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Glossary of Automotive Terms
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DEAD CENTER: The extreme top or bottom of the piston stroke.
DETERGENT: An additive in engine oil to improve its operating characteristics.
DETONATION: An unwanted explosion of the air/fuel mixture in the combustion chamber caused by excess heat and compression, advanced timing, or an overly lean mixture. Also referred to as "ping".
DEXRON: A brand of automatic transmission fluid.
DIAPHRAGM: A thin flexible wall separating two cavities, such as in a vacuum advance unit.
DIESELING: The engine continues to run after the car is shut off; caused by fuel continuing to be burned in the combustion chamber.
DIFFERENTIAL: A geared assembly which allows the transmission of motion between drive axles, giving one axle the ability to rotate faster than the other, as in cornering.
Figure 12 Differential action during cornering
DIGITAL VOLT OHMMETER: An electronic diagnostic tool used to measure voltage, ohms and amps as well as several other functions, with the readings displayed on a digital screen in tenths, hundredths and thousandths.
DIODE: An electrical device that will allow current to flow in one direction only.
DIRECT CURRENT (DC): Electrical current that flows in one direction only.
DISC BRAKE: A hydraulic braking assembly consisting of a brake disc, or rotor, mounted on an axleshaft, and a caliper assembly containing usually two brake pads which are activated by hydraulic pressure. The pads are forced against the sides of the disc, creating friction which slows the car.
DISPLACEMENT: The total volume of air that is displaced by all pistons as the engine turns through one complete revolution.
DISTRIBUTOR: A mechanically driven device on an engine which is responsible for electrically firing the spark plug at a pre-determined point of the piston stroke.
Figure 13 Typical distributor assembly
DOHC: Double overhead camshaft.
DOUBLE OVERHEAD CAMSHAFT: The engine utilizes two camshafts mounted in one cylinder head. One camshaft operates the exhaust valves, while the other operates the intake valves.
DOWEL PIN: A pin, inserted in mating holes in two different parts, allowing those parts to maintain a fixed relationship.
DRIVE CYCLE TEST: A function of the vehicle on board computer that includes the testing of the monitored systems that requires a start up -- from a cold engine to a warm engine -- and the vehicle being driven.
DRIVE TRAIN: The components that transmit the flow of power from the engine to the wheels. The components include the clutch, transmission, driveshafts (or axle shafts in front wheel drive), U-joints and differential.
DRUM BRAKE: A braking system which consists of two brake shoes and one or two wheel cylinders, mounted on a fixed backing plate, and a brake drum, mounted on an axle, which revolves around the assembly.
DRY CHARGED BATTERY: Battery to which electrolyte is added when the battery is placed in service.
DVOM: Digital volt ohmmeter
Figure 14 Performing a resistance check using a DVOM
DWELL: The rate, measured in degrees of shaft rotation, at which an electrical circuit cycles on and off.
DYNAMOMETER: Various devices used in testing a motor or engine for such characteristics as efficiency and torque, especially an instrument that measures current or the power of a motor by calculating the force between a fixed coil and a moving coil.
EBCM: See Electronic Control Unit (ECU).
ECM: See Electronic Control Unit (ECU).
ENGINE CONTROL SYSTEM: The computer control system that regulates fuel delivery, ignition timing, engine idle speed and on some vehicles the transmission shift points.
ECU: Electronic control unit.
ELECTRODE: Conductor (positive or negative) of electric current.
ELECTROLYTE: A solution of water and sulfuric acid used to activate the battery. Electrolyte is extremely corrosive.
ELECTRONIC ACTUATION SYSTEM: The electronic controls for an anti-lock braking system or electronic suspension system. This unit contains the control computer for the individual system.
ELECTRONIC CONTROL UNIT: A digital computer that controls engine (and sometimes transmission, brake or other car system) functions based on data received from various sensors. Examples used by some manufacturers include Electronic Brake Control Module (EBCM), Engine Control Module (ECM), Powertrain Control Module (PCM) or Vehicle Control Module (VCM).
ELECTRONIC IGNITION: A system in which the timing and firing of the spark plugs is controlled by an electronic control unit, usually called a module. These systems have no points or condenser.
ENAMEL: Type of paint that dries to a smooth, glossy finish.
END-PLAY: The measured amount of axial movement in a shaft.
ENGINE: The primary motor or power apparatus of a car, which converts liquid or gas fuel into mechanical energy.
ENGINE BLOCK: The basic engine casting containing the cylinders, the crankshaft main bearings, as well as machined surfaces for the mounting of other components such as the cylinder head, oil pan, transmission, etc..
EP LUBRICANT: EP (extreme pressure) lubricants are specially formulated for use with gears involving heavy loads (transmissions, differentials, etc.).
ETHYL: A substance added to gasoline to improve its resistance to knock, by slowing down the rate of combustion.
ETHYLENE GLYCOL: The base substance of antifreeze.
EXHAUST MANIFOLD: A set of cast passages or pipes which conduct exhaust gases from the engine.
Figure 15 Typical exhaust manifold
FAST IDLE: The speed of the engine when the choke is on. Fast idle speeds engine warm-up.
FEDERAL ENGINE: An engine certified by the EPA for use in any of the 49 states (except California).
FEELER GAUGE: A blade, usually metal, of precisely predetermined thickness, used to measure the clearance between two parts.
FILAMENT: The part of a bulb that glows; the filament creates high resistance to current flow and actually glows from the resulting heat.
FINAL DRIVE: See axle ratio.
FIRING ORDER: The order in which combustion occurs in the cylinders of an engine. Also the order in which spark is distributed to the plugs by the distributor.
FLAME FRONT: The term used to describe certain aspects of the fuel combustion in the cylinders. The flame front should move in a controlled pattern across the cylinder, rather than simply combusting immediately.
FLAT ENGINE: Engine design in which the pistons are horizontally opposed. Porsche, Subaru and some old VWs are common examples of flat engines.
FLAT RATE: A dealership term referring to the standard fee charged by a technician for a particular repair or diagnostic service versus the actual labor time.
FLAT SPOT: A point during acceleration when the engine seems to lose power for an instant.
FLOODING: The presence of too much fuel in the intake manifold and combustion chamber which prevents the air/fuel mixture from firing, thereby causing a no-start situation.
FLYWHEEL: A heavy disc of metal attached to the rear of the crankshaft. It smoothes the firing impulses of the engine and keeps the crankshaft turning during periods when no firing takes place. The starter also engages the flywheel to start the engine.
Figure 16 The flywheel is mounted to the rear of the crankshaft
FOOT POUND (ft. lbs. or sometimes ft. lb.): The amount of energy or work needed to raise an item weighing one pound a distance of one foot.
FREEPLAY: The amount of travel in a clutch pedal or brake pedal before movement of the clutch or brakes take place. This adjustment is critical to the proper operation of the clutch or brakes.
FREEZE PLUG: A plug in the engine block which will be pushed out if the coolant freezes. Sometimes called expansion plugs, they protect the block from cracking should the coolant freeze.
FRONT END ALIGNMENT: A service to set caster, camber and toe-in to the correct specifications. This will ensure that the car steers and handles properly and that the tires wear properly.
FRONTAL AREA: The total surface area of the front of a car that is exposed to air flow.
FUEL FILTER: A component of the fuel system containing a porous paper element used to prevent any impurities from entering the engine through the fuel system. It usually takes the form of a canister-like housing, mounted in-line with the fuel hose, located anywhere on a car between the fuel tank and engine.
Figure 17 The fuel filter is mounted in-line with the fuel hose
FUEL INJECTION: A system replacing the carburetor that sprays fuel into the cylinder through nozzles. The amount of fuel can be more precisely controlled with fuel injection.
FULL FLOATING AXLE: An axle in which the axle housing extends through the wheel giving bearing support on the outside of the housing. The front axle of a four-wheel drive car is usually a full floating axle, as are the rear axles of many larger (3/4 ton and over) pick-ups and vans.
FULL-TIME FOUR-WHEEL DRIVE: A four-wheel drive system that continuously delivers power to all four wheels. A differential between the front and rear driveshafts permits variations in axle speeds to control gear wind-up without damage.
FUSE: A protective device in a circuit which prevents circuit overload by breaking the circuit when a specific amperage is present. The device is constructed around a strip or wire of a lower amperage rating than the circuit it is designed to protect. When an amperage higher than that stamped on the fuse is present in the circuit, the strip or wire melts, opening the circuit.
FUSIBLE LINK: A piece of wire in a wiring harness that performs the same service as a fuse. If overloaded, the fusible link will melt and interrupt the circuit.
FWD: Front wheel drive.
GAS ANALYZER: A tool used to test the exhaust emissions of a vehicle. Gases tested include CO (carbon monoxide), CO2 (carbon dioxide), HC (hydrocarbons), O2 (oxygen) and sometimes NOx (oxides of nitrogen).
GAWR: Gross axle weight rating. The total maximum weight an axle is designed to carry.
GCW: Gross combined weight. The total combined weight of a tow car and trailer.
GEAR RATIO: A ratio expressing the number of turns a smaller gear will make to turn a larger gear through one revolution. The ratio is found by dividing the number of teeth on the smaller gear into the number of teeth on the larger gear.
GEL COAT: A thin coat of plastic resin covering fiberglass body panels.
GENERATOR: A device which produces direct current (DC) necessary to charge the battery.
GVWR: Gross vehicle weight rating. The total maximum weight a car is designed to carry including the weight of the car, passengers, equipment, gas, oil, etc.
HALOGEN: A special type of lamp known for its quality of brilliant white light. Originally used for fog lights and driving lights.
HEADER TANK: An expansion tank for the radiator coolant. It can be located remotely or built into the radiator.
HEAT RANGE: A term used to describe the ability of a spark plug to carry away heat. Plugs with longer nosed insulators take longer to carry heat off effectively.
HEAT RISER: A flapper in the exhaust manifold that is closed when the engine is cold, causing hot exhaust gases to heat the intake manifold providing better cold engine operation. A thermostatic spring opens the flapper when the engine warms up.
HEATER CONTROL VALVE: The device that controls the flow of hot engine coolant through the heater core.
HEMI: A name given an engine using hemispherical combustion chambers.
HORSEPOWER: A measurement of the amount of work; one horsepower is the amount of work necessary to lift 33,000 lbs. one foot in one minute. Brake horsepower (bhp) is the horsepower delivered by an engine on a dynamometer. Net horsepower is the power remaining (measured at the flywheel of the engine) that can be used to turn the wheels after power is consumed through friction and running the engine accessories (water pump, alternator, air pump, fan etc.)
HUB: The center part of a wheel or gear.
HYDROCARBON (HC): Any chemical compound made up of hydrogen and carbon. A major pollutant formed by the engine as a by-product of combustion.
HYDROMETER: An instrument used to measure the specific gravity of a solution.
HYDROPLANING: A phenomenon of driving when water builds up under the tire tread, causing it to lose contact with the road. Slowing down will usually restore normal tire contact with the road.
HVAC: Heating, ventilation and air conditioning system.
IDLE MIXTURE: The mixture of air and fuel (usually about 14:1) being fed to the cylinders. The idle mixture screw(s) are sometimes adjusted as part of a tune-up.
IDLER ARM: Component of the steering linkage which is a geometric duplicate of the steering gear arm. It supports the right side of the center steering link.
INCH POUND (inch lbs. or sometimes in. lb. or in. lbs.): One twelfth of a foot pound.
INDUCTION: A means of transferring electrical energy in the form of a magnetic field. Principle used in the ignition coil to increase voltage.
INJECTOR: A device which receives metered fuel under relatively low pressure and is activated to inject the fuel into the engine under relatively high pressure at a predetermined time.
INPUT SHAFT: The shaft to which torque is applied, usually carrying the driving gear or gears.
INTAKE MANIFOLD: A casting of passages or pipes used to conduct air or a fuel/air mixture to the cylinders.
Figure 18 Typical intake manifold for a 4-cylinder engine
JOURNAL: The bearing surface within which a shaft operates.
JUMPER CABLES: Two heavy duty wires with large alligator clips used to provide power from a charged battery to a discharged battery mounted in a car.
JUMPSTART: Utilizing the sufficiently charged battery of one car to start the engine of another car with a discharged battery by the use of jumper cables.
KEY: A small block usually fitted in a notch between a shaft and a hub to prevent slippage of the two parts.
KNOCK: Noise which results from the spontaneous ignition of a portion of the air-fuel mixture in the engine cylinder caused by overly advanced ignition timing or use of incorrectly low octane fuel for that engine.
KNOCK SENSOR: An input device that responds to spark knock, caused by overly advanced ignition timing.
LABOR TIME: A specific amount of time required to perform a certain repair or diagnostic service as defined by a car or after-market manufacturer .
LACQUER: A quick-drying automotive paint.
LIMITED SLIP: A type of differential which transfers driving force to the wheel with the best traction.
LITHIUM-BASE GREASE: Chassis and wheel bearing grease using lithium as a base. Not compatible with sodium-base grease.
LOAD RANGE: Indicates the number of plies at which a tire is rated. Load range B equals four-ply rating; C equals six-ply rating; and, D equals an eight-ply rating.
LOCKING HUBS: Accessories used on part-time four-wheel drive systems that allow the front wheels to be disengaged from the drive train when four-wheel drive is not being used. When four-wheel drive is desired, the hubs are engaged, locking the wheels to the drive train.
LOCK RING: See Circlip or Snapring
MANIFOLD: A casting of passages or set of pipes which connect the cylinders to an inlet or outlet source.
MANIFOLD VACUUM: Low pressure in an engine intake manifold formed just below the throttle plates. Manifold vacuum is highest at idle and drops under acceleration.
MASTER CYLINDER: The primary fluid pressurizing device in a hydraulic system. In automotive use, it is found in brake and hydraulic clutch systems and is pedal activated, either directly or, in a power brake system, through the power booster.
Figure 19 Master cylinder
McPHERSON STRUT: A suspension component combining a shock absorber and spring in one unit.
Figure 20 McPherson struts combine shocks and springs in one assembly
MISFIRE: Condition occurring when the fuel mixture in a cylinder fails to ignite, causing the engine to run roughly.
MODULE: Electronic control unit, amplifier or igniter of solid state or integrated design which controls the current flow in the ignition primary circuit based on input from the pick-up coil. When the module opens the primary circuit, high secondary voltage is induced in the coil.
MULTI-WEIGHT: Type of oil that provides adequate lubrication at both high and low temperatures
NEEDLE BEARING: A bearing which consists of a number (usually a large number) of long, thin rollers.
NITROGEN OXIDE (NOx): One of the three basic pollutants found in the exhaust emission of an internal combustion engine. The amount of NOx usually varies in an inverse proportion to the amount of HC and CO.