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Glossary of Automotive Terms
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OCTANE RATING: A number indicating the quality of gasoline based on its ability to resist knock. The higher the number, the better the quality. Higher compression engines require higher octane gas.
OEM: Original Equipment Manufactured. OEM equipment is that furnished standard by the manufacturer.
OFFSET: The distance between the vertical center of the wheel and the mounting surface at the lugs. Offset is positive if the center is outside the lug circle; negative offset puts the center line inside the lug circle.
OHM: The unit used to measure the resistance of conductor-to-electrical flow. One ohm is the amount of resistance that limits current flow to one ampere in a circuit with one volt of pressure.
OHMMETER: An instrument used for measuring the resistance, in ohms, in an electrical circuit.
Figure 21 Checking resistance using an ohmmeter
OSCILLOSCOPE: A piece of test equipment that shows electric impulses as a pattern on a screen. Engine performance can be analyzed by interpreting these patterns.
O2 SENSOR: See oxygen sensor.
OUTPUT SHAFT: The shaft which transmits torque from a device, such as a transmission.
OVERDRIVE: (1.) A device attached to or incorporated in a transmission that allows the engine to turn less than one full revolution for every complete revolution of the wheels. The net effect is to reduce engine rpm, thereby using less fuel. A typical overdrive gear ratio would be .87:1, instead of the normal 1:1 in high gear. (2.) A gear assembly which produces more shaft revolutions than that transmitted to it.
OVERHEAD CAMSHAFT (OHC): An engine configuration in which the camshaft is mounted on top of the cylinder head and operates the valve either directly or by means of rocker arms.
OVERHEAD VALVE (OHV): An engine configuration in which all of the valves are located in the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.
OVERSTEER: The tendency of some cars, when steering into a turn, to over-respond or steer more than required, which could result in excessive slip of the rear wheels. Opposite of understeer.
OXIDES OF NITROGEN: See nitrogen oxide (NOx).
OXYGEN SENSOR: Used with a feedback system to sense the presence of oxygen in the exhaust gas and signal the computer which can use the voltage signal to determine engine operating efficiency and adjust the air/fuel ratio.
PARK NEUTRAL SAFETY SWITCH: On vehicles with automatic transmissions, a neutral safety switch (often referred to by various names by the different manufacturers, such as: transmission range sensor, neutral safety switch, park/neutral switch, etc.) on the side of the transmission is wired to the relay or solenoid. Its function is to prevent activation of the starter (by creating an open circuit) when the transmission is in any gear other than P (park) or N (neutral). The vehicle can only be started in P or N. Most manual transmission vehicles have a clutch switch to prevent starting the vehicle unless the clutch is depressed.
PARTS WASHER: A basin or tub, usually with a built-in pump mechanism and hose used for circulating chemical solvent for the purpose of cleaning greasy, oily and dirty components.
PART-TIME FOUR-WHEEL DRIVE: A system that is normally in the two-wheel drive mode and only runs in four-wheel drive when the system is manually engaged because more traction is desired. Two or four-wheel drive is normally selected by a lever to engage the front axle, but if locking hubs are used, these must also be manually engaged in the Lock position. Otherwise, the front axle will not drive the front wheels.
PASSIVE RESTRAINT: Safety systems such as air bags or automatic seat belts which operate with no action required on the part of the driver or passenger. Mandated by Federal regulations on all cars sold in the U.S. after 1990.
PAYLOAD: The weight the car is capable of carrying in addition to its own weight. Payload includes weight of the driver, passengers and cargo, but not coolant, fuel, lubricant, spare tire, etc.
PCM: Powertrain control module. See Electronic Control Unit (ECU).
PCV VALVE: A valve usually located in the rocker cover that vents crankcase vapors back into the engine to be reburned.
PERCOLATION: A condition in which the fuel actually "boils" due to excessive heat. Percolation prevents proper atomization of the fuel causing rough running.
PICK-UP COIL: The coil in which voltage is induced in an electronic ignition.
PING: A metallic rattling sound produced by the engine during acceleration. It is usually due to incorrect ignition timing or a poor grade of gasoline.
PINION: The smaller of two gears. The rear axle pinion drives the ring gear which transmits motion to the axle shafts.
PISTON RING: An open-ended ring which fits into a groove on the outer diameter of the piston. Its chief function is to form a seal between the piston and cylinder wall. Most automotive pistons have three rings: two for compression sealing; one for oil sealing.
PITMAN ARM: A lever which transmits steering force from the steering gear to the steering linkage.
Figure 22 Steering linkage
PLY RATING: A rating given a tire which indicates strength (but not necessarily actual plies). A two-ply/four-ply rating has only two plies, but the strength of a four-ply tire.
POLARITY: Indication (positive or negative) of the two poles of a battery.
POWER-TO-WEIGHT RATIO: Ratio of horsepower to weight of car.
POWERTRAIN: See Drivetrain.
Ppm: Parts per million; unit used to measure exhaust emissions.
PREIGNITION: Early ignition of fuel in the cylinder, sometimes due to glowing carbon deposits in the combustion chamber. Preignition can be damaging since combustion takes place prematurely.
PRELOAD: A predetermined load placed on a bearing during assembly or by adjustment.
PRESS FIT: The mating of two parts under pressure, due to the inner diameter of one being smaller than the outer diameter of the other, or vice versa; an interference fit.
PRESSURE PLATE: A spring-loaded plate (part of the clutch) that transmits power to the driven (friction) plate when the clutch is engaged.
PRIMARY CIRCUIT: The low voltage side of the ignition system which consists of the ignition switch, ballast resistor or resistance wire, bypass, coil, electronic control unit and pick-up coil as well as the connecting wires and harnesses.
PROFESSIONAL TECHNICIAN: A repair technician that has been properly trained in a vehicle’s systems. Usually affiliated with ASE or other certification system. This technician will also have the proper tools to diagnose and repair your vehicle.
PROFILE: Term used for tire measurement (tire series), which is the ratio of tire height to tread width.
Psi: Pounds per square inch; a measurement of pressure.
PUSHROD: A steel rod between the hydraulic valve lifter and the valve rocker arm in overhead valve (OHV) engines.
QUARTER PANEL: General term used to refer to a rear fender. Quarter panel is the area from the rear door opening to the tail light area and from rear wheelwell to the base of the trunk and roof-line.
RACE: The surface on the inner or outer ring of a bearing on which the balls, needles or rollers move.
RACK AND PINION: A type of automotive steering system using a pinion gear attached to the end of the steering shaft. The pinion meshes with a long rack attached to the steering linkage.
Figure 23 Rack and pinion steering
RADIAL TIRE: Tire design which uses body cords running at right angles to the center line of the tire. Two or more belts are used to give tread strength. Radials can be identified by their characteristic sidewall bulge.
RADIATOR: Part of the cooling system for a water-cooled engine, mounted in the front of the car and connected to the engine with rubber hoses. Through the radiator, excess combustion heat is dissipated into the atmosphere through forced convection using a water and glycol based mixture that circulates through, and cools, the engine.
REAR MAIN OIL SEAL: A synthetic or rope-type seal that prevents oil from leaking out of the engine past the rear main crankshaft bearing.
RECALL: When a manufacturer recalls vehicles it has manufactured back to the dealership for specific repairs related to unplanned mechanical problems and/or safety issues. Recalls are usually voluntary and are made in conjunction with regulatory control of the National Highway Traffic Safety Agency (NHTSA). They can originate with the manufacturer or with the NHTSA. Repairs performed under a recall are usually free to the consumer.
RECIRCULATING BALL: Type of steering system in which recirculating steel balls occupy the area between the nut and worm wheel, causing a reduction in friction.
RECTIFIER: A device (used primarily in alternators) that permits electrical current to flow in one direction only.
REFRIGERANT 12 (R-12) or 134 (R-134): The generic name of the refrigerant used in automotive air conditioning systems.
REGULATOR: A device which maintains the amperage and/or voltage levels of a circuit at predetermined values.
RELAY: A switch which automatically opens and/or closes a circuit.
RELUCTOR: A wheel that rotates inside the distributor and triggers the release of voltage in an electronic ignition.
RESIN: A liquid plastic used in body work.
RESISTANCE: The opposition to the flow of current through a circuit or electrical device, and is measured in ohms. Resistance is equal to the voltage divided by the amperage.
RESISTOR SPARK PLUG: A spark plug using a resistor to shorten the spark duration. This suppresses radio interference and lengthens plug life.
RESISTOR: A device, usually made of wire, which offers a preset amount of resistance in an electrical circuit.
RETARD: Setting the ignition timing so that spark occurs later (fewer degrees before TDC).
RING GEAR: The name given to a ring-shaped gear attached to a differential case, or affixed to a flywheel or as part of a planetary gear set.
ROCKER ARM: A lever which rotates around a shaft pushing down (opening) the valve with an end when the other end is pushed up by the pushrod. Spring pressure will later close the valve.
Figure 24 Typical rocker arm and shaft assembly
ROCKER PANEL: The body panel below the doors between the wheel openings.
ROLLER BEARING: A bearing made up of hardened inner and outer races between which hardened steel rollers move.
ROTOR: (1.) The disc-shaped part of a disc brake assembly, upon which the brake pads bear; also called brake disc. (2.) The device mounted atop the distributor shaft, which passes current to the distributor cap tower contacts.
Figure 25 Rotor mounted on top of distributor shaft
ROTARY ENGINE: See Wankel engine.
RPM: Revolutions per minute (usually indicates engine speed).
RUN-ON: Condition when the engine continues to run, even when the key is turned off. See dieseling.
SEALED BEAM: An automotive headlight. The lens, reflector and filament form a single unit.
SEATBELT INTERLOCK: A system whereby the car cannot be started unless the seatbelt is buckled.
SECONDARY CIRCUIT: The high voltage side of the ignition system, usually above 20,000 volts. The secondary includes the ignition coil, coil wire, distributor cap and rotor, spark plug wires and spark plugs.
SEMI-FLOATING AXLE: In this design, a wheel is attached to the axle shaft, which takes both drive and cornering loads. Almost all solid axle passenger cars and light trucks use this design.
SENDING UNIT: A mechanical, electrical, hydraulic or electromagnetic device which transmits information to a gauge.
SENSOR: Any device designed to measure engine operating conditions or ambient pressures and temperatures. Usually electronic in nature and designed to send a voltage signal to an on-board computer, some sensors may operate as a simple on/off switch or they may provide a variable voltage signal (like a potentiometer) as conditions or measured parameters change.
SERPENTINE BELT: An accessory drive belt, with small multiple v-ribs, routed around most or all of the engine-powered accessories such as the alternator and power steering pump. Usually both the front and the back side of the belt come into contact with various pulleys.
SHEATH: The outer casing for clutch or brake cables.
SHIM: Spacers of precise, predetermined thickness used between parts to establish a proper working relationship.
SHIMMY: Vibration (sometimes violent) in the front end caused by misaligned front end, out of balance tires or worn suspension components.
SHORT CIRCUIT: An electrical malfunction where current takes the path of least resistance to ground (usually through damaged insulation). Current flow is excessive from low resistance resulting in a blown fuse.
SINGLE OVERHEAD CAMSHAFT: See overhead camshaft.
SKIDPLATE: A metal plate attached to the underside of the body to protect the fuel tank, transfer case or other vulnerable parts from damage.
SLAVE CYLINDER: In automotive use, a device in the hydraulic clutch system which is activated by hydraulic force, disengaging the clutch.
SLUDGE: Thick, black deposits in engine formed from dirt, oil, water, etc. It is usually formed in engines when oil changes are neglected.
SNAP RING: A circular retaining clip used inside or outside a shaft or part to secure a shaft, such as a floating wrist pin.
SOHC: Single overhead camshaft.
SOLENOID: An electrically operated, magnetic switching device.
SPARK PLUG: A device screwed into the combustion chamber of a spark ignition engine. The basic construction is a conductive core inside of a ceramic insulator, mounted in an outer conductive base. An electrical charge from the spark plug wire travels along the conductive core and jumps a preset air gap to a grounding point or points at the end of the conductive base. The resultant spark ignites the fuel/air mixture in the combustion chamber.
SPECIFIC GRAVITY (BATTERY): The relative weight of liquid (battery electrolyte) as compared to the weight of an equal volume of water.
SPLINES: Ridges machined or cast onto the outer diameter of a shaft or inner diameter of a bore to enable parts to mate without rotation.
SPONGY PEDAL: A soft or spongy feeling when the brake pedal is depressed. It is usually due to air in the brake lines.
SPRUNG WEIGHT: The weight of a car supported by the springs.
SRS: Supplemental restraint system
STABILIZER (SWAY) BAR: A bar linking both sides of the suspension. It resists sway on turns by taking some of the added load from one wheel and putting it on the other.
STARTER: A high-torque electric motor used for the purpose of starting the engine, typically through a high ratio geared drive connected to the flywheel ring gear.
Figure 26 Starter motor assembly location
STEERING GEOMETRY: Combination of various angles of suspension components (caster, camber, toe-in); roughly equivalent to front end alignment.
STRAIGHT WEIGHT: Term designating motor oil as suitable for use within a narrow range of temperatures. Outside the narrow temperature range, its flow characteristics will not adequately lubricate.
STROKE: The distance the piston travels from bottom dead center to top dead center.
SUPERCHARGER: An air pump driven mechanically by the engine through belts, chains, shafts or gears from the crankshaft. Two general types of supercharger are the positive displacement and centrifugal types, which pump air in direct relationship to the speed of the engine.
SUPPLEMENTAL RESTRAINT SYSTEM: See air bag.
SYNCHROMESH: A manual transmission that is equipped with devices (synchronizers) that match the gear speeds so that the transmission can be downshifted without clashing gears.
SYNTHETIC OIL: Non-petroleum based oil.
TAMPERING: Used in conjunction with a vehicles emission control system. Tampering is used to describe any alterations to the original design of the vehicles emission control system.
TAMPERING INSPECTION: An inspection done by State or Local authorities to determine if a vehicle's emission control system has been tampered with.
TACHOMETER: A device used to measure the rotary speed of an engine, shaft, gear, etc., usually in rotations per minute.
TDC: Top dead center. The exact top of the piston's stroke.
THERMOSTAT: A valve, located in the cooling system of an engine, which is closed when cold and opens gradually in response to engine heating, controlling the temperature of the coolant and rate of coolant flow.
THROW-OUT BEARING: As the clutch pedal is depressed, the throwout bearing moves against the spring fingers of the pressure plate, forcing the pressure plate to disengage from the driven disc.
TIE ROD: A rod connecting the steering arms. Tie rods have threaded ends that are used to adjust toe-in.
TIMING BELT: A square-toothed, reinforced rubber belt that is driven by the crankshaft and operates the camshaft.
TIMING CHAIN: A roller chain that is driven by the crankshaft and operates the camshaft.
TIRE ROTATION: Moving the tires from one position to another to make the tires wear evenly.
TOE-IN (OUT): A term comparing the extreme front and rear of the front tires. Closer together at the front is toe-in; farther apart at the front is toe-out.
Figure 27 Wheel toe-in (top view)
TOP DEAD CENTER (TDC): The point at which the piston reaches the top of its travel on the compression stroke.
TORQUE: Measurement of turning or twisting force, expressed as foot-pounds or inch-pounds.
TORQUE CONVERTER: A turbine used to transmit power from a driving member to a driven member via hydraulic action, providing changes in drive ratio and torque. In automotive use, it links the driveplate at the rear of the engine to the automatic transmission.
TORSION BAR SUSPENSION: Long rods of spring steel which take the place of springs. One end of the bar is anchored and the other arm (attached to the suspension) is free to twist. The bars' resistance to twisting causes springing action.
Figure 28 Torsion bar front suspension
TRACK: Distance between the centers of the tires where they contact the ground.
TRACTION CONTROL: A control system that prevents the spinning of a car's drive wheels when excess power is applied.
TRANSAXLE: A single housing containing the transmission and differential. Transaxles are usually found on front engine/front wheel drive or rear engine/rear wheel drive cars.
TRANSDUCER: A device used to change a force into an electrical signal.
TRANSFER CASE: A gearbox driven from the transmission that delivers power to both front and rear driveshafts in a four-wheel drive system. Transfer cases usually have a high and low range set of gears, used depending on how much pulling power is needed.
TRANSISTOR: A semi-conductor component which can be actuated by a small voltage to perform an electrical switching function.
TREAD WEAR INDICATOR: Bars molded into the tire at right angles to the tread that appear as horizontal bars when
Figure 29 Location of tire tread wear indicators
TREAD WEAR PATTERN: The pattern of wear on tires which can be "read" to diagnose problems in the front suspension.
TSB: Acronym for Technical Service Bulletin. This bulletin is produced by the vehicle manufacturer and alerts automotive technicians about specific service problem areas, repair procedures, and new service techniques for a vehicle.
TUNE-UP: A regular maintenance function, usually associated with the replacement and adjustment of parts and components in the electrical and fuel systems of a car for the purpose of attaining optimum performance.
TURBOCHARGER: An exhaust driven pump which compresses intake air and forces it into the combustion chambers at higher than atmospheric pressures. The increased air pressure allows more fuel to be burned and results in increased horsepower being produced.
Figure 30 The turbocharged engine principle uses exhaust gas to spin the turbocharger, increasing maximum engine power output
TURN OVER: Synonymous with "cranks over", the action of the engine internal components rotating during the starting cycle. This is what happens when you turn the key before the engine starts.
U-JOINT (UNIVERSAL JOINT): A flexible coupling in the drive train that allows the driveshafts or axle shafts to operate at different angles and still transmit rotary power.
ULTRAVIOLET LIGHT: A light used in conjunction with a fluorescent dye to detect leaks in the engine, transmission or cooling system.
UNDER LOAD: Used to describe the condition of a vehicle when climbing a steep incline or hill. When the vehicle is placed under a higher than normal amount of load when carrying excessive cargo.
UNDERSTEER: The tendency of a car to continue straight ahead while negotiating a turn.
UNIT BODY: Design in which the car body acts as the frame.
UNLEADED FUEL: Fuel which contains no lead (a common gasoline additive). The presence of lead in fuel will destroy the functioning elements of a catalytic converter, making it useless.
UNSPRUNG WEIGHT: The weight of car components not supported by the springs (wheels, tires, brakes, rear axle, control arms, etc.).
VACUUM ADVANCE: A device which advances the ignition timing in response to increased engine vacuum.
VACUUM MODULATOR: A device used to control the transmission shift points based on the amount of load placed on the engine.
VACUUM GAUGE: An instrument used to measure the presence of vacuum in a chamber.
VALVE: A device which control the pressure, direction of flow or rate of flow of a liquid or gas.
VALVE CLEARANCE: The measured gap between the end of the valve stem and the rocker arm, cam lobe or follower that activates the valve.
VALVE GUIDES: The guide through which the stem of the valve passes. The guide is designed to keep the valve in proper alignment.
VALVE LASH (CLEARANCE): The operating clearance in the valve train.
VALVE STEM SEALS: Synthetic rubber seals that are used to control the oil that lubricates the valve stems in the valve guides. Worn valve stem seals can cause blue smoke from the exhaust when first starting the engine.
VALVE TRAIN: The system that operates intake and exhaust valves, consisting of camshaft, valves and springs, lifters, pushrods and rocker arms.
VAPOR LOCK: Boiling of the fuel in the fuel lines due to excess heat. This will interfere with the flow of fuel in the lines and can completely stop the flow. Vapor lock normally only occurs in hot weather.
VARNISH: Term applied to the residue formed when gasoline gets old and stale.
VCM: See Electronic Control Unit (ECU).
VISCOSITY: The ability of a fluid to flow. The lower the viscosity rating, the easier the fluid will flow. 10 weight motor oil will flow much easier than 40 weight motor oil.
VOLT: Unit used to measure the force or pressure of electricity. It is defined as the pressure needed to move one amp through a resistance of one ohm.
VOLTAGE REGULATOR: A device that controls the current output of the alternator or generator.
VOLTMETER: An instrument used for measuring electrical force in units called volts. Voltmeters are always connected parallel with the circuit being tested.
WANKEL ENGINE: An engine which uses no pistons. In place of pistons, triangular-shaped rotors revolve in specially shaped housings.
WATER PUMP: A belt driven component of the cooling system that mounts on the engine, circulating the coolant under pressure.
WEAR INDICATORS: A metal tab mounted on disc brake pads that touch the brake rotor when the brake linings need replacement.
Figure 31 Water pump mounting onto front of the engine block
WHEEL ALIGNMENT: Inclusive term to describe the front end geometry (caster, camber, toe-in/out).
WHEEL CYLINDER: Found in the automotive drum brake assembly, it is a device, actuated by hydraulic pressure, which, through internal pistons, pushes the brake shoes outward against the drums.
Figure 32 Drum brake components
WHEEL WEIGHT: Small weights attached to the wheel to balance the wheel and tire assembly. Out-of-balance tires quickly wear out and also give erratic handling when installed on the front.
WHEELBASE: Distance between the center of front wheels and the center of rear wheels.