anti-Skid System
Another name for anti-lock braking system.

Apprentice Mechanic
A beginner who is learning under direction of an experienced auto technician.

Armature
Rotating support for multiple windings in a motor.

Asbestos
A mineral material once widely used on clutches and brake linings. Asbestos dust is a known cancer-causing agent.

Asbestos Lining
Anti-friction material once widely used for brake linings.

ASE
Abbreviation for National Institute for Automotive Service Excellence, which certifies auto technicians.

Aspect Ratio
The relationship of tire height to width, or profile.

Asphyxiation
Death resulting From lack of oxygen to breathe.

Atmospheric Pressure
The pressure exerted by the Earth's atmosphere (14.7 psi at sea level).

Automatic Clutch Adjuster
A mechanism designed to automatically remove any play from a clutch cable system.

Automatic Temperature Control System
A method of climate control using sensors and a computer to maintain a preset temperature in the vehicle's passenger compartment.

Automatic Transmission
One that does not have to be shifted manually.

Automatic Transmission Electronic Controls
Computerized selection of shift points, based on input from sensors.

Automatic Transmission Fluid
Oil with special additives to make it compatible with Friction clutches arid bands.

Automatic Transmission Slippage
Condition in which engine races as transmission shifts (in severe cases, the car may not move at all).

Automotive Clutch
A mechanical device used to connect and disconnect a manual I transmission from engine power.

Auto Technician's Certification
A program in which technicians take written tests to become certified by the National Institute for Automotive Service Excellence (ASE).

Auxiliary Chain
A chain and sprocket combination used to drive engine auxiliaries, such as the oil pump.

Axle C-clip
Spring steel retainer that fits in a groove on the axle end. 

Axle End Play
In-and-out movement OF the axle, adjusted to specification by using shims.

Axle Retainer Plate
See axle shaft retainer.

Axle Shaft Retainer
Devices that attach to the outside of an axle housing to prevent axles from sliding out. 

Axle Shims
Used between axle housing and retainer to limit end play of the axle.

Backfiring
Condition caused by air-fuel mixture igniting in the intake manifold or exhaust system.

Backing Plate
Component that holds the shoes, wheel cylinder, and other parts inside a drum brake. 

Back Pressure
Pressure developed in the exhaust system when the engine is running.

Backup Light Switch
An electrical switch that completes a circuit to the back up lights whenever the reverse gear if engaged.

Bad Hydraulic Lifter
A cause of valve clatter.

Balancer Shaft
A Rotating component used in some engines to cancel vibration produced by another rotating shaft such as the crankshaft.

Ball and Trunnion U-joint
A seldom-used type of constant velocity U-joint.

Ball Joint
Swivel joint that provides free movement for steering knuckle and control arm.

Ball Joint Balloon Seal
An airtight seal used to protect the ball joint

Ball Joint Wear Indicator
An indicator on the shoulder of the ball joint that shows the amount of wear.

Ball Nut
Component that rides up and down a worm gear as the worm rotates in a recirculating ball gearbox.

Ball Sockets
Components that allow motion in up-and-down and side-to-side direction.

Band
A metal strap with a frictional material lining that can clamp a clutch drum in an automatic transmission to stop its rotation.

Band Adjustment
Check and adjusting the tightness of automatic transmission band for proper automatic transmission shifting and driving conditions.

Bare Cylinder Head
A head with all of its components removed including valves, springs, seals, etc.

Barrel
A cylinder. 

Basic Circuit Problem
One caused by something in the circuit itself that increases or decreases an electrical value.

Battery Activation
To fill with electrolyte then charging a dry battery.

Battery Cables
Heavy gauged wires connected to the battery. Typically two are used. One, connects the battery to the starter (positive) and the second from the battery to a grounding point on the vehicle.

Battery Charge Condition
The state of its plates and electrolyte.

Battery Charger
Device for restoring battery to a proper electrical charge.

Battery Drain Test
A method of checking for unusual current draw with ignition key off.

Battery Leakage Test
Check to determine if current is discharging across top of battery case.

Battery Load Test
A test for battery capacity, made under full electrical load.

Battery Terminal Test
A test for good contact between cables and terminals.

Battery Voltage
For batteries used in modern cars, 12.6V; older cars, 6.3V.

Battery Voltage Test
Check of battery charge with a voltmeter.

BDC
Bottom Dead Center. The lowest position a piston can reach with in its cylinder. 

Bearing Clearance
Small space between moving parts, permitting oil to enter for lubrication.

Bearing Crush
Slight oversize of bearings to jam them in place when cap is tightened on connecting rod. 

Bearing Housing
Section of the turbocharger housing containing the shaft bearings and oil seals. 

Bearing Noise
Constant whir or humming sound due to damage or wear of bearings in the carrier or axle assemblies.

Bearing Packer
Tool used to simplify packing of grease into wheel bearings.

Bearing Spread
Practice of making bearing halves slightly wider than necessary to hold them in place during installation.

Bell Housing
The metal shell surrounding the clutch assembly that bolts to the rear of the engine.

Belted Bias Tire
A bias-ply tire with extra belts added beneath the tread area.

Belts
Fabric made of steel or other material that is placed between body plies and tread.

Belt Squeal
Noise resulting from a slipping drive belt.

Bench Bleed
Method of filling and hand-pumping a master cylinder before installation to remove trapped air.

Bent Push Rod
A cause of valve clatter. Bent rods usually must be replaced.

bhp
Abbreviation for brake horsepower, a measurement of usable horsepower of an engine.

Bias Ply Tire
One with plies running at an angle from bead to bead.

Bleeder Screw
Fitting on top of brake caliper that allows air to be bled from system.

Bleeding
Process of removing any trapped air from a hydraulic system.

Block Diagram
A simple drawing, using rectangles and lines, that shows how sensors, computers, and actuators are interconnected.

Block Heater
A heating device used to warm the block of a diesel engine in cold weather.

Block Line Boring
A technique used to "true" misaligned main bearing bores.

Blow-By
Oil vapors and other emissions that leak past piston rings into the crankcase.

Blower
A fan that forces air to move tough the evaporator of an air conditioning system.

Blow Gun
An air-powered device used for cleaning and drying parts washed in solvent, or blowing away accumulations of dust and dirt.

Blown Head Gasket
Leak of compression from one or more cylinders results from failure of the gasket.

Blow-through Turbo
A turbocharger located ahead of the compressor or throttle body. It compresses only air.

Body Lubrication
Applying oil and grease to such friction points as hinges and latches.

Boiling a Block
Cleaning technique in which strong solvents and heat are used to remove mineral deposits from the water jacket.

Bolt Size
Measurement of the outside thread diameter. 

Boost Pressure
The additional amount of pressure supplied to the intake system produced by a turbocharger or blower.

Boot
Flexible pleated covers placed over the CV joints of a front-wheel-drive vehicle to keep road dirt out of them.

Boot Deterioration
Cracking, tearing, or other damage to a boot, which would allow road dirt and moisture to enter the CV-joint.

Bowl Vent
Passageway that prevents a pressure buildup in fuel bowl.

Box End
A wrench that has completely closed ends that surround and grip a bolt head.

Brake Booster
Component operated by vacuum or power steering system to decrease braking effort needed.

Brake Lines
Metal tubing and rubber hoses connecting master cylinder to wheel brake assemblies.

Brake Pads
Replaceable friction surfaces mounted on caliper of disc brake system.

Brake Pedal Assembly
Foot lever for operating brake system.

Brake Pedal Vibration
Pulsing movement of brake pedal, usually caused by out-of-round brake drum or warped rotor.

Brake Sensor
Provides braking data to computer in electronic shock absorber system.

Brake Shoes
Curved, replaceable friction surfaces used with drum-type brakes.

Brake System
Components that are used to stop a vehicle.

Brake System Flushing
Removal of all old fluid by pressure bleeding, then replacing it with fresh fluid.

Brake Warning Light
Dashboard indicator that warns of low brake system hydraulic pressure. 

Braking Ratio
Comparison of front wheel to rear wheel braking effort.

Break-Away Torque
In a limited-slip differential, the amount of torque needed to make one axle rotate the clutches.

Brushes
Sliding electrical contacts that ride on the slip rings of a generator.

BTU
British thermal units; the units used to measure heat transfer.

Bubbling Sight Glass
A sign that the air conditioning system is low on refrigerant and that air may be present.

Burned Fluid
Condition caused by overheating due to slippage of transmission bands.

Burned Piston
Actual melting or breakage of a part of the piston, resulting in a hole.

Burned Valve
Valve face damaged by heat, allowing leakage of air-fuel mixture from cylinder during compression.

Bus
A pathway for data inside a computer.

Bypass Lubrication System
One that filters only some of the oil going to bearings.

Bypass Valve
Method used to permit coolant circulation in engine when thermostat is closed.

Calibration Fluid
A fluid that is less flammable than diesel fuel, used when testing injectors out of the engine.

Caliper
The component that houses disc brake pads on both sides of the brake rotor and are forced together through applied brake fluid pressure to stop the rotor from turning, thus stopping the car. It is also the name of a tool to measure small inside and outside diameters.

Camber
The inward or outward tilt of a wheel assembly.

Cam Ground Piston
One that is ground slightly out of round to compensate for different rates of expansion.

Cam Lobe Wear
Reduces valve lift, causing reduced engine power and rough idle.

Camshaft
Rotating shaft with lobes that open valves at the correct times for proper engine operation.

Camshaft Drive
Gears, a chain and sprockets, or a belt and sprockets that run the camshaft at half the speed of crankshaft rotation.

Camshaft Lift
Amount of valve train movement produced by the cam lobe.

Cam Thrust Plate
Component that limits front-to-rear movement of camshaft.

Cap Pressure Rating
Pressure at which the cap valve opens to vent coolant to the overflow tube or recovery tank.

Caps
Covers over cell openings on top of a battery.

Carbon Trace
Small line of conductive carbon-like material on a distributor cap or rotor.

Carburetor Cleaner
A chemical used to dissolve gums and material from carburetor parts.

Carburetor Flooding
Excess fuel flow, preventing starting of engine.

Carburetor Fuel System
One that mixes air and fuel and directs it to engine cylinders through an intake manifold.

Carburetor Kit
Gaskets, needle valves, and other parts needed to rebuild a carburetor.

Carburetor Rebuild
Overhaul, with cleaning and part replacement.

Carburetor System
Network of passages that control the air-fuel ratio under specific engine conditions. 

Cartridge Oil Filter
One with a replaceable filter element.

Case
The outer protective shell of a battery, or any other component or device.

Case Bearing Preload
Amount of force pushing differential case bearings together.

Caster
Forward or backward tilt of steering knuckle.

Caster-Camber Bubble Gauge
Instrument with bubbles that indicate degree of tilt.

Catalytic Converter
Device in the exhaust system that oxidizes most harmful emissions from the engine.

Cell
An electrical energy storage device, consisting of negative and positive plates immersed in a conductive fluid (electrolyte).

Cell Voltage Test
Check of individual battery cells for correct charge.

Center Support Bearing
A ball or roller bearing unit that supports the middle of a two-piece drive shaft. 

Centrifugal Advance
A system that uses pivoting weights to advance timing as engine speed increases.

Certified Master Technician
Auto technician who has passed all eight ASE tests.

Certified Technician
Auto technician who has passed at least one ASE test.

Cetane Number
An indication of the cold-starting ability of a diesel fuel.

CFM
Cubic feet of air per minute.

Chain Slap
Flapping motion of chain if slack is permitted.

Charcoal Canister
Unit that traps and stores fuel vapors when engine is not running.

Charge Indicators
Dash-mounted warning light, voltmeter, or ammeter used to show charging system status. Also the visual symbol (often a green dot) that shows state of charge in a battery.

Charging
Current flowing into a battery from an alternator. Also, adding new or recycled refrigerant to an air conditioning or refrigeration system.

Charging Station
Usually, a wheeled cart containing a tank of refrigerant, vacuum pump, and manifold set for adding refrigerant to an air conditioning system.

Charging System
One that uses an alternator to replace the electrical energy drawn from the battery during starting.

Charging System Output Test
A measurement of current and voltage output of the charging system under load.

Charging Voltage
Alternator output that is higher than battery voltage (usually 1 3V- I 5V).

Chassis
The frame and other parts of a vehicle other than the body.

Check Valve
Valve that permits fuel flow in only one direction.

Chemical Burns
Injury to the skin from exposure to toxic or corrosive chemical substances.

Chemical Flushing
Method of removing scale buildup, using a strong chemical and water flush.

CID
Cubic inch displacement.

Circuit Board
A base upon which computer circuits are printed and components are mounted.

Circuit Breaker
Device that interrupts current if a circuit is overloaded or a short occurs. Unlike a fuse, it can be reset.

Circuit Resistance Tests
Measurements of resistance in the insulated and ground circuits of the system.

Clear Sight Glass
A sign that the air conditioning system is operating properly.

Climate Control Computer
ECM that controls temperature levels in the vehicle's passenger compartment.

Clogged Heater Core
One that restricts or blocks flow of coolant, due to an accumulation of rust particles.

Closed Loop
Control system that is constantly changing, based on inputs from sensors.

Closed System
Cooling system using a recovery tank for coolant.

Cloud Point
Temperature at which wax separates out of the fuel.

Cloudy Sight Glass
A sign that desiccant from the receiver-drier is circulating through the air conditioning system.

Clutch
Device that allows the driver to engage or disengage the engine and transmission.

Clutch Adjustment
Process of setting the correct amount of free play in the release mechanism.

Clutch Cable
A simple mechanical arrangement that uses a cable to transmit clutch pedal movement to the clutch fork.

Clutch Chatter
A condition in which clutch severely vibrates as car accelerates.

Clutch Disc
A disc that is splined to transmission input shaft and pressed against the face of the flywheel.

Clutch Fork
Lever that forces the throw-out (release) bearing into pressure plate of clutch.

Clutch Lining
Frictional material riveted to the face of the clutch disk.

Clutch Linkage
A mechanical arrangement of levers and rods that transmits force from the clutch pedal to the clutch fork.

Clutch Master Cylinder
The device that produces the hydraulic pressure needed to operate the clutch.

Clutch Pedal Free Travel
Distance the pedal moves before the throw-out bearing acts on the pressure plate.

Clutch Pilot Shaft
Tool used to keep clutch disc from falling as pressure plate is loosened during clutch disassembly.

Clutch Piston
A piston that moves back and forth inside the clutch drum to clamp driving and driven discs together.

Clutch Release Mechanism
Cable or linkage permitting driver to disengage clutch with foot pedal.

Clutch Slippage
Condition in which engine rpm increases without increase in car's road speed.

Clutch Start Switch
Safety switch that prevents starting motor operation until the clutch is disengaged.

CO
Abbreviation for carbon monoxide, a toxic byproduct of partial burning of fuel.

C02 Readings
Used to evaluate the air-fuel ratio of the engine; measured in percent by volume.

Coast Side
Concave side of gear tooth.

Cogged Belt
Rubber belt with built-in cogs (teeth) that engage the teeth of camshaft and crankshaft sprockets to form a positive drive.

Coil Oscillations
Voltage fluctuations shown on an oscilloscope after the spark plug stops firing.

Coil Spring
Length of spring steel rod wound into a spiral.

Coil Spring Compressor
Tool used to safely compress a spring for removal or installation on a vehicle.

Coil Wire
Conductor carrying high voltage from the coil to the distributor.

Cold Cranking Rating
The amount of current a battery can deliver for 30 seconds at 0~F

Cold Plug
One with a short insulator tip, used in engines operated at high speed.

Cold Soak Cleaner
Cleaning method using unheated solvents to remove oil and grease from disassembled engine parts.

Cold Start Injector
Valve that supplies extra fuel for cold starts.

Combination Valve
One that includes both a suction valve and an expansion valve (used in air conditioning systems). Also braking system valve that can function as a metering or proportioning valve and a brake warning light switch.

Combination Wrench
One that has an open jaw on one end and a box (closed end) on the other.

Combustion Chamber
Area at the top of the cylinder where a spark plug ignites the compressed air-fuel mixture.

Combustion Leak Test
Test that checks for presence of combustion gases in cooling system.

Commutator
Sliding electrical connection between motor windings and brushes.

Commutator End Frame
The end housing on a motor, holding the brushes, brush springs, and shaft bushing.

Compact Spare
Small-diameter spare tire for use in emergencies.

Compression Gauge
Dial indicator used to measure cylinder compression.

Compression Ignition
Ignition of an air-fuel mixture by heat that results from high pressure (compression).

Compression Pressure
The amount of pressure produced in the cylinder by the compression stroke.

Compression Ratio
The relationship of cylinder volumes with the piston at TDC and at BDC.

Compression Ring
Ring that fills the gap between the piston and cylinder wall, preventing blow-by of compression pressure into the crankcase.

Compression Stroke
Piston stroke that compresses the air-fuel mixture in the cylinder.

Compression Test
To check of the compression developed in each engine cylinder and is a means of determining engine condition.

Compressor
Device that pressurizes a refrigerant and forces it through a refrigeration system.

Compressor Assembly
Motor-powered air pump that produces pressure for an electronic height-control system.

Compressor Housing
Outer case around the compressor wheel section of turbocharger.

Compressor Shaft Seal
A common point of refrigerant leakage in a system.

Compressor Wheel
Fan-like wheel that forces air, under pressure, into the engine's intake manifold.

Computer
Electronic device used to control many systems of modern vehicles.

Computer Amplifier
Device that strengthens signals inside the computer.

Computer Analyzer
A more complex testing instrument than a scanner.

Computer Clock
Pacing device that keeps all input, processing, and output operations "in step."

Computer-Coil Ignition
A distributor less ignition system using sensors, a control unit, and multiple ignition coils.

Computer Conditioner
Interface that converts digital signals to analog and vice versa.

Computer Controlled Carburetor
One that calculates and sets air-fuel ratio based on input from sensors.

Computer Harness Connector
Plug at the end of a wiring harness, with one prong for each wire.

Computerized Emission Control System
A computer controlled system that uses sensors, a catalytic converter, and fuel injection to minimize emissions of harmful substances.

Computer Memory
Storage area where data is held electronically.

Computer Menu
The list of tasks or programs initially shown on a computer screen

Computer Scan Values
Output from the vehicle's computer system that can be read by a diagnostic scanner.

Computer Terminal Values
Voltage and resistance values at the ECU connector, read by a digital VOM.

Computer Trouble Codes
Numeric codes, shown on a display or as pulses, that indicate the nature or location of a problem.

Computer Voltage Regulator
Device that provides a very smooth DC voltage for circuits and devices controlled by the computer.

Condensation
Change from a gas to a liquid state.

Condenser
An electrical component in contact point distributors that prevents arcing as points open and close. Also, a device that allows refrigerant to give off absorbed heat and change from a gas back to a liquid state.

Conduction
Method of heat transfer through direct contact.

Conductor
A material that permits easy flow of electrons.

Connecting Rod
Part that fastens the piston to the crankshaft.

Connecting Rod Side Clearance
Distance between side of the connecting rod and the side of the crankshaft journal.

Constant Velocity Joint
One that uses two cross-and-roller joints connected by a centering socket and center yoke.

Contact Pattern
The area of a gear tooth where the matching gear's tooth physically contacts it.

Contact Point Regulator
An older type of voltage regulator that has largely been replaced by the electronic type.

Contact Points
In older distributors, the spring-loaded electrical "make/break" switch contacts.

Continuous Injection
Fuel injection that is constant.

Continuously Variable Transmission
One that has an infinite number of driving ratios and uses belts and pulleys, rather than planetary gear sets.

Control Arm Bushing
Sleeve that allows control arm to swing up and down.

Control Arms
Movable lever arm that forms part of a vehicle's suspension system.

Control Rod
A toothed shaft that rotates control sleeves to control the output of a diesel injector pump.

Control Sleeve
Rotating sleeve around the plunger of a diesel injection pump that helps control pump output.

Convection
A method of heat transfer through air or fluid medium.

Conventional Measuring System
The system of feet, inches, pounds, etc., traditionally used in the United States.

Converter Housing
Case containing the fluid coupling (torque converter) used with an automatic transmission.

Cooling System
Radiator and other components that allow a coolant to circulate and maintain a constant engine operating temperature.

Cooling System Hydrometer
Test instrument used to check specific gravity of coolant.

Cooling System Pressure Test
A method of checking for leaks by placing system under pressure. 

Cooperative Training
Work release program combining school classes with work experience.

CO Readings
When reading CO levels, an exhaust analyzer provides a percentage by volume output.

Cotter Pin
A soft metal pin that fits through hole in a nut and is then spread to lock components in place.

Countershaft
A Shaft on which cluster gears are mounted that transmits force from input gears to output gears.

Countershaft Gear
Cluster of gears in a manual transmission that transmit force from input gears to output gears.

Cradle
Sub-frame supporting the engine of some vehicles.

Crankcase
Lower portion of the block, containing the crankshaft.

Crank Position Sensor
Similar to a distributor pickup coil, the sensor is activated by the teeth on a pulse ring in a crankshaft triggered ignition system.

Crankshaft
Component that changes the up-and-down motion of the pistons into rotating motion. 

Crankshaft End Play
End-to-end movement of crankshaft in block.

Crankshaft Front Oil Seal
Component that keeps oil from leaking between the crankshaft snout and the engine front cover.

Crankshaft Position Sensor
Device to monitor engine speed.

Crankshaft Pulley
Pulley, attached to harmonic balancer, that drives belts for alternator and other units.

Crankshaft Triggered Ignition
System that places trigger wheel and pickup coil on the crankshaft damper at the front of the engine.

Crankshaft Turning
Grinding or otherwise smoothing the surface of crankshaft rod and main journals to repair damage or wear.

Creeper
A low, wheeled cart used to work under a car supported by jackstands.

Crimping Pliers
Special tool used to mechanically fasten connectors to wires.

Crisscross Pattern
Typical pattern used to tighten cylinder head bolts to provide even pressure.

Crossflow
Type of radiator arrangement with tubes running horizontally.

Crossover Pipe
Connector between left and right header pipes in a dual exhaust system, used to equalize backpressure.

Curb Height
Distance from a given point on the car to the ground.

Curb Weight
Weight of vehicle with full gas tank and no passengers or cargo.

Current
The flow of electrons through a conductor.

Cushioning Springs
Flat springs under the friction material on the clutch disc that help smooth the clutch engagement.

CV
Constant velocity.

CV-Joint Repair Kit
Kit containing joint components, replacement boot, grease, and other necessary items for joint repair.

Cybernetics
The study of how electrical-mechanical devices can duplicate actions of the human body.

Cylinder Balance Test
Oscilloscope test that measures the power output from each of the engines cylinders. 

Cylinder Block
Another name for the engine block, or main body of the engine.

Cylinder Bore
Diameter of the engine cylinder.

Cylinder Boring
Machining of cylinder walls to make them perfectly straight and smooth, removing signs of wear and damage.

Cylinder Head
Component that bolts to the top of the engine, enclosing the tops of the cylinders.

Cylinder Head Stand
A fixture used to hold the cylinder head off the workbench surface.

Cylinder Hone
A tool used to develop a pattern of fine scratches on cylinder walls to aid in new ring break-in.

Cylinder Leakage Tester
Instrument used to determine amount of air leakage from combustion chamber.

Cylinder Out-of-Roundness
Condition in which a cylinder diameter is different when measured front-to-back than when measured side-to-side.

Cylinder Sleeving
The practice of inserting a liner into a bored cylinder to restore it to its original diameter.

Cylinder Taper
The slight difference in diameter, due to wear, from the top to the bottom of an engine cylinder.

Cylinder Wear
When the natural up and down movement of the engines piston physically erodes the cylinder walls.

Dash Light Code
The Morse type trouble code conveyed by a flashing light on dashboard.

Dash Warning Light
A labeled indicator that lights to show there is a problem of malfunction.

DC
Direct current.

DC Generator
Device that produces direct current.

Dead Axle
A solid, straight rear axle on a front-wheel-drive vehicle.

Dead Battery
One that has become discharged.

Dead Cylinder
One in which the spark plug is not firing.

Decimal Conversion Chart
Table showing equivalent quantities in fractions of an inch, decimal fractions, and millimeters.

Deck Warpage
Twisting or distortion of engine block surfaces as a result of overhearing.

Delivery Valve
Spring-loaded valves in the outlet fittings to diesel injector pump.

Detonation
Explosive, uneven burning of fuel causing engine knock.

Diagnosis Chart
A listing of problem causes and corrections.

Diagnostic Connector
A terminal to which a diagnostic scanner can be connected to check vehicle operation.

Diagonally Split
Term for a braking system that has each master cylinder piston actuating wheel cylinders that are diagonally opposed.

Diagrams
Drawings that are used to show wiring, vacuum, or hydraulic systems.

Dial Bore Gauge
A tool used to quickly and accurately measure cylinder taper.

Dial Indicator
A device used to measure tiny movements and display the distance on a dial.

Diaphragm Spring Clutch
One that uses a single diaphragm spring, rather than several coil springs, to help release the clutch disk.

Die
Tool for cutting threads on the outside of a rod or shaft.

Diesel Cylinder Balance Test
Diagnostic procedure to determine whether cylinders are firing properly.

Diesel Engine Maintenance
Since diesel engines do not have ignition systems, maintenance (tune-up) work is primarily devoted to making adjustments, changing oil, and replacing filters.

Diesel Fuel Grade
Viscosity rating of a diesel fuel.

Dieseling
Continued engine operation with the ignition turned off (also called "run-on").

Diesel Injection
A mechanical system that forces diesel oil directly into combustion chambers.

Diesel Injection System Tester
Device used to test a number of diesel components and functions by checking pressures.

Diesel Maximum Speed Adjustment
A limitation that can be set by using an adjusting screw on the injection pump.

Diesel Tachometer
Device used to establish the revolutions per minute of a diesel engine.

Differential
An assembly of gears used to provide power to the rear axles and allow them to rotate at different speeds as necessary.

Differential Carrier
Component used to mount the differential assembly on the rear axle housing. 

Differential Case
Case that holds the ring gear, spider gear, and inner ends of the axles.

Differential Lubricant
High viscosity oil used to reduce friction between differential components.

Differential Yoke
Component that connects the rear universal of the drive line to the differential.

Digital Code
Trouble code displayed as actual digits (numbers), rather than flashes.

Digital Display
An oscilloscope that displays a numerical reading (digital display) on a separate screen.

Digital Pyrometer
An electronic device for making accurate temperature measurements.

Digital Signal
One made up of strictly on-off (or high-low) pulses.

Dimmer Switch
A switch that controls high-beam and low-beam headlamp functions.

Diode
Electronic device that allows current flow in only one direction.

Diode Test
Check for open or shorted conditions in a diode, using an ohmmeter or special test equipment.

Dipstick
Strip of stiff metal used to check fluid levels. Markings indicate whether more fluid must be added.

Disc Brakes
Brakes using a caliper that clamps against a rotor for stopping.

Disc Brake Service
Procedure involving worn pad replacement, caliper rebuilding, rotor surfacing, and system bleeding.

Discharging
Flowing out (describing current movement related to a battery).

Disc Resurfacing
Machining the rotor surface to remove wear marks or correct runout.

Disc Runout
Amount of side-to-side movement of brake disc (rotor).

Displacement
The volume displaced by the pistons in moving from BDC to TDC.

Display Output
Actuator that provides readable characters on a small screen or liquid crystal display. 

Distilled Water
Water that has been purified.

Distributor Cap 
A plastic, insulating cover that encloses the distributor rotor and other components.

Distributor Injection Pump
A pump that uses one or two cylinders to handle injection of diesel fuel for an engine, as compared to an inline pump with a plunger for each cylinder.

Distributor Point Gap
Recommended distance between points when fully open.

Distributor Rotor
A slotted shaft on a distributor injector pump that controls fuel flow to each nozzle. 

Distributor Tester
Test device used to check operation of an ignition system distributor.

Distributor Wrench
A specially shaped wrench that allows technician to reach under distributor housing to adjust timing.

Diverter Valve
Component that prevents air from entering the exhaust system during deceleration.

DOHC
Dual Overhead Cam engine.

DOT Number
The Department of Transportation code that indicates the tire has passed required safety rests. It also identifies manufacturer, construction type, and other data.

Double Lap Flare
Approved method of preparing steel fuel lines for connection.

Downflow
Type of radiator arrangement with tubes running vertically.

Dragging Brakes
Braking friction material that remains partially applied, even after the brake pedal is released.

Dragging Clutch
Failure of friction disc to fully disengage from flywheel, even though clutch pedal is depressed.

Draw-Through Turbo
A turbocharger located behind the compressor or throttle body. It compresses the air-fuel mixture.

Drilled Rod
Connecting rod with an oil passage drilled through its length to deliver oil to the piston pin.

Drive Chain
A chain used with some longitudinally mounted engines to transfer power from the engine crankshaft to the transaxle.

Drive Housing
Case surrounding the pinion gear on a starter motor.

Drive Line
The parts that transfer power from the transmission to the drive wheels.

Drivers
Power transistors in a computer that control current flow to actuators.

Drive Shaft
Steel tube that transfers rotating motion from transmission to rear wheels of a car.

Drive Shaft Angle
The angle at which the drive line meets the differential or the transmission.

Drive Shaft Assembly
Components between the transmission and differential, including front and rear yokes, universal joints, and a drive shaft.

Drive Shaft Balance
Equal weight distribution around the axis of the shaft.

Drive Shaft Noise
Sounds typically caused by worn U-joints, worn slip joints, or a faulty center support bearing.

Drive Shaft Runout
Lack of straightness, due to being bent or because of U-joint wear.

Drive Shaft Vibration
A rapid oscillation caused by a shaft imbalance or excessive shaft runout. 

Drive Side
Convex side of gear tooth.

Drive Size
The size of the square opening for the handle of a socket wrench set.

Driving Hub
Mounting for wheel on end of axle. 

Driving Range
Distance a vehicle can be driven without refueling.

Drop Light
An electrical extension cord with a light bulb in a safety cage attached.

Dropping Brake Pedal
Slow descent of brake pedal to floor when brakes are applied. Usually caused by internal leak in master cylinder.

Drum
The housing that holds the parts of a clutch assembly for an automatic transmission. 

Drum Brakes
System that forces brake shoes against the inside of a rotating drum to stop vehicle. 

Drum Brake Service
Process that involves dismounting, disassembling, cleaning, and replacing parts as necessary. Usually, shoes are replaced, wheel cylinders replaced or rebuilt, and the drum is turned (resurfaced). System is then reassembled, bled, and tested.

Drum Grinding
Sometimes done to remove hard spots on a brake drum.

Drum Maximum Diameter
Largest inside diameter allowed for safe operation of drum brakes. 

Drum Resurfacing
Machining of brake drum to remove surface damage.

Dry Charged
Battery that is filled with electrolyte just before being installed in a vehicle.

Dry Park Test
Visual check for looseness of steering components.

Dry Sleeve
A thin cylinder liner that is not exposed to coolant.

Dual Reservoir Master Cylinder
Brake system pump with two pistons and fluid reservoirs for safety.

Dummy Shaft
Tool used to drive out countershaft and reverse idler shaft when disassembling manual transmission.

Dust Cap
Metal cover on end of axle or spindle to keep grease in and road dirt out of bearings.

Dwell
The amount of time distributor points remain closed between openings.

Dwell Meter
One that measures point setting in degrees of distributor rotation.

Dwell Section
Section of oscilloscope pattern used to identify problems in the distributor or electronic control unit.

Dwell Signal
Electronic signal output by carburetor that can be read on a special meter for troubleshooting.

Dwell Variation
Change in meter readings indicating distributor wear.

Dye Penetrate
A testing material that can be sprayed on aluminum or cast iron engine parts to locate cracks. 

Dynamic Imbalance
Tire imbalance that causes both up-and-down and side-to-side movement while rotating.

Dynamometers
Instrument used to measure power output and performance of an engine.

Dyno
An engine dynamometer, used to measure brake horsepower.

ECA
Electronic control assembly; another name for an automotive computer.

ECM
Electronic control module; another name for an automotive computer.

ECU
Electronic control unit; another name for an automotive computer.

Effective Plunger Stroke
The amount of plunger movement that pressurizes fuel in a diesel injector pump.

EFI
Electronic fuel injection.

EFI Self-Diagnosis
The ability of an electronic fuel injection system to display trouble codes.

EFI Tester
A diagnostic device used with EFI systems that do not feature self-diagnosis.

EFI Trouble Code
A digital display or on-off sequence that shows a number code for problem identification.

EGR Valve
Air Gas Re-circulation Valve. A valve allowing exhaust gases to re-enter the intake manifold to be burned again, reducing cylinder temperature and some types of toxic emissions.

Electrical Fire
One that involves, or begins in, electrical wiring.

Electric Engine Fan
One operated by an electric motor, under control of a thermostatic switch.

Electric Fuel Pump
An electrical device that uses a rotary motion to move fuel.

Electrolyte
Liquid that surrounds the plates of a battery and allows a free flow of electrons.

Electronic Advance
A system that uses sensor input and the vehicle's computer to control spark timing. 

Electronic Airbag System
System using impact sensors to deploy an airbag that cushions vehicle operator (and in some cases, passengers) in a collision.

Electronic Coil Module
Grouping of ignition coils and the control unit needed to operate them.

Electronic Control Unit
Another term for a computer used in a vehicle.

Electronic Ignition System
One that uses an electronic control circuit and distributor pickup coil points and condensers to break the field of the ignition coil, sending voltage to the spark plug.

Electronic Ignition Tester
Instrument used to identify source of ignition problems.

Electronic Modules
Small computers in a vehicle, used for specific systems (such as anti-lock brakes or climate control).

Electronic Regulator
Solid-state regulator separate from the alternator.

Electronic Shock Absorber System
One that uses a computer, actuators, and adjustable shock absorbers to regulate stiffness of the vehicle's suspension.

Electronic Steering Assist
Power steering system using an electric motor, rather than hydraulic components.

Electronic Transmission/Transaxle Control System
A computer-based system that controls shift points and torque converter lockup for improved fuel economy.

Element
One of the cells that can be combined to form a battery.

Emergency Brake
Mechanical means of applying rear brakes.

Emission Control System
Components and adjustments used to reduce the amount of pollutants released by operation of an automobile.

Engine
The propulsion system that provides motive power for a vehicle.

Engine Analyzer
Grouping of instruments used to check various engine functions and components. 

Engine Blow-By
Leakage of unburned fuel and other pollutants past piston rings and into the crankcase.

Engine Bottom End
The block, crankshaft, connecting rods, pistons, and related components. 

Engine Break-in
Sequence of acceleration, deceleration, and other operations designed to properly seat and seal new piston rings.

Engine Crane
A portable, wheeled hoist used to lift and remove engines from vehicles.

Engine Efficiency
Ratio of power produced by the engine to the power supplied to that engine. 

Engine Firing Order
The sequence in which spark plugs are fired.

Engine Flooding
Excess fuel in the intake manifold, preventing starting.

Engine Front Cover
Housing of sheet metal or cast aluminum that covers the timing chain or gears.

Engine Front End
A combination of components that operates the camshaft.

Engine Miss
Roughness in engine operation, indicating failure of a spark plug to fire.

Engine Off Self-Test
Test conducted with engine off but ignition key on to cause displaying of trouble codes.

Engine On Self-Test
One conducted with the engine fully warmed up, allowing check of the sensors under normal conditions.

Engine Operating Temperature
Temperature reached by coolant during normal engine operation. 

Engine Overhaul
Process of servicing all internal parts of an engine.

Engine Sensors
Devices that monitor temperatures, fluid levels, and other engine conditions for computer input.

Engine Smoke
Emissions during acceleration or deceleration. Color can indicate engine condition. 

Engine Surge
Rise and fall of engine speed in cruising operation.

Engine Temperature Sensor
Component that measures temperature of engine coolant.

Engine Top End
The cylinder heads, valves, camshaft, and related parts.

Engine Torque
A rating of turning force at the engine crankshaft.

Engine Tune-up
A process of parts replacement and adjustment to return an engine to its peak performance.

EPROM
Erasable programmable read-only memory. 

Erasing Trouble Codes
Removing trouble codes from computer memory after they have been read and the problems corrected.

Ethyl Alcohol
Grain alcohol or ethanol.

Evacuation
A process of removing air and moisture from an emptied air conditioning system, using a vacuum pump.

Evaporator
A device in which refrigerant changes from a liquid to a gas and absorbs heat from its surroundings.

Exhaust Gas Analyzer
Testing device to measure the chemical content of exhaust gases.

Exhaust Gas Sensor
See oxygen sensor.

Exhaust Manifold
Component that directs output of the exhaust ports to the exhaust system.

Exhaust Manifold Heat Valve
A component that forces hot exhaust gases to circulate through intake manifold as an aid to cold-weather starting.

Exhaust Stroke
Piston stroke that forces byproducts of combustion out of the cylinder.

Exhaust System
Components that quiet engine operation and direct combustion products (exhaust gases) to the rear of the vehicle.

Expander-Spacer
Metal ring used with two rails to form oil control ring.

Expansion Plug
Freeze plug designed for installation in tight quarters.

Expansion Tube
A device with a small orifice that meters refrigerant into the evaporator of an air conditioning system.

Expansion Valve
Temperature-sensitive valve that controls refrigerant flow and air conditioner evaporator temperature.

Extension Housing
A separate housing bolted to the transmission housing, containing the output shaft and rear oil seal.

External Oil Leakage
Escape of oil from the engine, usually around gaskets or seals.

Face
Area of a gear tooth above the pitch line. 

Fast Charge(r):
A charger or charge that provides a high current flow for quickly recharging a battery.

Fastener
Devices that hold parts together.

Fast Flushing
Flushing system through a heater hose fitting, without removing the thermostat. 

Fast Idle Cam
A cam that increases idle speed when the choke is closed.

Fast Idle Solenoid
Device that holds throttle plates open when engine is operating, but lets them almost close when engine is shut off.

Feathered Edge
One side of each tread rib is worn sharp and raised; the other side is rounded or recessed.

Feeler Gauge
A thin metal strip or wire of identified thickness, used to measure clearance.

Fender Cover
Cloth or plastic blankets placed over auto body sections to protect the finish while repairs go on.

fhp
Frictional horsepower.

Field Frame
Housing on a motor that holds the field coils.

Field Windings
Stationary windings in a motor that creates a magnetic field to keep the armature rotating. 

Filler Neck Restriction
Metal piece preventing introduction of the larger fuel nozzle used for leaded fuel. 

Fill Ring
Electrolyte level indicator in older style batteries.

Firing Line
The tall spike shown on an oscilloscope, representing the voltage needed to make the spark jump the plug gap.

Firing Order
Sequence in which the spark plugs fire in cylinders.

Fixed Caliper
Brake caliper rigidly mounted to steering knuckle.

Flank
Area on a gear tooth below the pitch line.

Flex Fan
One with blades that alter airflow with engine speed.

Float
Device that rises and falls with fuel level in bowl, opening and closing the needle valve.

Floating Caliper
Brake caliper mounted on t\VO rubber bushings, allowing some movement.

Fluid Contamination
Inclusion of foreign matter in transmission fluid.

Fluid Coupling Fan Clutch
Clutch designed to slip at higher fan speeds.

Fluid Varnish
Sticky, glue-like substance resulting from breakdown of automatic transmission fluid.

Flywheel
Large, heavy wheel mounted on the rear end of the crankshaft. Usually includes a ring gear that is engaged by the starter pinion.

FM
Frequency modulation (type of radio broadcasting).

Force
A pushing or pulling action.

Forward Bias
Arrangement in which diode acts as a conductor.

Four-Gas Analyzer
Measuring instrument that provides the reading of oxygen and carbon dioxide, as well as HC and CO, levels in exhaust.

Four-Stroke Cycle
An engine that takes four piston movements (intake, compression, power, exhaust) to complete a cycle.

Frame
The strong steel structure that supports the body of a vehicle.

Friction Bearing
A Plain bearing with two smooth surfaces sliding on each other. Not a tapered bearing with an outside cone used as the race or riding surface.

Front Cover Gasket
Thin sealing component that is compressed between the cover and the engine.

Front Drive Axles
Shafts that transfer power from the transaxle differential to the vehicle's wheels. 

Front End Rack
Alignment stand.

Fuel Accumulator
Diaphragm that dampens pressure pulses in a fuel injection system.

Fuel Distributor
Hydraulically operated valve used to control fuel flow in a continuous injector system.

Fuel Evaporization System
A combination of technologies and components that prevents fuel vapors from entering the atmosphere.

Fuel Heater
A device used to warm diesel fuel and keep it from jelling into a semi-solid during cold weather. 

Fuel Injection System
General term for systems used with either gasoline or diesel fuel.

Fuel Injector
Fuel valve controlled by a coil or solenoid.

Fuel Pressure Regulator
System that controls pressure of fuel entering injector valves.

Fuel Pump Pressure
The pressure of a fuel pump's output.

Fuel Pump Vacuum
The amount of "pull" exerted on fuel by the pump.

Fuel Pump Volume
The amount of fuel discharged by a pump in a measured period.

Fuel Rail
Tubing that connects several injectors to the main fuel line.

Fuel Return System
One that keeps cool fuel circulating to prevent vapor lock.

Fuel Tank Capacity
How much fuel a tank can hold. 

Full-Floating Piston Pin
One that is free to rotate. It is secured in place with snap rings.

Full Flow Lubrication System
One that forces all oil through a filter before it reaches the parts to be lubricated.

Fully Synchronized Transmission
One in which all forward gears are equipped with synchronizers to allow downshifting while in motion.

Fuse
Device that interrupts current if a circuit is overloaded or a short occurs.

MacPherson Strut
Suspension system that uses one control arm and one strut for each wheel assembly.

Magnafluxing
Testing procedure that uses a magnet and metal powder to find cracks in cast iron parts.

Magnetic Clutch
Device used to engage and disengage the compressor of an air conditioning system.

Magnetic Field
Field of force generated around an electrical conductor.

Magnetic Sensor
One that uses part movement (such as rotation) and induced current to produce a signal for a computer.

Mag-Tach
A magnetically triggered tachometer usable on both gasoline and diesel engine.

Main Bearing Bores
Holes machined into the bottom of the block for the crankshaft and its bearings.

Main Bearing Cap Torque
The factory recommended degree of tightness for main cap bolts.

Main Bearing Knock
eep, resonant sound caused by wear of bearing and possibly crankshaft journal.

Main Body
Central portion of the carburetor, forming the air horn and fuel bowl.

Main Caps
Pieces the bolt to the bottom of the block to hold the crankshaft in place.

Main Computer
The largest and most powerful microprocessor in a vehicle's system. It can control other computers.

Main Discharge Tube
Passage between fuel bowl and venturi.

Mainframe Computer
A large, centralized computer with a great deal of memory and computing power.

Main Jet
Fuel inlet metering device for normal-speed engine operation.

Main Journals
Carefully machined surfaces on the ends of the crankshaft that fit into the block main bearings.

Maintenance-Free Battery
One without removable filler caps that does not require periodic filling with water.

Maintenance Interval
The specific number of miles or months that should elapse between tune-ups as described in the vehicle owner's manual.

Main Thrust Bearing
Flanged version of main bearing, designed to limit crankshaft endplay.

Major Tune-Up
A tune-up requiring some decree of repair to systems, in addition to the work done for a minor tune-up.

Manifold Pressure Sensor
Measures pressure inside the intake manifold. Also called MAP sensor.

Manual Bleeding
A method of system bleeding using only master cylinder pressure.

Manual Transaxle
One with a manual (driver-operated) transmission.

Manual Transmission
One that is shifted from gear to gear by the vehicle operator.

Manual Valve
In an automatic transmissions a valve actuated by the gear shift lever that routes oil pressure to the components required for the selected gear.

Manufacturer's Manual
Service manual produced by an auto maker and restricted to its vehicles.

Master Cylinder
Hydraulic piston type pump that develops pressure for the braking system.

Mechanical Choke Unloader
A linkage that opens the choke plate whenever the throttle swings fully open.

Mechanical Efficiency
A comparison of brake horsepower to indicated horsepower, measuring frictional loss.

Mechanical Fuel Pump
A Device that uses a reciprocating motion to move fuel.

Mechanical Lifter
Solid lifters that must be adjusted periodically.

Metering Rod
Stepped rod that moves in and out of main jets to alter fuel flow.

Metering Valve
Valve designed to equalize pressure at wheel cylinders on vehicles with front disc and rear drum brakes. Is also known as a Proportioning Valve.

Methyl Alcohol
Wood alcohol, or methanol.

Metric Measuring System
System of measure, based on units of 10, that is used by most of the world.

Mica
Mineral used as an insulator between commentator segments.

Micrometer
A precision measuring device for very small distances.

Microprocessor
A small computer, sometimes called a "chip" or "IC" (integrated circuit).

Milky Fluid
Condition caused by contamination of transmission fluid by engine coolant.

Milky or White Oil
Lubricant that has become contaminated by the presence of coolant.

Milling
A machining process that removes a thin layer of metal; often used to repair warped engine cylinder heads.

Millisecond
Fraction (1000th) of a second.

Minimum Disc Thickness
Thinnest rotor dimension allowed for proper and safe operation of disc brakes. 

Minor Tune-Up
A tune-up done on an engine in good condition, involving primarily adjustment and ignition parts replacement or maintenance.

Missing
Failure of one or more cylinders to fire. 

Mixture Control Solenoid
Electromechanical; device that opens and closes air and fuel passages in carburetor.

Modem
A modulator-demodulator; a device that changes analog signals to digital, and vice versa, for communications.

Modem Analyzer System
System that allows a shop computer analyzer to communicate via telephone line with a large mainframe that stores information needed for advanced troubleshooting.

Modulated Injection
Injection of fuel intermittently, without reference to intake valve timing.

Modulator Vacuum Leakage
Cause of incorrect shift point operation in an automatic transmission.

Movable Pole Shoe
Device that uses a yoke lever to move the pinion gear into contact with the flywheel gear. 

Muffler
An exhaust-system component that decreases the noise of vehicle operation.

Muffler Clamps
U-shaped connectors for fastening parts of the exhaust system together.

Multi-Cylinder Engine
One that has two or more cylinders.

Multi-Point
Fuel injection system that sprays fuel into port for each cylinder.

Multi-Weight
Motor oil that exhibits different viscosity characteristics under different conditions.

Multi-Meter
An electrical test device that can be used to measure voltage, current, or resistance.

Multiple Disc Clutch
One with several discs that can be used to drive planetary gear sets.

Mushroomed Valve Stem
Stem end that had been enlarged and spread outward by rocker arm contact. 

Mushy Shifts
Transmission changes gears too slowly.

Needle Valve
A precisely machined rod used to control fuel flow from in the injector.

Net hp
Maximum horsepower developed with all accessories in use.

Neutral Safety Switch
Switch that prevents engaging the starter when the vehicle is in gear.

Neutral Safety Switch Adjustment
Altering position of the switch to permit starting of the engine when gear selector is in the "park" position.

NIASE
National Institute for Automotive Service Excellence.

Noise Suppressor
Capacitors that absorb voltage fluctuations in a car's electrical system, reducing radio noise.

Nonadjustable Rocker Arm
One that does not allow clearance to be changed. Used with hydraulic lifters.

Non-Driving Hub
One that rotates freely on spindles (axle ends).

Non-Independent Suspension
System in which wheels are attached to each end of a solid axle.

Normal Aspiration
System that provides air to the engine at normal atmospheric pressure.

No Start Problem
Engine turns over but refuses to fire. 

NOx
Abbreviation for oxides of nitrogen, pollutants resulting from high combustion temperatures.

Number Set
Punches used to indent identifying numbers in metal.

Nut Lock
Thin, slotted nut that fits over main spindle nut on a nondriving wheel.

O2 Readings
Oxygen must be present for proper catalytic converter functioning. Analyzers measure it in percentage by volume.

Octane Number
Indicators of the antiknock value of a gasoline.

OHC Follower
Component that fits between camshaft and lifter.

Ohm's Law
A simple formula for computing unknown electrical values when two values are known.

Oil Change Interval
Mileage or period of time after which oil should be changed (3000 miles or three months, typically).

Oil Consumption
The loss of oil caused by internal or external leakage.

Oil Cooler
A radiator-like device used to regulate oil temperature.

Oil Dye
Additive used to help locate leaks. 

Oil Filler Cap
A metal or plastic cap used to cover the opening on an engine where oil is added.

Oil Film
Thin layer of lubricant between parts, preventing metal-to-metal contact.

Oil Filter Housing
Part of the engine on which the filter is mounted.

Oil Gallery
Passage in the engine block through which oil can flow to point requiring lubrication.

Oil Pressure Gauge
Instrument that provides a direct reading of engine oil pressure.

Oil Pressure Indicator
Warning light on control panel to alert driver to low pressure conditions.

Oil Pressure Switch
Safety device that shuts off the fuel pump if engine oil pressure drops.

Oil Pressure Test
Measurement of actual oil pressure using a special testing device.

Oil Pump
Device for forcing oil under pressure to the points where lubrication is needed.

Oil Ring
Piston ring that scrapes excess oil off the cylinder wall.

Oil Service Rating
Identification of type of service for which an oil is suited.

Oil Slinger
Washer-shaped part mounted on crankshaft sprocket to throw oil onto timing chain during operation.

Oil Spurt Hole
Small hole drilled in connecting rod for improved cylinder lubrication.

Oil Streaked Sight Glass
A sign that the air conditioning system is low on refrigerant and allowing excess oil to circulate.

One-Wire Circuit
One that uses the vehicle frame as a return wire to the power source.

Open Circuit
Electrical circuit with a gap or break in continuity so that current cannot flow.

Open End
A type of wrench with an open jaw on both ends.

Open Injector Coil
A broken wire in the solenoid coil is preventing operation.

Open Loop
Control system using preset values in the computer to operate engine.

Open System
Cooling system that does not use a recovery tank.

Operating Parameter
An acceptable maximum or minimum electrical value.

Opposed Engine
One with cylinders lying flat on either side of the crankshaft.

Organizing Tray
Tray or board with holes in it for holding push rods and lifters during engine disassembly.

O-Ring Seal
A synthetic rubber ring that fits into a groove and is compressed when parts are assembled.

O-Ring Valve Seal
Small round ring, usually of rubber, that fits in a groove on a shaft to prevent oil leakage.

Oscilloscope
Instrument that displays line patterns that relate voltages to time.

Outboard CV-Joint
The outer universal joint on a front-wheel drive vehicle.

Outer Stub Shaft
In a front-wheel drive vehicle, the short shaft connecting outer universal joint and the front wheel hub.

Output
The signal sent by a computer (for example, to an actuator) as a result of processing inputs it has received.

Output Shaft
Transmission shaft on which the out-put gears are mounted.

Output Shaft Gears
Gears that turn the output shaft of a manual transmission.

Overbore Limit
The largest possible diameter to which a cylinder can be bored without weakening its walls. 

Overdrive Ratio
The situation in which a large gear is used to drive a smaller gear, with an increase in speed as a result.

Overhead Valve
One located in the cylinder head, rather than the block.

Over Inflation
Operating tire with a higher-than-recommended air pressure.

Overrunning Clutch
Device that locks a pinion gear in one direction and releases it in the other.

Oversize Piston and Rings
Larger components sized to fit a re-bored cylinder.

Oxygen Sensor
Device that monitors oxygen content in engine exhaust to correct air-fuel ratio.

Packing Wheel Bearings
Filling the bearing shells with grease to prevent excessive wear.

Pad Wear Sensor
Metal tab on brake pad that makes a squealing noise to signal the need for pad replacement.

Pancake Chamber
Combustion chamber that forms a flat pocket over the piston.

Parade Pattern
Oscilloscope pattern that shows traces for each cylinder from left to right across the screen.

Parking Pawl
A latch that locks the transmission so that the vehicle will not roll when the selection lever is in the "Park" position.

Part Alignment Marks
Lines or other marks scribed on parts by a technician during disassembly, so that components can be reassembled properly.

Particulates
Solid particles of soot and other substances that result from combustion.

Part Warpage
Often causes gasket failure, since parts do not seal together properly.

Passive Sensor
One that changes an externally produced signal, but does not generate its own voltage.

PCV System
Positive crankcase ventilation, a system that decreases pollution by drawing toxic gases back through the combustion process.

Pedal Free Play
The amount of brake pedal movement before braking action begins to take place.

Pedal Height
Distance of brake pedal above floor of vehicle.

Performance Test
A measurement of air conditioning system performance made with the vehicle engine running.

Petroleum
Oil taken directly out of the ground.

Pickup Coil
Component that sends pulses to the control unit of an electronic ignition system as a result of trigger wheel rotation.

Pickup Coil Air Gap
Space between the pickup coil and trigger wheel tooth.

Pilot Bearing
The bushing or bearing that supports the forward end of the transmission input shaft.

Ping
A mild knock (light tapping noise) caused by pre-ignition of fuel.

Pinion Gear
Differential gear turned by the driveline. It meshes with the ring gear. Also, gearbox component that meshes with rack, or a small gear on a starter motor that engages a larger gear to rotate the engine flywheel.

Pinion Gear Bearing Preload
Degree of tightness of bearings, adjusted by compressing a spacer or using shims.

Pinion Gear Clearance
Distance between the pinion gear and drive end frame when gear is engaged.

Pinion Gear Depth
The distance the pinion gear extends into the carrier to mesh with the ring gear.

Pinion Pilot Bearing
A bearing used to support the pinion gear in the differential.

Pinion Shaft
Shaft holding the two differential idler (pinion) gears.

Pinpoint Tests
Specific tests of individual components of a vehicle, using the electronic scanner.

Pintle
An inward-opening diesel injector nozzle, with fuel flow controlled by a pintle (needle tip).

Pipe Expander
Tool used to slightly enlarge inner diameter of steel exhaust pipes to make assembly easier.

Pipe Shaper
Tool used to remove dents from pipe ends.

Piston
Component that rides up and down in the cylinder.

Piston and Rod Markings
Identifying marks on pistons and connecting rods to aid in proper assembly.

Piston Clearance
Difference between cylinder bore and piston diameter.

Piston Knurling
A technique in which the piston skirt is grooved, pushing up metal a few thousandths of an inch to slightly increase piston diameter.

Piston Pin
Fastening device that holds piston onto the connecting rod.

Piston Pin Knock
Double knock caused by excessive clearance between pin and connecting rod bushing.

Piston Pin Offset
Positioning of the piston pinhole slightly off the piston centerline for quieter operation.

Piston Ring Gap
Clearance between ends of rings when installed on cylinder.

Piston Size
Diameter of the piston, measured on the skirt.

Piston Slap or Knock
A loud metallic sound caused by excessive wear to piston skirt or cylinder.

Piston Stroke
The distance the piston moves from BDC to TDC.

Piston Taper
A slight top-to-bottom difference in piston diameter to adjust for differences in expansion rates.

Pitch Line
Imaginary line along the center of a gear tooth.

Pitman Arm
Component that transfers gearbox motion to the steering linkage.

Pitman Shaft Over-Center Adjustment
Adjustment of clearance between sector gear and ballnut teeth in re-circulating ball gearbox.

Planetary Gear Set
A set of gears consisting of several "planet" gears rotating around a central "sun" gear.

Plastigage
A plastic measuring clearance tool that is compressed between bearing surfaces, then compared to a scale to find thickness.

Plate
A grid, covered with porous lead, that will store electrical energy.

Pleated Paper Filter
An in-line fuel filter.

Plug Gap
Distance between the center and side electrodes on a spark plug.

Plug Heat Range
Numeric indicator of how hot a spark the plug will develop.

Plug Reach
Length of the threaded portion of a spark plug.

Ply Separation
Pulling apart of tire plies as a result of overheating due to under inflation, or other causes.

Pneumatic
Filled with air.

POA
Pilot operated absolute valve.

Pole Piece
Magnetic component of motor that keeps the armature rotating.

Poor Fuel Economy
High fuel usage caused by such conditions as overly rich air-fuel mixture or incorrect engine timing.

Pop Tester
Unit used to test diesel injectors when they are out of the engine.

Port Injection
Another name for multi-point injection.

Potentiometer Sensor
One that changes resistance in response to external change (such as part movement).

Power
The rate or speed at which work is done.

Power Antenna
A radio antenna equipped with a small electric motor for raising and lowering.

Power Brush
Cleaning tool used with an electric or air-driven drill.

Power Door Lock
One that is opened or closed with a solenoid.

Power Steering Fluid
A hydraulic oil usually automatic transmission fluid.

Power Steering Pressure Test
Use of a pressure gauge to check pump and associated component for correct pressure. 

Power Steering Pump
The unit that provides the hydraulic pressure needed in a power steering system.

Power Stroke
The stroke in which the piston is driven downward by the explosion of the air-fuel mixture in the cylinder.

Power Train
Gearing system and other components used to transfer energy from the engine to the vehicle's  wheels.

Power Valve
 A device that performs the same function as a metering rod.

Power Window
One that is raised and lowered through use of a small electric motor.

Pre-Chamber Cup
Pressed in combustion area equipped with a glow plug for easier w inter starting of a diesel engine.

Pre-Combustion Chamber
Used in diesel engines with a glow plug for easier winter starting.

Pre-Ignition
Ignition of fuel before it is fully compressed in the cylinder.

Pre-Lubricator
Pressure tank used to force oil through a lubrication system without running the engine, as a means of testing for worn engine bearings.

Press-Fit Piston Pin
One forced into the connecting rod end. The piston can move freely, however.

Pressed-in Valve Guide
One that uses a sleeve pressed into a hole machined in the cylinder head.

Pressure Bleeding
A method of system bleeding using additional pressure supplied by an external air tank.

Pressure Cap Test
Measurement of the opening pressure of a radiator cap.

Pressure Chamber
Area around the needle valve of an injector where pressure builds up to open valve. 

Pressure Fed Oiling
Oil provided to high-friction areas by means of a pump.

Pressure Gauge
Test instrument used to read positive pressure values.

Pressure Gauge or Manifold Assembly
A combination of pressure gauges, valves, and hoses used to check air conditioning system pressures and remove or add refrigerant.

Pressure Plate
Spring-loaded device that clamps clutch disc against flywheel.

Pressure Plate Covers
Lid that bolts on the pressure plate to hold various components in place.

Pressure Plate Face
A large ring that contacts the friction disk as the clutch engages.

Pressure Plate Release Levers
Levers hinged inside the pressure plate that help move the pressure plate face away from the clutch disk and flywheel.

Pressure Regulator
A limiting device in an automatic transmission, regulating maximum hydraulic oil pressure.

Pressure Relief Valve
Spring-loaded bypass that operates when pressure reaches a preset point.

Pressure Tests
Diagnostic test using gauge to check fluid pressures in various transmission hydraulic circuits.

Pressure Valve
Spring-loaded disc inside radiator cap that opens when system pressure increases past its set-point.

Primary
In a carburetor, the components that operate under normal driving conditions.

Primary and Secondary Pistons
The two pistons in a brake system dual master cylinder.

Primary and Secondary Shoes
Front and back shoes in a drum brake system. The secondary shoe has a larger surface area.

Primary Circuit
In an ignition system, all components operating on battery (low) voltage.

Primary Pattern
Pattern of low-voltage (ignition primary) changes, as shown on an oscilloscope. 

Primary Wire
Small insulated conductor that carries battery or alternator voltage.

Printed Circuit
One that consists of conductors that are flat metallic strips applied to an insulating board base. Other components are mounted on the board, as well.

Printer
A device attached to an engine analyzer that can print out a "hard copy" of test results.

Probe Tools
Used to view or retrieve items in hard-to-reach areas. Mirrors and magnetic pickups are examples.

Process
The action taken by a computer program as a result of information from inputs.

Programmed
Provided with a specific set of directions for actions to be taken.

PROM
Programmable read-only memory. 

PROM Carrier
A plastic case used to protect a PROM making installation easier.

Proportioning Valve
Valve designed to equalize pressure at wheel cylinders on vehicles with front disc and rear drum brakes.

Pulling Brakes
Situation in which car veers to one side when brakes are applied.

Pulse Air System
System that produces the same results as an air injection system, but uses natural pressure pulses in the exhaust system to provide airflow.

Pulse Ring
A trigger wheel placed on the crankshaft damper in a crankshaft triggered ignition system. 

Pulse Width
An indication of how long an injector is energized and kept open.

Pumping Plunger
Small pistons used to pump and pressurize diesel fuel.

Purge Line
Line connecting the charcoal canister and engine intake manifold.

Push Rod
When camshaft is located in block. the long push-rod transmits motion from lifter to rocker arm.

Quick Charge Test
A method of determining whether battery plates are sulfated (no longer able to hold a charge).

R-12 Frostbite
Injury resulting from contact with extremely cold refrigerant

Rack
Flat toothed bar that is moved left or right by rotation of pinion gear.

Rack and Pinion Steering Gear Adjustment
Tig htening or loosening rack adjustment screws as necessary for optimum steering.

Radial Runout
Uneven rotation caused by differences in diameter.

Radial Tire
A tire that has horizontal material cords (stripes) from the inside tire wall to the outside tire wall with the tire tread molded on top.

Radiation
The method of heat transfer through infrared radiation.

Radiation Interference
Unwanted voltage that can cause a computer to malfunction.

Radiator
An arrangement of tubes and cooling fins that serves as a heat exchanger on a vehicle.

Radiator Cap
Closure that seals and pressurizes the cooling system of a vehicle.

Radiator Hoses
Flexible tubes that carry coolant between the engine and radiator.

Radiator Shop
Specialized repair facility for radiators.

Radio
The receiving unit for broadcast AM or FM signals.

Radio Noise
Static or interference that interferes with signal reception.

RAM
Random access memory.

Raster Pattern
Oscilloscope pattern that shows the traces for the cylinders stacked vertically, bottom to top.

Reading Oxygen Sensor Tip
A visual inspection of tip color, which indicates engine condition.

Reading Spark Plugs
Determining cause of a problem by examining condition of the spark plug. 

Reading Tires
Identifying alignment, suspension, and other problems through the wear patterns on tire treads.

Reading Trouble Code
Identifying the code number for the indicated condition from any of the various display systems.

Reaming
Process of cutting valve guide to slightly larger diameter. Valves with oversize stems can then be installed.

Rear Axle Assembly
A combination of gears and axles converting rotary motion of the drive shaft to forward or backward motion of the vehicle.

Rear Axle Ratio
The relationship between the numbers of teeth on the pinion gear and ring gear. Ratio affects acceleration, pulling power, and fuel economy.

Rear Drive Axle Assembly
Differential, axles, and other components transferring power from drive line to rear wheels.

Rear Drive Axles
The components that transmit power from the differential gears to the wheels.

Rear Main Oil Seal
Seal that fits around the rear of the crankshaft to prevent oil leakage.

Rear Wheel Bearing
Ball or roller-type bearings that reduce friction between the axle and axle housing. 

Receiver-Drier
Air conditioning system component that removes moisture and stores extra refrigerant.

Re-Circulating Ball
Most common type of gearbox used with linkage steering system.

Rectified
Term used to describe AC current that has been changed to DC.

Reduction Starter
One that uses extra gears to increase the torque applied to the flywheel gear.

Reference Voltage
A known voltage (usually 0.5 V) fed to passive sensors by a computer. Changes in sensor resistance can then be read by the computer. 

Refractometer
Test instrument used to measure antifreeze protection.

Refrigerant
Substance with a very low boiling point that can be used to absorb heat.

Refrigerant Oil
Lubricant used in the compressor of an air conditioning system.

Regulator Bypass Test
Test that connects full battery voltage to the alternator field, leaving the regulator out of the circuit.

Regulator Voltage Test
Test of the charging system under low output, low load conditions.

Relay
Electrically operated switch.

Relay Actuator
One that uses a smaller current flow to operate a switch controlling a larger current flow.

Relief Valve
Valve that opens to protect steering or other hydraulic system when pressure becomes too high.

Reserve Capacity Rating
The amount of time a battery will continue to provide an acceptable current flow when not being recharged by the alternator.

Reserve Distance
Amount of travel remaining between pedal and floor when brakes are applied. 

Reservoir
A tank or other container to hold a supply of fluid (such as the brake master cylinder reservoir).

Resistance
Opposition to current flow.

Resistance Plug Wire
Special type of spark plug wire that eliminates most radio interference.

Retracting and Hold-Down Springs
Springs that pull the shoes away from the brake drum surface when the pedal is released.

Retreads
Tire bodies that have had new tread rubber applied to extend useful life.

Reverse Bias
Arrangement in which diode acts as an insulator.

Reverse Flushing
System cleaning done by forcing water backward through the radiator and block to remove scale and sediments.

Reverse Idler Shaft
Shaft in a manual transmission on which the reverse idler gear is mounted.

Reverse Polarity
Accidental backward connection of primary wires.

Rich Fuel/Air Ratio
A fuel/air mixture that contains more fuel than needed..

Ridge Reamer
Device used to remove metal ridge formed at top end of cylinder due to metal wear.

Ring and Pinion Backlash
The amount of space between the meshing gear teeth.

Ring and Pinion Noise
Whining or howling sounds that change pitch with speed changed, usually caused by wear or damage to differential components.

Ring Expander
Tool used to spread a ring sufficiently to slip it over the piston.

Ring Gap
Space between the ends of a piston ring.

Ring Gap Spacing
Staggered alignment of ring gaps to reduce ring wear.

Ring Gear
Large gear in differential that is driven by the pinion gear and, in turn, drives the spider gears. 

Ring Gear Runout
Amount of wobble that occurs as the gear rotates.

Ring Groove Cleaner
Special scraper used to loosen and remove deposits from piston grooves.

Ring Markings
Information cast into the ring to show the top side or proper positioning on piston. 

Ring Spacers
Thin steel rings inserted next to compression rings to restore proper side clearance.

Ring-to-Groove Clearance
Also called ring side clearance, this is the space between a compression ring and the edges of the groove in the piston.

Rocker Arm
Pivoted mechanism that operates valves. 

Rod Bearing Knock
Rapping sound resulting from excessive wear of bearings.

Rod Bolt Covers
Temporary protective coverings, such as pieces of rubber hose, used when inserting piston and connecting rods in cylinders.

Rod Bolt Torque
Recommended degree of tightness for connecting rod bolts.

Rod Cap Numbers
Numbers used to match sets of rods and rod caps.

Rod Journals
Machined and polished surfaces on the crankshaft to which the connecting rods are attached.

Rollover Valve
Safety feature that prevents gasoline from leaking out tank vent if car rolls over.

Roller Lifter
One with a roller riding on the cam lobe to reduce wear.

Rolling Resistance
A measure of the amount of resistance that is generated as a tire rolls on the road surface.

ROM
Read-only memory.

Rosin Core Solder
Soft metal compound used to join electrical wires. The rosin is a non-corrosive flux to aid bonding.

Rotary Brush
A stiff brush, used with an air tool for cleaning parts.

Rotary Pump
Oil pump using star-shaped rotors.

Rotor
A rotating contact inside the distributor that routes electrical pulses from the coil to the spark plugs. Also, the metal disc against which brake pads are forced to stop vehicle.

Rotor Current Test
Method used to check alternator windings for an internal short.

Rotor Winding Open
An open (broken) winding in an alternator rotor.

Rotor Winding Short
 short-to-ground fault in an alternator rotor.

RTV
Room Temperature Vulcanizing - a type of sealant that cures at approximately 72F.

Rust Penetrant
Lubricant that helps loosen rusted joints or fasteners.

Rzeppa CV-Joint
Ball-and-cage type constant velocity joint used on front-wheel drive vehicles

Safety Rim
Wheel designed with small ridges that hold a tire in place if a blowout or flat occurs.

Safety Washer
On a non-driving wheel, flat washer that keeps wheel bearing from rubbing on spindle adjusting nut.

Satellite Face
Valve face coated with hard metal to withstand high temperatures.

Saving Memory
A method of preventing loss of information in computer memory by connecting a small voltage source to it before disconnecting the vehicle's battery cables.

Scanner
Electronic system used to analyze engine and computer operations.

Schrader Valve
A spring-loaded valve, similar to a tire valve, used in air conditioning systems.

Screw Extractor
Tool threaded opposite normal direction, so it can be used to unscrew a broken bolt or screw.

Seal
A formed material or paste-like substance used to prevent leaks.

Seal Alignment Tool
Device used to center seal around the crankshaft snout when mounting some engine front covers.

Sealant
A material applied, in liquid or paste form, to prevent leakage between parts.

Sealing Rings
Rings placed around either end of a turbo shaft to keep oil from leaking into the turbocharger housing.

Seal Part Number
Code number stamped on a seal that is used when ordering a replacement.

Seat Width
The area of the valve seat that is actually in contact with the valve face.

Secondary
In a carburetor, the components that operate under high engine output conditions.

Secondary Circuit
In an ignition system, all components operating on coil (high) voltage.

Secondary Pattern
Pattern of high-voltage (ignition secondary) changes, as shown on an oscilloscope. 

Secondary Wire
Wire used in a vehicle ignition system. It carries high voltage from coil to spark plugs.

Secondary Wire Resistance
A type of test performed to check condition of a spark plug wire or coil wire. 

Sector Shaft
Output gear in a re-circulating ball gearbox.

Select Fit Parts
Parts that have been selected and installed to improve fit or clearance.

Self-Diagnosis
The ability of a computer system to check circuits and output a code showing the nature or location of the problem.

Self-Sealing Tire
One with a scaling compound applied to its liner to stop air leakage in case of puncture.

Semi-Centrifugal Clutch
One that uses weighted release levers or rollers on the pressure plate, and the effects of centrifugal force, to increase clamping pressure on the clutch disk.

Semiconductor
Substance that acts as an insulator or a conductor, depending upon conditions.

Semi-Floating Axle
Most common type of rear axle for automobiles.

Sensor
Device that monitors and reports a condition (such as engine temperature) to the vehicle computer.

Sensor Rotor
A toothed wheel that operates at the same rpm as the vehicle wheel.

Separator
An insulating material placed between plates of a battery.

Service Manager
Person responsible for the complete service and repair operation.

Service Manual
Book with detailed information on specific car repairs. Also called a shop manual.

Service Publications
In addition to service manuals, there are other publications (such as owner's manuals and technical bulletins) that provide information on vehicle servicing.

Service Valves
Points at which pressures in an air conditioning system can be checked, and refrigerant removed or replaced.

Servo
Piston that operates a band in an automatic transmission.

Servo Action
Situation in which primary shoe of a drum brake system helps apply the secondary shoe. 

Servo Motor
One that can be stopped in exact positions (degrees of rotation).

Servo Motor Actuator
A small DC motor that can turn or move parts.

Set Screw
Headless fastener used to secure a part on-to a shank.

Shaft Run-Out
Wear or damage (bending) causing a shank to not run true around its axis.

Sharp Valve Margin
A result of excessive grinding of valve face, leaving no margin.

Shell
A component of an automatic transmission clutch that connects the front clutch drum and the sun gear of a planetary gear set.

Shift Fork
Device that physically moves synchronizer and gear together as a result of shift lever (gearshift) movement.

Shift Lever
The handle operated by the vehicle driver to manually shift from gear to gear.

Shift Linkage Adjustment
Making sure transmission linkage positions match the gear selector positions.

Shift Linkage Alignment Pin
Tool used to properly align shift linkage rods for proper operation.

Shift Rail
A manual transmission linkage that is contained within the transmission case.

Shock Absorber
Device that uses air or hydraulic pressure to dampen up-and-down motion of vehicle.

Shock Actuators
Solenoid-operated valves that control fluid flow inside shock absorbers in an electronic shock absorber system.

Shock Bounce Test
Method of quickly checking whether shock absorbers need replacement.

Shock Compression and Extension
Actions of shock absorbers resulting from the vehicle traveling over road bumps.

Shock Mode Switch
Dash switch that allows driver to select desired ride stiffness in an electronic shock absorber system.

Shop Supervisor
A person in charge of a group of technicians in a large garage.

Short Block
The bottom end of the engine, including the cylinders, pistons, and crankshaft.

Short Circuit
Excess current flow that occurs when a conductor touches ground.

Shorted Condenser
One with a direct electrical connection to ground.

Shorted Injector Coil
A short-circuited solenoid coil is preventing injector operation.

Shroud
Enclosure around a fan, used to direct airflow through the radiator.

Sidewall
Portion of tire between tread and bead. 

Simple Circuit
One consisting of a power source, a load, and conductors.

Single-Point
Injection system with fuel sprayed from a single location into the intake manifold.

Sintered Bronze Filter
A porous metal fuel filter.

Six-Point
Box end wrench with six indentations, or gripping teeth.

Slant Engine
One with a single bank of cylinders tilted to one side.

Slave Cylinder
Hydraulic cylinder that produces the movement of the clutch fork.

Slide Hammer Puller
Tool used to break loose and remove an axle that is stuck in its housing.

Slipper Skirt
Material remaining after part of piston below pin is removed for better crankshaft clearance.

Slip Rings
Components mounted on the rotor shaft of a generator to provide current to rotor windings.

Slip Yoke
Component that connects transmission to the front universal joint of the driveline.

Slow Charger
One that feeds a small current into the battery over a long period of time.

Smog
The cloud of airborne pollutants visible over major population centers.

Smoke Meter
Device for testing the amount of smoke (ash or soot) in diesel exhaust.

Smooth DC
Direct current without "ripples" or "spikes" due to fluctuating voltages.

Snap Ring
Spring steel ring that snaps into a groove to act as a retainer on a shaft.

Sodium Filled Valve
Hollow construction accepts sodium filling for more even cooling.

Soft Failure
One that is intermittent, such as the make/break connection from a loose terminal.

SOHC
Single overhead cam engine.

Soldering Gun
Tool that applies heat to joined wires so that solder can be melted into the joint.

Solenoid Actuator
One with a moving metal core that is actuated by an induced magnetic field.

Solvent Tank
Container holding a cleaning solution used for removing grease and other dirt from a part.

Sonar Sensor
Sound wave sensor used to detect road conditions in some electronic shock absorber systems.

Spark Ignition
System that uses an electric arc to ignite fuel.

Spark Knock
Noise caused by spark plug firing too early.

Spark Line
Oscilloscope line showing voltage needed to maintain an arc across the spark plug gap.

Spark Plug
Devices that emit an electrical arc at the tip to ignite the air-fuel mixture in an engine cylinder.

Spark Plug Gap
See Plug gap.

Spark Test
Check of the spark intensity (brightness and length of arc).

Spark Tester
Device used to cheek operation of ignition system.

Specialized Manual
Service manual devoted to a specific vehicle area, such as engines or braking systems.

Specialized Mechanic
A technician who is an expert on one system of a car. 

Special Tools
Fixtures or tools needed for certain repairs or adjustments. They are described in service manuals.

Specific Gravity
Weight or density of a liquid.

Speed Control System
Method of regulating a car's throttle to maintain a preset speed. Also called cruise control.

Spider Gears
Idler and axle gears in the differential that drive the rear axles of a vehicle.

Spindle
Stationary shaft used to support rotating wheel assembly on non-driving wheels.

Spindle Adjusting Nut
A nut threaded on end of wheel spindle to adjust wheel bearing.

Spin On Oil Filter
One that is replaced as a unit.

Splash Oiling
Oil distributed to needed areas by spraying or splashing.

Splines
A series of slots cut into a shaft and mating part.

Spongy Brakes
Braking system that is "soft" feeling, usually as a result of air trapped in the hydraulic system.

Spring Bind
Situation that can occur when a valve spring is fully compressed and locks the valve train.

Spring Fatigue
Weakening of springs that allows the height of the car (and thus road clearance) to decrease.

Spring Free Length
The length of a spring when removed from the engine.

Spring Rate
The stiffness or tension; amount of weight needed to compress or bend a spring.

Spring Tension
The stiffness of a valve spring.

Sprocket Timing Marks
Lines, circles, or dots on crankshaft and camshaft sprockets, aligned to set engine timing.

Spur Gears
Those with teeth cut parallel to the centerline of the drive shaft.

Squish Area
Part of a wedge-shaped combustion chamber.

Staking
Making a small dent in cylinder head metal next to a valve seat to hold it in place after replacement.

Stall
Work area of a repair shop.

Stalling
Condition in which the engine merely stops running.

Stall Speed
Highest speed of impeller rotation in a torque converter without rotation of the turbine.

Stall Test
Method used to shop-test for transmission slippage.

Starter Current Draw Test
Starting test that establishes the number of amps used by the starting system.

Starter Ground Circuit Resistance Test
Check of all parts between the battery negative and the starting motor ground for excess resistance.

Starter Mounted Solenoid
One with a plunger that moves to engage the pinion gear with the flywheel gear.

Starter Relay
Device that uses a small current flow from the ignition switch to control a larger current flow to the starter solenoid.

Starter Shims
Thin metal pieces used to adjust the space between the pinion gear and the flywheel gear.

Starter Solenoid
A high current relay that energizes the starter motor.

Starting Headlight Test
Starting test conducted with headlights turned on to provide a load on the battery.

Starting Motor Rebuild
Process involving disassembly, cleaning, parts replacement, and reassembly of the motor.

Starting System
Electric motor and other components used to rotate the engine until it starts.

Star Wheel
Adjusting screw assembly for drum brakes.

States of Matter
Forms in which a substance can exist (solid, liquid, gas).

Static Imbalance
Lack of balance that causes a wheel to vibrate up and down as it rolls.

Static Pressure Reading
A reading made with the engine off to determine whether a system has an adequate refrigerant charge.

Stator
The stationary magnetic field in a generator. Also component of torque converter that improves oil circulation and thus, torque.

Stator Support
A stationary tube surrounding the input shaft of a torque converter and supporting the stator.

Stator Test
Ohmmeter check for open or shorted windings in the stator.

Steering Axis Inclination
Angle formed by the inward tilt of ball joints, king pin, or struts. 

Steering Column
Assembly consisting of the steering wheel, steering shaft, ignition key mechanism and associated parts.

Steering Gearbox
Gear assembly that turns rotary motion into linear (straight line) left-right motion. 

Steering Knuckle
Component that provides support for wheel spindle or bearings surrounding an axle. 

Steering Linkage
Components connecting steering gearbox to steering knuckles.

Steering Sensor
Provides wheel orientation and speed data to computer in electronic shock absorber system.

Steering Shaft
Component that transfers turning motion from steering wheel to steering gearbox.

Steering System
The components that let the driver change direction of a vehicle.

Steering Wheel Play
Excessive movement of wheel without causing front wheel movement.

Stem Type Valve
Manual valve that is opened or closed by screwing the valve stem in or out.

Stethoscope
A medical device, also used by auto technicians to better hear internal engine noises. 

Stiff Clutch Pedal
A condition caused by binding or other restriction in the clutch mechanism, making the pedal hard to depress.

Stoichiometric Fuel Mixture
A perfect (chemically correct) air-fuel mixture.

Straightedge
A metal ruler or bar, with one edge known to be smooth and straight, used to determine flatness of surfaces.

Strap
Connector between cells of a battery. 

Stratified Charge
A combustion chamber design that first ignites the air-fuel mixture in a small chamber connected to the main chamber.

Strut Assembly
Suspension component combining shock absorber, coil spring, and upper damper unit. It replaces the upper control arm.

Strut Cartridge
Replaceable shock absorber unit on a MacPherson strut.

Strut Rod
A rod that fastens to the control arm and frame to keep arm properly oriented.

Stuck Valve
One that will not move up and down freely in the guide; usually, a condition occurring after long storage.

Stumble
See Hesitation.

STY
Suction throttling valve.

Supercharger
Air pump used to push denser fuel-air charge into combustion chambers for increased power.

Superheat Switch
One that shuts down an air conditioning system if refrigerant or oil levels are low.

Superimposed Pattern
Oscilloscope pattern in which the traces for all cylinders are placed one on top of the other.

Surging
Condition in which engine power fluctuates up and down.

Suspension Leveling System
Suspension system designed to keep vehicle level and at proper height even when carrying a heavy load in the truck.

Suspension System
Components that let the wheels move up and down without body movement. 

Suspension System Computer
One that accepts sensor input and regulates the stiffness of the vehicle's suspension system.

Sway Bar
A stabilizer that keeps the vehicle body from leaning excessively in turns.

Swing Axle
Axle provided with U-joints to allow for up-and-down suspension movement. Used on vehicles with differential mounted solidly on frame.

Swirl Chamber
Combustion chamber shape that causes the air-fuel mixture to spin as it enters, for better mixing.

Switching Sensor
One that opens or closes a switch in response to a change in condition.

Switch Self Test
Systematic actuation of various switches while using the scanner, providing a check of switch operation.

Synchronizer
Assembly of hub, sleeve, and other components that locks the selected output gear to the output shaft to transmit power. It permits meshing of gears without grinding.

Synthetic Fuels
Liquid fuels made from such solids as coal or tar sand.

System
A group of related parts that perform a specific function.

Systematic Approach
Combining knowledge and a logical process of elimination to solve a problem.

System Bleeding
Removing any trapped air from the entire braking system.

Tach-Dwell:
Combined meter that measures engine rpm and degrees of breaker cam rotation for distributor point adjustment.

Tailpipe
Tubing that carries exhaust from muffler to point at rear or side of the vehicle, where it can be dispersed.

Tank Pickup-Sending Unit
Component that extends into fuel tank to withdraw fuel and send fuel-level information to the fuel gauge.

Tap
Tool for cutting threads inside a hole. 

Tappet Clearance
The proper degree of tension (neither too tight nor too loose) for the valve train of an engine.

Taxable hp
General rating of engine size.

TBI
Throttle body (single-point) fuel injection.

TBI Rebuild
Similar to a carburetor rebuild.

TDC
Top Dead Center. When the engine cylinder piston achieves its highest position within the cylinder.

Telescoping Gauge
Spring-loaded device for measuring inside dimensions (such as a cylinder bore).

Temperature Gauge Tester
Electronic device for testing accuracy of engine temperature gauge.

Tensile Strength
The amount of stretching a material can withstand before breaking.

Terminals
The positive and negative posts or threaded connectors on a battery.

Test Light
Device that will light up to show the presence of voltage (electrical potential).

Test Light Code
Trouble code read by counting the flashes of a test light.

Thermal Efficiency
A comparison of fuel burned to horsepower output.

Thermo-Time Switch
Control circuit that energizes cold start injector when temperature is low enough.

Thermostatic Air Cleaner
System that heats the air being drawn through the engine air inlet to prevent carburetor icing.

Thermostatic Fan Clutch
Clutch that locks up for maximum airflow when it reaches operating temperature. 

Thermostatic Spring
Bimetal coil spring that responds to engine heat and opens or closes the choke. 

Thermostatic Switch
Electrical component that shuts off an air conditioning Compressor when the evaporator temperature approaches the freezing point.

Thermostat Rating
Temperature at which the thermostat opens.

Thread Pitch
Number of threads per inch or (metric sizes) distance between threads.

Throttle Body
Section of the carburetor containing the throttle valves.

Throttle Positioner
Device that works with idle air control valve in a TBI system to control idle speed.
Throttle Position Sensor Device that senses how much throttle is opening or closing.

Throttle Return Dashpot
Diaphragm device that prevents engine stalling when returning from high speed to idle operation.

Throttle Valve
Disc-shaped valve that controls airflow through the air horn.

Throw-Out Bearing
Bearing that decreases friction between clutch fork and pressure plate.

Thrust Bearing
Special bearings with flanges that limit crankshaft forward and rearward movement.

Thrust Washers
Metal pieces that can be slipped between block and crankshaft to limit endplay.

Tie Rod
Connectors between rack ends and steering knuckles.

Tightening Sequence
Recommended pattern for tightening multiple fasteners to obtain even tension. 

Timed Injection
System timed to inject fuel as the intake valves open.

Timing Advance
Making the spark plug fire sooner in the compression stroke.

Timing Belt
Rubber belt performing the same job as timing chain or timing gears.

Timing Belt Cover
See Engine front cover.

Timing Belt Sprockets
Toothed wheels driving a cogged timing belt.

Timing Belt Tension
The proper degree of tightness; one that will transmit power efficiently without causing excessive belt wear.

Timing Belt Tensioner
Part performing same function as timing chain tensioner.

Timing Chain
Sprocket-and-chain combination that performs same function as timing gears.

Timing Chain Guide
Channel that helps support chain and prevents chain slap.

Timing Chain Tensioner
Spring-loaded plastic or fiber block that pushes on chain to eliminate slack. 

Timing Gear Backlash
The amount of clearance between timing gear teeth.

Timing Gear Marks
Markings on differential ring and pinion gear sets that permit proper alignment. 

Timing Gear Runout
The amount of wobble that occurs when gear is rotating.

Timing Gears
Meshing gears on crankshaft and camshaft that rotate camshaft at half crankshaft speed.

Timing Light
Strobe-like light that makes moving parts appear to stand still, allowing timing marks to be observed.

Timing Marks
Calibrating marks on timing gears or other timing devices.

Timing Retard
Making the spark plug fire later in the compression stroke.

Timing Sprockets
Gear-like toothed wheels used with timing chain.

Tire Bead
Wire ring, encased in rubber, that helps hold tire sidewall against the rim.

Tire Changer
Air-powered machine that automates many steps of tire changing.

Tire Gauge
Small instrument used to check inflation pressure of a tire.

Tire Impact Damage
Punctures, cuts, or tears caused by running over debris in road.

Tire Markings
Information shown on the sidewall to indicate inflation pressure, load carrying ability, size, and other data.

Tire Plug
Rubber insert sometimes used to repair punctures. The tire industry discourages the use of plugs under most circumstances.

Tire Ply
Layer of fabric or other material that forms the carcass or body of the tire.

Tire Rotation
Moving tires to different wheels periodically to even out wear.

Tire Wear Pattern
Areas of tread that are worn off, which can provide information on causes of the wear.

Toe
Degree to which opposing wheels are on converging or diverging lines (not parallel). Also, the narrow part of a gear tooth.

Toe-out On Turns
Steering feature that turns inside wheel more sharply than outside wheel.

Torque Converter
Fluid coupling that acts as a clutch on an automatic transmission.

Torque Multiplication
Variation in torque achieved by turning the impeller of a torque converter faster than the turbine.

Torque Specifications
Information on the correct amount of force to be applied in tightening a fastener.

Torque Tube
A solid steel drive shaft enclosed in a hollow tube, with a single swivel joint at the front.

Torque Wrench
A tool that is used to indicate the amount of force being applied to a fastener.

Torsion Bar
Spring steel rod that operates by twisting and untwisting.

Torsion Springs
Small coil springs that help absorb the shock and vibration that occur when the clutch engages.

Tracking
The position or direction of the front wheels in relation to the rear wheels.

Track Rod
Metal rod used to prevent axle side-to-side movement when cornering.

Tram Gauge
Instrument used to compare distances between the front and rear of a set of tires for toe adjustment.

Transaxle
A combination of transmission and differential in one case, used on front-wheel-drive vehicles.

Transaxle Differential
Transaxle assembly that transfers torque to driving axles and allows them to rotate at different speeds.

Transaxle Dipstick
Metal rod used to check level of lubricant in transaxle.

Transaxle External Adjustments
Clutch, shift linkage, bands, and other components often can be adjusted from outside the transaxle housing.

Transaxle Fill and Drain Plugs
Removable plugs that can be used to drain and refill lubricant in a transaxle. 

Transaxle Filter
Replaceable element used to trap metal particles and other debris circulating with the lubricant.

Transaxle Gearbox
The transmission section of the transaxle, housing the forward and reverse gears. 

Transaxle Input Shaft
Main shaft that turns the gears in a transaxle.

Transaxle Output Shaft
Shaft that transfers power to the ring and pinion gears of the differential. 

Transaxle Pan
Lubricant sump at bottom of a transaxle.

Transducer
A device that changes an action or signal from one medium to another (an electrical pulse into a physical movement, for example).

Transfer Case
A power takeoff unit that sends power to both the front and rear axle assemblies on a four-wheel-drive vehicle.

Transfer Case Fill Plug
Removable plug to allow checking and adjusting of fluid level in the transfer case.

Transfer Pump
A small pump that supplies fuel to a diesel injector.

Transistor
Tiny electronic component that functions as a switch, but has no moving parts.

Transmission Case
Metal housing surrounding and supporting the transmission.

Transmission Cooler
A small separate radiator used to cool transmission oil in vehicles pulling heavy loads.

Transmission Diagnosis Chart
A diagnostic aid listing symptoms and probable causes.

Transmission ID Tag
A separate tag or set of' stamped numbers identifying the exact type of transmission being serviced.

Transmission Jack
A special tool used to support the weight of a transmission as it is being removed from a vehicle.

Transmission Linkage
System that connects the shift lever with the transmission shift forks.

Transmission Oil Cooler
Small tank within the radiator, used to regulate transmission fluid temperature. 

Transverse
Crosswise; term used to identical an engine rotated 90 degrees from the traditional longitudinal mounting, typically found in front wheel drive cars.

Tread
Outer surface of tire that contacts the road.

Trigger Wheel
Rotating component with one tooth for each cylinder.

Tripod CV-Joint
Constant velocity joint used on front wheel drive vehicles, consisting of a spider and ball arrangement inside a housing.

Trouble Code Chart
Diagnostic aid that lists the trouble for each code.

Troubleshooting Chart
A guide that lists methods for finding and correcting vehicle problems.

Tubeless
Tire that does not have a separate inner tube to hold air.

Tumbler
Ignition switch lock mechanism.

Tune-Up Parts
Spark plugs, filters, and other periodic replacement items.

Turbine
The driven fan assembly in a torque convener. 

Turbine Housing
The outer case that routes gases around the turbine wheel.

Turbine Wheel
A fan-like wheel driven by the exhaust gases of an engine. It turns a shaft and compressor wheel.

Turbo Bearing
Bearings supporting the turbo shaft.

Turbocharger
A form of supercharger that is driven by exhaust gases.

Turbo Lag
A short delay before turbo charging becomes effective.

Turbo Shaft
Shaft connecting turbine and compressor wheels in a turbocharger.

Turning
Term usually used for machining a brake drum or rotor, since process is carried out on a lathe.

Turning Radius Gauge
Instruments that measure how many degrees left or right the front wheels are turned.

Turn Signal Flasher
Bimetallic strip and heater unit that makes and breaks contact to cause on-off operation of the turn signals.

Twelve-Point
Box end wrench with twelve indentations, or gripping teeth.

Twin I-Beam
Double-axle suspension system used on some pickup truck models.

Two-Gas Analyzer
Measuring instrument that provides reading~ of NC and CO levels in exhaust.

Two-Stroke Cycle Engine
One that completes a full power-producing cycle with only one full crankshaft revolution.

U-Joint Alignment Marks
Scribed marks made on u-joint components before disassembly, allowing the joint components to be reassembled in the same positions to avoid possible imbalance and vibration.

Umbrella Valve Seal
Rubber or plastic seats that fit over opening at top of valve guide to keep oil out of them.

Under Inflation
Operating tire with a lower-than recommended air pressure.

Undersize Hearing
One designed for use on a crankshaft journal that has been machined to a smaller diameter.

Uni-Body:
A vehicle structure in which body and frame are one unit.

Universal Joint
A flex joint allowing limited up-and down and side-to-side movement.

Unleaded Gasoline
Fuel that does not contain lead-based antiknock additives.

Un-Sprung Weight
The weight of vehicle parts that are not supported by springs, such as the wheels.

Up-Date PROM
A PROXI with new programming that is plugged into the computer in place of the old one.

Vacuum
A pressure lower than atmospheric, in an enclosed area.

Vacuum Advance
System that provides additional ignition advance when engine load is low at medium throttle positions.

Vacuum Booster
Braking system booster actuated by vacuum.

Vacuum Choke Break
A device that uses vacuum to open and close the choke to prevent engine flooding at startup.

Vacuum Delay Valve
Mechanism that restricts airflow to slow down action of vacuum on a device.

Vacuum Gauge
Test instrument used to read negative pressure values.

Vacuum Leak
Loss of negative pressure due to crack or hole in a vacuum hose.

Vacuum Pump
A device that provides vacuum for operation of the engine accessories.

Vacuum Switch
One that opens or closes on a change in vacuum.

Vacuum Valve
Valve inside radiator cap that allows flow of coolant from recovery tank back into radiator.

Valve
Component that opens or closes a port to permit flow into and out of the combustion chamber.

Valve Adjusting Shims
Thin metal plates used to alter the cam-to-valve clearance.

Valve Adjustment
Correcting any malfunction in the opening and closing of valves actuated by mechanical lifters.

Valve Body
Housing containing most of the valves used in operation of an automatic transmission.

Valve Core
Threaded air valve that screws into place in a valve stem.

Valve Cover
A metal or plastic cover over the top of the cylinder head.

Valve Float
A tendency for valves to remain partly open, especially at high speeds. It usually results from a weak or broken valve spring.

Valve Grind Gasket Set
Set of gaskets needed for top end reassembly after valve grinding.

Valve Grinding
Process of machining a smooth surface on valve faces and valve stem tips.

Valve Guide
Holes machined into the engine block to support the valve stems as they slide up and down.

Valve Guide Cleaner
Rotating tool use to remove deposits from valve guides.

Valve Guide Insert
Replacing the worn guide with a new one.

Valve Guide Knurling
Grooving process that raises metal to restore inside diameter of guide.

Valve Job
Servicing of the cylinder head and valve train.

Valve Lifter
Component that is moved by the camshaft lobe and in turn moves the push rod or the rocker arm.

Valve Overlap
The short period when both valves in a cylinder are open.

Valve Reliefs
Indentations in a piston crown to provide valve clearance.

Valve Rotator
Device that turns the valve to prevent carbon buildup.

Valve Seal
Seals that fit over valve stems to prevent oil leakage.

Valve Seat
Machined surfaces on the intake and exhaust ports, against which the valve rests and seals.

Valve Seat Angle
The angle formed by the finished face of the seat.

Valve Seat Insert
One that is pressed into a recess cut into the head.

Valve Seat Reconditioning
Grinding or cutting valve seats to obtain a smooth surface.

Valve Seat Runout
Improper centering, so that the seat is not centered around the valve guide after grinding or cutting.

Valves in Receiver
Expansion and POA valves enclosed within the receiver drier of an air conditioning system.

Valve Spring
Assembly that closes the valve when rocker arm pressure is removed.

Valve Spring Compressor
A Tool used to compress valve springs during disassembly.

Valve Spring Installed Height
The distance from top to bottom of the spring as installed in the cylinder head.

Valve Spring Seat
Cup-shaped washer that holds the bottom of the valve spring.

Valve Spring Shim
Precisely machined washer used to increase valve spring tension.

Valve Spring Shims
Thin metal pies used to adjust spring. tension after valve grinding.

Valve Spring Tester
Device for measuring the tension or pressure exerted by the spring.

Valve Stem
A rubber inflation tube with a threaded metal core that snaps into a hole on the rim of a wheel designed for use with tubeless tires.

Valve Stem Cap
Cap placed over end of valve stem to prevent stem wear.

Valve Stem Seal
A seal placed over the end of the valve stem to prevent engine oil from entering the combustion chamber 

Valve Timing
Intervals at which valves open and close, determined by camshaft configuration.

Valve Train
The parts that operate the engine valves, camshaft, lifters, push rods, rocker arms, and springs.

Vaporization
A rapid change of state from liquid to gas.

Vapor Lock
Condition caused by bubbles in fuel due to overheating. Can cause stalling, hard starting, or failure to start.

Vapor Separator-Filter
Fuel filter that collects bubbles of vaporized fuel and returns them to the tank.

Variable Resistance Sensor
One with internal resistance that changes in response to changes in a condition (such as temperature).

Variable Venturi Carburetor
One in which the venturi size adjusts to maintain a constant air speed in the carburetor.

Vehicle Identification Number (VIN)
Individual number identifying a vehicle and displayed on a plate attached to the body.
Vehicle Maintenance Term for all operations performed to keep a vehicle in good running condition.

Vehicle Sensor
Device that changes a condition into an electrical signal that can be used as an input by a computer.

Venturi
Restriction (narrowed area) in air horn.

Vibration Damper
Heavy, rubber-mounted wheel on front of crankshaft to counter harmonic vibrations.

VIN
See Vehicle identification number.

Viscosity
A measure of the thickness, or ability to flow, of a lubricant.

Voice Alert System
Computer-based system that provides audible messages, in an electronically generated voice, to warn of safety problems or vehicle malfunctions.

Voltage
Electrical pressure that causes current flow. 

Voltage Drop
Reduction of the amount of current flowing in a circuit.

Voltage Drop Tests
Starting system tests that identify parts showing high resistance.

Voltage Generating Sensor
One that changes voltage in response to external change.

Voltage Regulator
Device used to control alternator output.

VOM
Volt-ohm-milliammeter for determining electrical values.

V-type Engine
One with two banks of cylinders, arranged in a "V" configuration.

Wankel
A rotary engine with few moving parts.

Warped Cylinder  Head
A head with a  distorted (twisted or curved) surface as a result of overheating. 

Waste Gate
A valve that limits the amount of boost developed by the turbocharger by venting excess exhaust gases.

Water Corrosion
Injector damage caused by the presence of water in diesel fuel.

Water Detector
Warning device that tells the driver there is water in diesel fuel.

Water Pump
A pump (usually of the centrifugal type) that circulates coolant through the engine and radiator of a vehicle.

Water Pump Rebuild
Process of disassembly, cleaning, parts replacement, and reassembling a water pump.

Wear Bar
Solid bars of rubber across the tread that appear when a tire has worn to the safe limit.

Wear Sleeve
Bushing between the axle and seal on a front-wheel drive vehicle.

Wedge Chamber
Combustion chamber with a triangular shape.

Wet Charged
Battery that is filled with electrolyte and fully charged at the factory.

Wet Compression Test
Test made by placing small amount of oil in a cylinder with a low reading, to determine whether worn rings are cause of low compression.

Wet Sleeve
A thick cylinder liner exposed to coolant.

Wheel Alignment
Adjusting wheels of a vehicle to roll in a straight line.

Wheel Balancing Machine
Device used to identify locations where weights must be placed to balance a tire.

Wheel Bearing
Ball or roller bearing assemblies that reduce friction as wheels or axles rotate.

Wheel Brake Assemblies
Components that use hydraulic pressure to apply friction for stopping vehicle.

Wheel Cover
Metal or plastic disk designed to fit over center section of wheel for better appearance.

Wheel Cylinder
Hydraulic piston that actuates braking at each wheel.

Wheel Cylinder Rebuild
Process that typically involves honing the cylinder and replacing all rubber parts (cups and boots).

Wheel Hop
A bouncing or up-and-down movement.

Wheel Puller
A device used to remove pressed-on parts, such as gears.

Wheel Shimmy
A side-to-side movement caused by dynamic imbalance.

Wheel Speed Sensors
Magnetic pickups to detect wheel speed (used on anti~lock braking systems). 

Wheel Weight
Small pieces of lead that are clipped to the wheel rim to balance the wheel and tire combination.

Wiggle Test
Physically moving wires and connectors to locate broken wires or other causes of intermittent problems.

Winding
Loop of wire on a motor armature that generates a magnetic field.

Wire Wheel
Cleaning tool with wires arranged radially, rotated at moderately high speed to remove carbon deposits from parts.

Wiring Diagram
Drawings that show relationships of components in an electrical circuit.

Wiring Harness
A group of primary wires enclosed in a protective plastic covering.

Work
The result of force causing movement; measured in foot-pounds, watts, or joules.

Worm Shaft
Input gear in a re-circulating ball gearbox.

Worn Cam Bearings
After prolonged engine use, this condition can cause reduced oil pressure.

Worn Idler Arm
Condition that can cause steering wheel play.

Worn Piston Rings
Many engine problems arc caused by failure of worn rings to properly seal against cylinder walls.

Worn Rocker Arms
Primarily cause noise (valve clatter).

Worn Tie Rod End
A cause of excessive play in the steering wheel.

Worn Timing Chain
Permits slack between camshaft and crankshaft sprockets, so valves no longer keep proper timing with the pistons.

Worn Joint
Excessive play between the cross and yoke.

Worn Valve Guide
Enlarged guide that allows valve to move excessively, causing various engine problems.

WOT Switch
Device that shuts off air conditioning compressor