Glossary Term

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  • 4

  • 4WD
    Four-Wheel Drive
  • a

  • A-Circuit
    Refers to a voltage regulator wired on the groundside of the field circuit. This type of circuit is common on alternators that use electronic voltage regulators.
  • Acceleration Simulation Mode (ASM)
    A dynamometer test that checks a vehicle's HC, CO, and NOx emissions at a steady speed under a fixed load.
  • Accelerometer
    A device used to sense the degree of lateral acceleration on certain vehicles equipped with electronic ride control.
  • Accounts Receivable
    Money owed by customers who charge their purchases. Accounts receivable are listed among the assets of the business.
  • Accumulator
    A storage device for liquid refrigerant located on the low side of the air conditioning system. The accumulator contains a desiccant to absorb any moisture in the system.
  • Accumulator (Refrigeration)
    A storage device for liquid refrigerant located on the low side of the air conditioning system. The accumulator contains a desiccant to absorb any moisture in the system.
  • Accumulator (Transmission)
    A spring-loaded device used to dampen pressure fluctuations within the hydraulic system. The accumulator prevents shock loads by slowing the application rate of a clutch or band.
  • Actuator
    A device controlled by the ECM/TCM, such as a solenoid or relay. Most actuators are controlled by low-side (ground) drivers in the control module.
  • Actuator (Operating Systems)
    A device used to operate the doors inside the HVAC plenum. Heating and air conditioning systems use vacuum actuators, motorized actuators, or a combination of both.
  • Actuator (Transmission)
    A computer controlled output device that provides mechanical action in response to an electrical signal. An electronic pressure control valve is an example of an actuator.
  • Actuator Motor
    An electric motor that regulates the damping rate of the shocks/struts on certain vehicles with electronic ride control.
  • Air Shock/Strut
    A shock absorber or strut with an inflatable rubber membrane or bladder. The bladder connects the piston rod dust tube to the shock/strut body. Air shocks/struts are used on vehicles with automatic level control.
  • Air Spring
    An inflatable bag that substitutes for a traditional coil spring. Air springs are used on certain vehicles equipped with an automatic leveling or ride control system.
  • Alloy
    The material that results from the fusion of two or more different metals. For example, the fusion of copper and zinc produces brass.
  • Alphanumeric
    A combination of letters and numbers. Alphanumeric codes are typically used in catalogs to identify parts.
  • Alternating Current
    An electric current whose polarity changes on a regular basis.
  • Alternating Current (AC)
    An electric current whose polarity constantly changes from positive to negative and then back again.
  • Ambient Air
    The air that surrounds us.
  • Ammeter
    A device used to measure current flow in an electrical circuit.
  • Amperage
    A circuit's total current flow.
  • Ampere
    The unit of measurement for electron flow abbreviated as 'amp.'
  • Ampere-Hour (AH)
    A capacity measurement that indicates the amount of steady current a battery can deliver for 20 hours at 80° F (26.6°C) while maintaining a terminal voltage of 10.5V. For example, a 100 AH battery is capable of delivering 5 amps for 20 hours (100/20 = 5).
  • Amplifier
    A device used to increase signal strength.
  • Amplitude (ABS)
    The maximum voltage level of an electrical signal. In the case of alternating current, amplitude refers to the maximum peak-to-peak voltage of the AC voltage. In a direct current signal however, amplitude refers to the highest voltage reached above the zero volt line.
  • Amplitude (Powertrain)
    The peak value of an electrical signal. When viewed on a scope, the amplitude of a DC signal is the maximum voltage above the zero line. The amplitude of an AC signal is the peak-to-peak (positive-to-negative) signal voltage.
  • Analog Signal
    An electrical signal that changes in direct proportion to a measured quantity.
  • Annealing
    A process of heating metal and then allowing it to cool slowly. Annealing reduces a metal's brittleness and increases its durability.
  • API Service Rating
    The American Petroleum Institute's coding system for classifying engine oils. Gasoline engines are identified with an 'S' prefix for 'spark' ignition, and are rated from 'SA' through 'SJ.' Diesel engines are identified with a 'C' prefix for 'compression' ignition, and are rated from 'CA' through 'CF.' The second letter in the code indicates the oil's specific lubricating qualities.
  • Applied Voltage
    The voltage level present at a circuit load.
  • Apply Side
    Refers to the side of a piston on which force or pressure is exerted.
  • Armature
    A conductor inside a motor that rotates as a result of a reaction between its own magnetic field and the lines of force generated by stationary coils or permanent magnets.
  • Asbestos
    A fibrous incombustible substance used for friction material in brake linings and clutch discs. Be aware that repeated exposure to airborne asbestos fibers has been linked to cancer in humans.
  • ASM 25/25
    A steady load test in which the vehicle runs at 25 mph with a load equivalent to 25% of the power needed to accelerate at 3.3 miles per second. Hydrocarbon and NOx readings are measured in ppm (parts per million), while CO and CO2 readings are expressed as a percentage.
  • ASM 50/15
    A steady load test in which the vehicle runs at 15 mph with a load equivalent to 50% of the power needed to accelerate at 3.3 miles per second. Hydrocarbon and NOx readings are measured in ppm (parts per million), while CO and CO2 readings are expressed as a percentage.
  • Aspect Ratio
    The height of a tire's sidewall with respect to its tread width. For example, a '70s series tire indicates that the sidewall is 70 percent as high as the tread is wide.
  • Aspirator
    A device used to draw air across the in-car temperature sensor of an ATC system.
  • ATC
    Automatic Temperature Control
  • ATDC
    After Top Dead Center
  • Available Voltage
    A circuit's source voltage.
  • AWD
    All-Wheel Drive
  • b

  • B-Circuit
    Refers to a voltage regulator wired on the feed side of the field circuit. This type of circuit is common on older vehicles with electromagnetic voltage regulators.
  • Backlash
    Refers to the clearance or play that exists between two meshing gears.
  • Backpressure
    A term used to indicate the resistance to gas flow in a vehicle's exhaust system. Excessive backpressure reduces an engine's breathing ability,which can result in symptoms ranging from a lack of power to a no-start condition.
  • Backpressure Transducer
    A device used in an EGR control system that modulates the vacuum signal to the EGR valve based on exhaust system backpressure.
  • Ball Bearing
    An anti-friction bearing containing a series of steel balls sandwiched between an inner and outer race.
  • Ball Joint
    A component consisting of a ball-and-socket installed on the control arm. The ball joint is used as both a support and pivot point for the spindle.
  • Band
    A circular brake that wraps around a drum to stop the rotation of a planetary gear component. Bands are hydraulically operated by servos.
  • BAR31
    A 31-second transient load test that includes an acceleration ramp similar to the I/M 240 test.
  • Barometric Pressure
    Atmospheric pressure expressed in terms of 'inches of mercury.'
  • Base Circle
    The bottom part of the cam lobe where no valve lift occurs.
  • Battery Capacity Test
    A test used to determine a battery's ability to handle a sustained high-current load for a period of 15 seconds. When tested at 80¡F, a good battery will maintain a terminal voltage of at least 9.6V for the duration of the test.
  • BDC
    Bottom Dead Center
  • Bearing Crush
    Refers to the load applied to a bearing insert when a main or rod cap is torqued into position. Crush results from the slight distance the bearing insert rises above the cap's parting line.
  • Bell Housing
    The casting attached to the back of the engine on rear-drive vehicles that encloses the clutch disc, pressure plate and related components; also known as the clutch housing.
  • Bellhousing
    The casting attached to the back of the engine on rear-drive vehicles that encloses the clutch disc, pressure plate and related components; also known as the clutch housing.
  • Bench Bleeding
    The method used to remove trapped air from a master cylinder prior to installing the unit on the vehicle.
  • Bi-metallic Strip
    A strip consisting of two dissimilar metals that expand at different rates when heated. As a result, heating the strip causes it to bend. Bi-metallic strips are used in turn signal and hazard flashers to open and close the electrical circuit at regular intervals.
  • Bill of Lading
    A written receipt provided by a carrier for goods accepted for transportation.
  • Bleeder Valve
    A hollow bolt with a tapered seat. The bleeder valve is installed on a caliper or wheel cylinder to allow trapped air to be released from the hydraulic system during brake bleeding.
  • Blend Door
    The door in the plenum assembly that controls discharge temperature by varying the amount of air that passes through the heater core.
  • Blends
    A term used to describe alternative refrigerants that contain more than one chemical.
  • Blocking Ring
    The brass or bronze member of a synchronizer assembly used to slow the rotation of a speed gear.
  • Blowby
    Combustion gases that leak past the piston rings and flow into the crankcase during the engine's power stroke.
  • Bonded Linings
    Friction material attached to drum brake shoes with adhesive.
  • Bonded Pads
    Friction material attached with adhesive to the steel backing of a disc brake.
  • Boss
    An extension or reinforced section of certain components, such as the projections on the outside of the block that accept the engine mount bolts.
  • Brake Caliper
    The device used in a disc brake system that converts master cylinder pressure into a mechanical force. The caliper operates similar to a vise by clamping the rotor on either side when the brake pedal is depressed.
  • Brake Fluid
    A non-petroleum type fluid used to transfer pressure in a hydraulic brake system. Virtually all passenger cars and light trucks use DOT 3 or DOT 4 brake fluids, which are amber colored glycol-based liquids. Many commercial vehicles, motorcycles, and German-made cars use the silicone-based DOT 5 brake fluid, due to its high boiling point. This fluid satisfies the demanding braking requirements associated with these vehicles (repeated stop-and-go or high speed braking). DOT 5 can be recognized by its purple color.
  • BTDC
    Before Top Dead Center
  • Bulletin
    A notice issued by the manufacturer that corrects catalog errors.
  • Burnishing
    A process that involves repeated and controlled moderate braking from approximately 30 mph (48 km/h) to properly seat the brake linings.
  • Bushing
    A sleeve press-fitted into a bore that serves as the bearing surface for a shaft.
  • c

  • Calculated Load Value
    A scan data value that indicates the engine's volumetric efficiency (actual airflow divided by maximum airflow).
  • Caliper
    The component in a disc brake system that converts hydraulic pressure into mechanical clamping force.
  • Cam Lobe
    The egg-shaped part of the camshaft that controls valve lift and duration.
  • Camber
    Camber is the inward or outward tilt of the tire with reference to true vertical, as viewed from either end of the vehicle. When the top of the tire tilts in (toward the vehicle), camber is negative. When the top of the tire tilts away from the vehicle, camber is positive. Camber is not adjustable on every vehicle.
  • Capillary Tube
    A tube filled with a liquid or gas that expands as temperature rises. The capillary tube serves as a link between a remote sensing bulb and the expansion valve or thermostatic switch.
  • CARB
    California Air Resources Board
  • Carbon Dioxide (CO2)
    A byproduct of the combustion process consisting of one molecule of carbon and two molecules of oxygen. CO2 is used to indicate combustion efficiency, and is not considered a pollutant.
  • Carbon Monoxide
    A colorless, odorless, poisonous gas.
  • Carbon Monoxide (CO)
    A byproduct of the combustion process that results when there is a lack of oxygen in the mixture. CO consists of one molecule of carbon and one molecule of oxygen. High carbon monoxide emissions are always the result of a rich fuel mixture.
  • Cardan Joint
    A universal joint with four machined trunnions equally spaced around the center axis.
  • Carrying Cost
    The cost of buying and stocking parts to maintain adequate inventory.
  • Case
    A selling unit increment that refers to several parts packaged together.
  • Cash Discount
    A discount extended to charge customers who pay their bill on or before a specified date.
  • Caster
    Looking from the side of the vehicle, caster refers to the forward or rearward tilt of the steering axis from true vertical. The weight of the vehicle is supported through the steering axis. When caster is positive, it allows the tire to be pulled along by the vehicle's weight, since the load is projected out to a point in front of the tire's contact area. When caster is negative, the load is moved behind the tire. This position lessens steering effort, but upsets the vehicle's directional stability. Caster does not directly affect tire wear, and is not always adjustable.
  • Catalog Maintenance
    The process of updating catalogs by inserting the appropriate bulletins and/or supplements, or by replacing outdated catalogs with current editions.
  • Catalyst
    An element or compound capable of causing a chemical reaction without being consumed in the process.
  • CCOT
    Cycling Clutch Orifice Tube
  • Centerlink
    Part of a parallelogram style steering system that transfers pitman arm movement to the tie rods.
  • Chamfer
    A beveled edge.
  • Charcoal Canister
    The heart of the evaporative emissions control system. The canister keeps fuel vapors from the gas tank and carburetor float bowl (if applicable) suspended in activated charcoal until the purge cycle begins.
  • Choke Pull-off
    A vacuum operated diaphragm mounted on the outside of a carburetor. The pull-off is used to open the choke valve a predetermined amount in order to prevent excessively rich fuel mixtures during cold starts.
  • Circuit Breaker
    A device used to protect an electrical circuit from current overloads. An automotive circuit breaker will automatically reset once the internal bi-metallic strip cools down.
  • CKP
    Crankshaft Position
  • Clean Air Act
    A law passed in 1990 that established national policy regarding the reduction and elimination of substances known to damage the earth's ozone layer.
  • Close
    The point at which a salesperson gets the customer to commit to a purchase.
  • Closed Loop
    A fuel control strategy that allows the computer to adjust the mixture based on signals it receives from the O2 sensor(s).
  • Clutch Pack
    A series of alternately stacked metal and composition plates.
  • Clutch Shaft
    The shaft splined to the clutch disc; also called the input shaft.
  • CMP
    Camshaft Position
  • Coefficient Of Friction
    A measurement equal to the force required to slide an object across a surface divided by the weight of the object.
  • Coefficient of Friction (COF)
    The ratio between the force needed to move one object against another, to the force holding the two objects together. The COF can be calculated by dividing the force required to move the object by the weight of the object. (e.g. if 40 pounds of force is needed to move a 100 pound object then 40 ÷ 100 = .40 COF)
  • Coil-Over-Plug (COP)
    A type of distributorless ignition system that has an individual coil mounted above each spark plug. While most COP systems have no secondary cables, there are a few where the coils are mounted remotely and therefore require short secondary cables.
  • Cold Cranking Amps (CCA)
    A capacity measurement that indicates the amount of current a battery can supply for 30 seconds at 0°F (17.7°C) without the terminal voltage dropping below 7.2V.
  • Cold Inflation Pressure
    The air pressure of a tire that has not been driven more than 1 mile or has been idle for at least 3 hours. The tire's load rating is always based on cold inflation pressure.
  • Cold Start Injector
    A non-computer controlled injector that delivers additional fuel during cold engine starts.
  • Combination Valve
    A component of the hydraulic brake system that integrates the metering, proportioning, and brake warning light functions into a single assembly.
  • Compression Fitting
    A flareless fitting that uses a crushable brass sleeve for sealing.
  • Compression Ratio
    Indicates the difference in cylinder volume between BDC and TDC.
  • Concentric Slave Cylinder
    A hydraulic cylinder located within the clutch housing that acts directly on the release bearing.
  • Conflict
    A condition in which the simultaneous execution of two monitors would result in one or both monitors failing incorrectly.
  • Contaminated Refrigerant
    This term applies to refrigerant containing air, dirt, or moisture. Refrigerant is also considered contaminated when it contains more than one chemical, such as when R12 and R134a are mixed, or when a 'blend' refrigerant is added to an R12 or R134a system.
  • Coolant
    A blend of water and ethylene glycol-based antifreeze. A 50/50 mixture provides freeze protection down to -34¡F (-37¡C), and raises the boiling point to 265¡F (129¡C) when used in a system with a 15-psi pressure cap.
  • Core Charge
    A charge applied to the sale of remanufactured parts. A core charge is added to an invoice to cover the cost of the old part, should the customer fail to return it after they install the replacement. For example, a remanufactured starter may sell for $150.00 plus a $25 core charge. The core charge will be refunded to the customer once they return the old part. Cores are valuable since they must be returned to the remanufacturer.
  • Core Plugs
    Steel discs used to fill the core holes in the engine block. The core holes allow residual sand to be removed from the block following the casting process.
  • Core Plugs (a.k.a. Freeze Plugs)
    Steel discs used to fill the core holes in the engine block. The core holes allow residual sand to be removed from the block following the casting process.
  • Corrosive
    Anything that burns; or eats away skin, metals, or other materials.
  • Coupe
    A vehicle with two doors.
  • Crank Pins
    The offset journals on the crankshaft that accept the connecting rods.
  • Crank Throw
    Refers to the distance between the centerline of a crankshaft's main bearing journals and the centerline of the crank pins (rod journals).
  • Crankcase Pressure
    Pressure that builds inside the crankcase during the power stroke as combustion gases leaks past the piston rings. Crankcase pressure is relieved through the PCV system.
  • Crankcase Vacuum
    A negative pressure that develops inside the crankcase as a result of a properly operating PCV system.
  • Crest
    The outermost part of a fastener's threads.
  • Crew Cab
    A truck with four full-size doors and a rear seating area.
  • Cross Reference
    A list of part numbers that cross over between manufacturers. For example, a filter catalog may list oil filters from several manufacturers so that if one brand is not available, another brand can be conveniently identified.
  • CV Boot
    An accordion pleat installed over a constant velocity joint. The boot is used to contain the joint grease as well as protect the joint from external contaminates.
  • Cycling Circuit Breaker
    A device used to protect electrical circuits from current overloads. A cycling circuit breaker, which is the most widely used type, resets automatically once the temperature of its internal bi-metallic strip drops to a pre-determined level.
  • Cylinder Leakage
    Refers to compression losses that occur as a result of worn rings, burned valves, blown head gasket, loose spark plugs, cracked block, or cracked head.
  • Cylinder Liner
    An iron sleeve used to line the cylinder bores of aluminum blocks. Liners are also used to restore a badly worn cylinder in a cast iron block.
  • d

  • Data Link Connector (DLC)
    The computer interface connector that accepts a scan tool; typically located below the instrument panel.
  • Denominator
    The lower number of a fraction.
  • Department of Transportation (DOT)
    A government agency responsible for establishing standards for vehicle components.
  • Deposit
    An amount of money left by a customer toward the purchase of a special ordered part.
  • Desiccant
    A drying agent contained in the accumulator or receiver-dryer that absorbs moisture in the air conditioning system.
  • Detonation
    A metallic rattling sound emanating from the cylinder as a result of two pressure fronts colliding during the combustion process. One pressure front is initiated through the firing of the spark plug. The second pressure front develops through spontaneous combustion, which results as temperature and pressure rise in the cylinder and cause the mixture to ignite in another location.
  • DI
    Distributor Ignition
  • Diaphragm Spring Pressure Plate Assembly
    An assembly that uses a steel plate and segmented convex spring to hold the clutch disc against the flywheel.
  • Dichlorodifluoromethane
    The chemical name for R12 refrigerant.
  • Digital Signal
    An electrical signal in one of two states: ON/OFF or HIGH/LOW.
  • Diode
    A one-way electrical check valve used in an alternator to convert AC to DC.
  • Discard Thickness
    A dimension cast into a brake rotor that indicates the wear limit of the disc.
  • Discharge Pressure
    The pressure of the vapor being expelled from the A/C compressor into the condenser.
  • Disposable Container
    A container used to store virgin refrigerant only. Disposable containers are not to be used for storing recycled refrigerant.
  • Distributor Ignition (DI)
    An ignition system that uses a distributor to deliver secondary voltage to the spark plugs.
  • DMM
    Digital Multimeter
  • Do-it-Yourselfer (DIY)
    Customers who service their own vehicles. DIYs generally perform light maintenance only and buy parts and supplies on an infrequent basis.
  • Dolly Block
    A tool used to straighten damaged body panels.
  • DOT
    Department of Transportation
  • DOT (Department of Transportation)
    A government agency responsible for establishing performance standards for vehicle components.
  • DOT 4BA or DOT 4BW
    Identification marks stamped on the tank collar of an approved portable container.
  • Double Flare
    The style of flare specified for brake line connections. With a double flare, the flared end of the tube is folded inward, creating a double wall that reinforces the flare.
  • Dowel
    A pin extending from one component that fits into a corresponding hole in a mating part.
  • Drive Cycle
    A driving routine that occurs within one key-on, key-off period, consisting of various vehicle-operating conditions such as idle, acceleration, cruise, and deceleration. A complete drive cycle is required in order for all of the OBD II monitors to run.
  • Drive Member
    A gear that transmits power to other members of a planetary gearset.
  • Driver
    A solid-state switch used to control the operation of computer output devices such as solenoids and relays.
  • Dry Boiling Point
    The boiling point specification of uncontaminated brake fluid (new fluid).
  • DTC
    Diagnostic Trouble Code
  • Duo-Servo
    A style of drum brake system where the motion of one shoe activates the other. A duo-servo design can be identified by the fixed anchor pin mounted at the top of the backing plate, and an adjustment screw that joins the bottom of the brake shoes together.
  • Dust Boot
    A round-shaped rubber component used to prevent contamination in a wheel cylinder or a caliper.
  • Dust Boot (Disc Brake)
    A circular-shaped rubber lip seal used to prevent contaminates from entering the brake caliper.
  • Dust Boot (Drum Brake)
    A circular-shaped rubber lip seal used to prevent contaminates from entering the wheel cylinder.
  • Duty Cycle
    The ON-time of a controlled device expressed as a percentage.
  • Dynamic Balance
    A condition that exists when the mass of a rotating tire/wheel assembly is equally distributed around its axis of rotation as well as its centerline. Dynamic balancing is also known as dual-plane balancing, since weights are placed on both sides of the wheel.
  • e

  • Early Fuel Evaporation (EFE)
    An emissions control system that improves fuel vaporization by preheating the air/fuel mixture. An EFE system may use a heat riser valve in the exhaust or an electric grid below the carburetor or throttle body.
  • ECT
    Engine Coolant Temperature
  • EEPROM
    Electronically Erasable Programmable Read Only Memory
  • EGR
    Exhaust Gas Recirculation
  • EGR Backpressure Transducer
    A device used to modify the vacuum signal to the EGR valve based on exhaust system backpressure.
  • EGR Valve
    A device that meters small amounts of exhaust gas into the induction system. The spent exhaust gases reduce combustion temperatures, which help lower oxides of nitrogen (NOx) emissions.
  • EI
    Electronic (distributorless) Ignition
  • Electrical Load
    Any device that draws current from the battery or the alternator.
  • Electrolyte
    A mixture of sulphuric acid and water.
  • Electromagnet
    A soft iron core that generates a strong magnetic field when current is passed through the wire encircling it. The rotor inside the alternator is an example of an electromagnet.
  • Electronic Ignition (EI)
    This type of ignition system does not use a distributor and is commonly referred to as a distributorless ignition system.
  • EMI
    Electro-Magnetic Interference
  • Enable Criteria
    A specific set of conditions that must be satisfied in order for the ECM to execute an OBD II monitor.
  • EPA
    Environmental Protection Agency
  • Equivalent Resistance
    The overall resistance in a parallel circuit.
  • Evacuation
    The process of creating a vacuum in the air conditioning system for removing air and moisture.
  • Event Cylinder
    Refers to the cylinder firing on the compression stroke in a waste spark ignition system.
  • Excise Tax
    An indirect tax on products such as tires produced, sold, used, or transported within a country.
  • Exhaust Backpressure
    The term for the resistance to the flow of exhaust gases.
  • Extended Cab
    A truck with two doors and an ample storage space behind the front seat. Extended cab trucks may come with a third or even fourth door to access the storage area.
  • Extended Warranty
    A warranty that provides coverage after the new car warranty expires.
  • f

  • Facing
    A merchandising technique used to simulate fully stocked shelves by bringing products to the front of the shelf with their labels forward.
  • Fastback
    A sports car with a sloping rear hatch that opens.
  • Federal Test Procedure (FTP)
    The FTP is a series of tests designed to measure the emissions output of new cars and light trucks sold in the USA.
  • Feeler Gauge
    A tool consisting of a series of blades with precise thickness used for measuring clearances.
  • Ferrous Metal
    A metal that contains iron or steel and is therefore magnetic.
  • Ferrule
    The brass sleeve inside a compression fitting.
  • Fixed Caliper
    A rigidly attached brake caliper that does not move when the brakes are applied. Fixed calipers have pistons on either side of the brake disc.
  • Flange
    The collar or rim found on certain parts for holding them in place. A crankshaft thrust bearing has a flange on either side.
  • Flash Code Diagnostics
    A feature of most OBD I systems that allows fault codes to be retrieved by counting the number of times the malfunction indicator light blinks when the diagnostic mode is activated.
  • Flat-Rate
    A system of compensating technicians based upon the number of labor hours they produce. Each service and repair procedure is assigned a specific time. For example, replacing the front brake pads on a particular model may be assigned 1.0 labor hours. With the flat-rate system, the technician will be paid 1.0 hour x their hourly rate regardless of the time required to actually perform the service.
  • Flexplate
    The name given to the steel disc and ring gear that joins the engine to an automatic transmission/transaxle.
  • Floating Caliper
    A brake caliper that uses one or more pistons on the inboard side of the rotor only. When the brakes are applied, the piston(s) pushes the inboard pad against the rotor, which causes the caliper to move in toward the vehicle. This action causes the outboard pad to be applied against the opposite side of the rotor with equal force.
  • Floor Traffic
    Customers who come into the store to make their purchases as opposed to shops who order their supplies over the phone.
  • Follower Joint
    The ball joint mounted to the unloaded control arm.
  • Freeze Frame Data
    A snapshot of information recorded by the ECM/TCM the moment it detects an emissions-related failure. Freeze Frame data is recorded for the first failure only, and may be overwritten by a different failure with a higher priority.
  • Frequency
    The number of signal periods occurring in one second. Signal frequency is expressed in Hertz (Hz), which represents cycles per second.
  • Frequency (ABS)
    A value measured in hertz (Hz), used to denote the number of times one cycle of a periodic electrical signal repeats within one second.
  • Friction Modifiers
    Additives used to enhance an oil's lubricating properties.
  • Fuel Cut Solenoid
    An electrical device found predominantly on import carburetors. The solenoid is used to stop fuel flow during deceleration and engine shutdown, which helps reduce HC emissions and prevents dieseling.
  • Fuel Trim (FT)
    The term used for adjustments made to injector pulse width, which are based on feedback signals from the upstream oxygen sensors. A reading of 0% indicates that no corrections are being made to the ECM's programmed fuel delivery calculations. Values greater than 0% indicate that the ECM is adding fuel to compensate for a lean exhaust (low HO2S voltage), while readings below 0% show that the PCM is reducing injector pulse width to compensate for a rich exhaust (high HO2S voltage).
  • Full-Fielding
    A diagnostic procedure that tests charging system output by bypassing the voltage regulator and applying maximum current to the rotor's field winding.
  • Full-Time 4WD
    A four-wheel drive system that incorporates a center differential inside the transfer case. This system allows the vehicle to be driven in 4WD all the time if desired. When maximum grip is needed on low traction surfaces, the center differential can be locked.
  • Fuse
    A one-time device used to protect electrical circuits from current overloads.
  • Fusible Link
    A length of wire used to protect electrical circuits from current overloads. Fusible links are typically four gauge sizes smaller than the circuit they're designed to protect.
  • FWD
    Front-Wheel Drive
  • g

  • GCWR
    The Gross Combination Weight Rating indicates the maximum allowable weight of a loaded tow vehicle and trailer.
  • Gear Pump
    An internal/external gear pump whose gears are separated by a crescent-shaped boss.
  • General Service Items
    Maintenance products such as oils, fluids, waxes, etc.
  • Generator
    OBD II (J1930 Standard) term for alternator.
  • Glaze
    A thin residue that forms on the cylinder wall through the combination of heat, oil, and friction over a long period.
  • Gondola
    A long shelving or rack system designed for displaying products as well as dividing floor space into aisles.
  • Governor
    A mechanical device consisting of weights and springs that is driven by a gear on the output shaft. The governor is used to modify line pressure based on changes in vehicle speed.
  • gpm
    grams per mile
  • gps
    grams per second (gm/sec)
  • Gross Vehicle Weight Rating (GVWR)
    Refers to the maximum allowable weight of a fully loaded vehicle including, the vehicle itself, passengers, and cargo.
  • GVWR
    The Gross Vehicle Weight Rating indicates the maximum allowable vehicle weight when loaded with driver, passengers, and cargo.
  • h

  • Halogen Lamps
    A lamp containing a tungsten filament surrounded by halogen gas. Halogen lamps produce 25% greater candlepower than comparable incandescent lamps.
  • Handling Charge
    A fee charged to a customer for ordering or returning certain parts.
  • Hatchback
    A coupe or sedan that has a hinged rear 'hatch' that provides access into the passenger compartment.
  • Hazardous
    Anything that causes illness, injury, or death; or pollutes air, land, or water.
  • Head Pressure
    Another term for discharge pressure.
  • Heater Control Valve
    A vacuum, cable, or thermostatically controlled gate valve used to regulate coolant flow through the heater core.
  • Height Sensor
    Used to determine the vehicle's actual ride height and signal the suspension computer to raise or lower the vehicle accordingly.
  • Helical Gear
    A gear whose teeth are cut at an angle.
  • High Side
    Refers to the points between the compressor discharge port and the metering device inlet.
  • High-Volume Low-Pressure (HVLP) Gun
    A spray gun that atomizes paint into low-speed particles before application. HVLP guns reduce overspray by as much as 80% compared to conventional spray guns.
  • HO2S
    Heated Oxygen Sensor
  • Horizontal
    Sideways, or parallel to floor.
  • Hot Idle Compensator
    A valve located in the air cleaner that helps lean out a rich fuel mixture during high ambient temperatures.
  • Hybrid Vehicle
    A vehicle that uses more than one energy source.
  • Hydraulic Modulator
    An electro-hydraulic component that regulates fluid pressure to the wheels in an anti-lock brake system.
  • Hydro-boost
    A type of brake booster used on some vehicles that uses hydraulic pressure from the power steering pump to operate. Hydro-boost was used on many Ford and Lincoln automobiles with gasoline engines in the 1970s. Today, it is commonly used in trucks or diesel-powered cars, and produces exceptional power for its small size when compared to the power produced by a comparable vacuum booster.
  • Hydrocarbons (HC)
    Unburned fuel molecules (hydrogen and carbon) that did not separate during the combustion process. High HC emissions can result from rich fuel mixtures, an engine misfire, or a leak in the evaporative emissions system.
  • Hygroscopic
    The property of attracting moisture. DOT 3 and 4 brake fluids are hygroscopic.
  • Hypereutectic Piston
    An aluminum alloy piston with a silicon content greater than 12 percent. The addition of silicon increases the piston's strength, wear resistance, and thermal stability.
  • Hypoid Gearset
    A ring and pinion whose teeth mesh below the ring gear centerline. Hypoid gears are spiral or bevel shaped.
  • i

  • I/M
    Inspection/Maintenance
  • I/M 240
    A dynamometer test that checks a vehicle's output of HC, CO, and NOx emissions under various speed and load conditions for a total of 240 seconds (4 minutes). In this test, vehicle emissions are measured by weight and test results indicated in grams per mile (gpm).
  • IAC
    Idle Air Control
  • IAT
    Intake Air Temperature
  • Idler Arm
    Part of a parallelogram style steering system that works in conjunction with the pitman arm to support the centerlink. The idler arm is rigidly mounted to the vehicle's frame.
  • IMACA
    International Mobile Air Conditioning Association
  • Impedance
    The total opposition to the flow of alternating current in a circuit. Impedance is measured in ohms and includes both resistance and reactance.
  • Impeller
    The rear half of the torque converter shell that drives the turbine. The impeller is also called a pump.
  • Impulse Items
    Relatively inexpensive products customers purchase on the spur of the moment.
  • Included Angle
    The included angle is a measurement that combines SAI and camber reading. The included angles on each side of the vehicle should be nearly the same unless a spindle or strut is bent.
  • Individually Marked
    Items with an attached price tag.
  • injury
    land
  • Inner Tie Rod
    Part of the steering linkage that attaches to the centerlink on recirculating ball systems with parallelogram style linkage. On rack and pinion systems, the inner tie rod is secured directly to the rack, either at the end of the steering gear or in the center of it.
  • Integral ABS
    A high-pressure anti-lock brake system that uses a single device to perform the functions of the hydraulic modulator, master cylinder, and booster.
  • Invoice
    An itemized list of merchandise or services rendered to a buyer.
  • ISC
    Idle Speed Control
  • ISO
    International Standards Organization
  • ISO Flare
    A style of flare established by the International Standards Organization. An ISO flare is shaped like a bubble rather than being folded inward like a double flare. This design makes an ISO flare less susceptible to damage when overtightened.
  • j

  • Jobber
    One who buys goods in bulk from a warehouse distributor and sells to the end-user.
  • k

  • Key-Off Battery Drain
    Refers to the current used to power the memory and timer functions of certain electronic devices (e.g. engine computer) when the vehicle is not in use.
  • kilovolt (kV)
    1000 volts (1kV)
  • Kingpin
    A heavy pin that operates like a door hinge and allows the spindle to pivot. Kingpins can be found on vehicles that have a solid front axle.
  • km/h (kilometers/hr.)
    1.6 km = 1 mile
  • Knurling
    A procedure used to restore the inside diameter of a worn valve guide.
  • KOEO
    Key on, engine off
  • KOER
    Key On/Engine Running
  • kPa (kilopascals)
    6.895 kPa = 1 psi
  • l

  • Land
    The area of metal on a piston that separates the rings from one another.
  • Lapping
    An alternative to machining the valves and seats. Lapping involves applying an abrasive compound between the valve face and seat, and then spinning the valve by hand to conform the two surfaces.
  • Lateral Runout
    The side-to-side (horizontal) movement of a rotating tire/wheel and hub assembly.
  • Lateral Runout (Disc Brake)
    The side-to-side movement of a rotor (also referred to as rotor wobble). Lateral runout is measured using a dial indicator.
  • LCD
    Least common denominator, the smallest number that all of the denominators will divide into.
  • Leading Shoe
    The shoe in a leading/trailing drum brake system positioned closer to the front of the vehicle. The leading shoe provides most of the forward braking power.
  • Leading/Trailing
    A type of drum brake system where the shoes operate independently of each other (non-servo).
  • Light-Emitting Diode (LED)
    A diode that produces light when forward biased due to the composition of its semiconductor material. LEDs are used as single indicator lights or can be grouped together for displaying numbers or messages.
  • Line
    A specific manufacturer's product offerings.
  • Line Pressure
    The name given to regulated pump pressure; also known as 'control pressure.'
  • Liquid Crystal Display (LCD)
    A type of segmented display used for instrumentation that consists of a liquid sandwiched between layers of glass or plastic. When voltage is applied to the crystal, the fluid becomes opaque and blocks an outside light source from illuminating the segment. When the voltage is interrupted, the segment illuminates. The computer turns the crystals on and off to form specific alphanumeric characters.
  • List Price
    The price the DIY typically pays for merchandise; also known as the retail price.
  • Live Axle
    A solid shaft that transmits power from the differential side gears to the wheels.
  • Loaded Joint
    The ball joint mounted to the control arm that is under tension by the coil spring.
  • Locking Hub
    A mechanism inside the front wheel hubs that allows the axle shafts to drive the front wheels; locking hubs can be manually or automatically operated.
  • Long Bed
    A pickup truck with an eight-foot bed.
  • Low Side
    Refers to the points between the metering device outlet and the compressor suction port.
  • m

  • MACS
    Mobile Air Conditioning Society
  • Magnetic Sensor
    A device commonly used for monitoring crankshaft position. The sensor contains a permanent magnet and produces an AC voltage when triggered.
  • Manifold Absolute Pressure (MAP) Sensor
    An electronic device that converts intake manifold pressure into an electrical signal. In some electronic transmission/transaxles, the computer uses the signal from the MAP sensor to adjust line pressure according to changes in altitude.
  • Manifold Vacuum
    The vacuum signal originating from below the closed throttle plate. Manifold vacuum is highest during deceleration and lowest at wide-open throttle.
  • Manual Valve
    A spool valve operated by the shift linkage. The manual valve directs hydraulic pressure to the appropriate location to apply bands and clutches.
  • MAP
    Manifold Absolute Pressure
  • Margin
    The area between the valve face and the head of the valve.
  • Markdown Item
    Products that have been significantly reduced due to damage or obsolescence.
  • Mass Airflow (MAF)
    A technique for determining engine airflow based on a direct measurement of air volume and density.
  • Mass Airflow (MAF) System
    A control strategy that determines engine airflow through direct measurement using a MAF sensor.
  • Match Mounting
    A procedure (also known as vectoring) used to offset excessive tire or wheel runout by aligning the tire's high spot to the wheel's low spot or vice-versa.
  • Material Safety Data Sheet (MSDS)
    A list of information regarding substances used in the workplace that pose a health and/or safety risk.
  • Maximum Diameter
    A dimension stamped on a brake drum indicating the maximum allowable inside braking surface diameter.
  • Maximum Refinish Diameter
    The maximum allowable inner diameter of a brake drum after machining.
  • Maximum Wear Diameter
    The maximum allowable inner diameter of a brake drum attributed to normal wear.
  • Memory Steer
    A condition generally associated with MacPherson strut suspensions where the vehicle will pull in the direction of the last completed turn. Memory steer is usually the result of an over-torqued top strut nut or a binding strut bearing.
  • Merchandising
    The art of marketing goods through advertising, product displays, inventory management, and various promotional campaigns.
  • Metering Valve
    A hydraulic component used to delay the application of the front disc brakes on a vehicle equipped with a disc/drum system.
  • MIL
    Malfunction Indicator Lamp
  • millisecond (mS)
    1 thousandth of a second
  • Min/Max Levels
    In a computerized tracking system, the min/max levels indicate the minimum and maximum quantity of a particular item the jobber should have on hand.
  • Minimum Thickness
    A dimension stamped on a brake rotor, indicating the minimum thickness the disc can reach and still be used safely.
  • Mode Door
    The doors in the plenum assembly that control air induction and distribution.
  • Monitor
    The term used for the OBD II diagnostic tests run by the ECM/TCM. Monitors are executed on a continuous or a non-continuous basis, and are used to evaluate the performance of emission-related components and sub-systems.
  • Monoleaf Spring
    A type of suspension spring that consists of a single flat piece of steel or fiberglass.
  • MSDS
    Material Safety Data Sheet
  • Multi-Leaf Spring
    A type of suspension spring that consists of multiple steel or fiberglass laminations linked together.
  • Multi-Port Fuel Injection (MPFI)
    Fuel injection systems that use one injector per cylinder are generally classified as MPFI systems.
  • Multiport Fuel Injection (MFI)
    An injector firing strategy in which all the injectors are fired simultaneously, which results in two shots of fuel per combustion cycle.
  • n

  • Negative Temperature Coefficient (NTC) (Emissions Control)
    The property of a thermistor that allows its resistance to fall as temperature rises. The ECT sensor is an example of an NTC thermistor.
  • Negative Temperature Coefficient (NTC) (Operating Systems)
    The term used to describe a thermistor whose resistance value decreases as the sensed temperature rises. The in-car and outside temperature sensors are both NTC thermistors.
  • Net 30, 60, or 90
    The terms indicating when a bill must be paid before interest charges accrue; 30, 60, or 90 days.
  • Net Price
    The price paid for merchandise by high-volume purchasers such as independent repair shops, fleet owners, and dealers.
  • Neutral Safety Switch
    An electrical device used to open the starter circuit when the transmission/transaxle is in gear.
  • Non-Integral ABS
    An anti-lock brake system that uses a traditional master cylinder and vacuum brake booster. Also known as 'add-on' ABS.
  • Normally Closed (N.C.)
    A notation used in wiring diagrams indicating that a relay's switch contacts are closed when the relay coil is de-energized. Also used to describe the status of a dedicated switch, such as a power steering pressure switch.
  • Normally Open (N.O.)
    A notation used in wiring diagrams indicating that a relay's switch contacts are open when the relay coil is de-energized. Also used to describe the status of a dedicated switch, such as a power steering pressure switch.
  • Nose
    The high part of the cam lobe.
  • Numerator
    the top number of a fraction.
  • o

  • O2S
    Oxygen Sensor
  • OBD I
    The first generation of mandated on-board diagnostic systems.
  • OBD II
    The second generation of mandated on-board diagnostic systems.
  • OE
    Original Equipment
  • OEM
    Original Equipment Manufacturer
  • Offset
    Positioned off center, as in the pinion gear-to-ring gear in a hypoid gearset.
  • OHC
    Overhead Cam
  • Ohm
    The unit of measurement for electrical resistance.
  • Ohm's Law
    The principle that defines the relationship between voltage, amperage, and resistance in an electrical circuit.
  • Ohmmeter
    A device used to measure the resistance in an unpowered electrical circuit or individual electrical components.
  • OHV
    Overhead Valve
  • Oil Pump Screen Bypass Valve
    A valve in the pickup screen that opens under low-temperature conditions when the oil is too cold to flow freely. The valve also opens in the event the pickup screen becomes clogged.
  • On-Board Diagnostic Test
    An Inspection/Maintenance test in which the vehicle is evaluated based on stored DTCs, the status of the MIL, and the readiness of the OBD II monitors.
  • Open Circuit
    An electrical circuit where continuity has been interrupted due to a broken wire, detached connection, or defective load.
  • Open Loop
    A computer operating strategy that controls fuel delivery based on pre-programmed instructions (no O2 sensor input).
  • Orifice
    A precisely machined hole that controls the flow of fluid.
  • Orifice Tube
    A device that meters liquid refrigerant from the high side to the low side of the air conditioning system.
  • OSHA
    Occupational Safety Hazard Act
  • Out-of-Round
    A term used to describe an oval or egg-shaped brake drum.
  • Outer Tie Rod
    The outer tie rod (also known as a tie rod end) connects the steering linkage to the spindle.
  • Overhead
    The operating expenses of a business.
  • Overselling
    Selling a customer products they don't really need in an effort to boost the sale.
  • Oxides of Nitrogen (NOx)
    An exhaust gas pollutant created by excessive combustion chamber temperatures (2500° or higher). High NOX emissions can be caused by any condition that raises engine or combustion temperature, such as a low coolant level, low oil level, or a lean air/fuel ratio.
  • p

  • Packing Slip
    Documentation listing the contents of a package.
  • PAG (Polyalkylene Glycol)
    The lubricant generally used in R134a air conditioning systems.
  • Parallel Circuit
    An electrical circuit that contains multiple paths for current to flow.
  • Parasitic Draw
    Current that flows from the battery to power up certain electronic devices during periods when the vehicle is not being operated.
  • Park Relay
    Controls current flow from the wiper switch to the wiper motor. When the wipers are turned on, the relay coil is energized and the switch contacts inside the park relay close. When the wipers are turned off, the switch contacts inside the park relay are held closed by a mechanical arm until the wipers return to the park position.
  • Parking Brake Equalizer
    A metal plate connected to an adjustable rod that simultaneously activates the parking brake mechanism in each rear wheel (or front wheel on certain applications).
  • Part-Time 4WD
    A four-wheel drive system that only operates in 4WD when the driver shifts the transfer case into the 4HI or 4LO position. Part-time four-wheel drive systems do not provide front-to-rear differential action, and should therefore only be used in the 4WD mode on reduced traction surfaces.
  • Pascal's Law
    A discovery made by a sixteenth century scientist that proved that liquid is non-compressible, and that a force applied to the top of a liquid in a closed container is exerted equally in all directions.
  • PCM
    Powertrain Control Module
  • PCV
    Positive Crankcase Ventilation
  • Pending DTC
    A temporary code recorded by the ECM/TCM. Pending codes are used to identify the first occurrence of a two-trip failure.
  • Period
    Refers to the length of time required for one cycle of an electrical signal to be completed.
  • Permanent Magnet Starter
    A starter that does not use field coils to generate a magnetic field around the armature.
  • Phase
    The installed positions of various elements in the driveline.
  • Photocell
    A light-sensitive electronic device used on vehicles with automatically controlled headlights.
  • PID
    Parameter Identification
  • Piston Slap
    The noise caused by excessive piston-to-cylinder wall clearance. Piston slap is loudest when the engine is cold, since the clearance is greatest under this condition.
  • Piston-To-Wall Clearance
    The distance measured between the piston and the cylinder wall.
  • Pitch
    The number of threads per inch on a standard fastener.
  • Pitman Arm
    Part of the steering linkage that attaches to the pitman shaft (or sector shaft) on vehicles with recirculating ball steering. The pitman arm transfers the rotary motion of the pitman shaft into lateral motion at the centerlink.
  • Plenum
    This component contains the heater core, evaporator, and the various doors that control air induction, air temperature, and air distribution in an HVAC system.
  • POE (Polyol Ester)
    The lubricant typically used with aftermarket retrofit kits.
  • Polyalkylene Glycol (PAG)
    The lubricant generally used in R134a air conditioning systems.
  • Polyol Ester (POE)
    The lubricant typically used with aftermarket retrofit kits.
  • Porosity
    Minute holes in a casting that result when air bubbles form during the casting process.
  • Portable Container
    A tank used with A/C service equipment for the recovery and storage of recycled refrigerant. New refrigerant may be transferred from a disposable container to a portable container, as would be the case when pre-charging new service equipment.
  • Ported Vacuum
    The vacuum signal generated above the closed throttle plate. Ported vacuum should measure nearly 0" Hg at idle, provided that engine idle speed is adjusted correctly. Ported vacuum is also known as control vacuum or timed vacuum, and is used to operate a variety of emission-related components.
  • Positive Crankcase Ventilation (PCV)
    A system used to remove blowby gases from the crankcase. Most PCV systems use a spring-loaded valve to regulate the volume of crankcase vapors entering the engine, while some use a fixed orifice.
  • Powermaster
    The name General Motors uses for its electro-hydraulic brake booster assembly.
  • Powertrain Control Module (PCM)
    An on-board computer that manages fuel delivery, spark timing, idle speed, emission control sub-systems, and other powertrain functions such as the torque converter clutch and transmission shift solenoids.
  • Preload
    The load applied to a bearing to eliminate looseness.
  • Pressure Cap
    A cap screwed on the radiator or surge tank that seals and pressurizes the cooling system.
  • Pressure Decrease Phase
    An anti-lock mode of operation in which fluid from the locking wheel or wheels is bled off and returned to the master cylinder.
  • Pressure Differential Switch
    A switch typically incorporated into the combination valve, which provides a ground for the brake warning light should a leak occur in one side of a split-system.
  • Pressure Increase Phase
    A mode of operation where the ABS system is in a standby mode, while full hydraulic pressure is applied to the wheels from the master cylinder.
  • Pressure Maintain Phase
    An anti-lock mode of operation in which fluid pressure to the locking wheel or wheels is isolated, meaning that hydraulic pressure in that circuit remains constant.
  • Price Sheets
    Price lists that accompany manufacturers' catalogs. Price sheets may be color-coded to identify their application. For example, yellow sheets may apply to DIYs, while green sheets may be used for professional installers.
  • Primary Piston
    The piston in the master cylinder nearest to the firewall. It is activated by the brake booster pushrod (power-assist) or pedal pushrod (manual brakes).
  • Primary Shoe
    The shoe in a duo-servo drum brake system positioned closest to the front of the vehicle. The length of the primary shoe lining is shorter than the friction material on the secondary shoe.
  • Primary Voltage
    The voltage produced in the primary winding of the ignition coil when primary current is interrupted and the magnetic field collapses. Primary voltage is typically 200-300 volts.
  • Proportioning Valve
    A valve in the hydraulic brake system that limits fluid pressure to the rear brakes once a pre-determined pressure is achieved. The proportioning valve prevents rear wheel lockup under severe braking.
  • Prussian Blue
    A material applied to the valve face for checking the seat contact area.
  • Public Relations (PR)
    Promotional efforts by management or other employees meant to establish favorable attitudes in the minds of the general public on behalf of the store.
  • Pulse Width
    Refers to the ON-time of an actuator, such as a solenoid.
  • Pulse Width Modulated
    A computer command signal that allows a controlled device to be cycled on and off many times per second to achieve a specific output.
  • Pulse Width Modulation
    An ECM control strategy used to vary the duty-cycle of an actuator.
  • Purchase Order
    A form submitted by a buyer detailing the requested goods, conditions of sale, and terms of delivery.
  • r

  • R-12
    A refrigerant containing chlorofluorocarbon.
  • R-134A
    A refrigerant containing hydrofluorocarbon.
  • Race
    The inner or outer polished steel ring of a ball bearing or roller bearing.
  • Rack & Pinion
    A self-contained steering gear that houses the essential elements of the entire steering system except for the tie rods and power steering pump.
  • Radial Runout
    The up and down (vertical) movement of a rotating tire/wheel and hub assembly.
  • Radio Frequency Interference (RFI)
    An electrical force that causes the disruption of radio wave transmissions.
  • Rain Check
    A voucher assuring a customer that they will only be charged the advertised sale price of a sold-out item once it becomes available.
  • Ravigneaux Gear Train
    Also known as a compound gear train, it consists of a single internal gear that houses two sets of planetary pinions mounted on a common carrier along with two sun gears.
  • RBS Tie Rod End (Rubber Bonded Socket)
    A type of tie rod end where the ball stud is enclosed in a rubber-filled socket. RBS tie rod ends require no lubrication.
  • Reaction Member
    The part of a planetary gearset that is kept stationary so that output motion can occur.
  • Rear Axle Ratio
    A comparison between the number of teeth on the ring gear and the number of teeth on the pinion gear.
  • Receiver-Dryer
    A storage device for liquid refrigerant located on the high side of the air conditioning system. The receiver-dryer contains a desiccant to absorb any moisture in the system.
  • Recirculating Ball Steering
    A steering gear containing a worm shaft and ball nut as its primary components. These parts are separated by steel balls to reduce friction.
  • Recovery Cylinder
    A container used exclusively for recovering refrigerant that will be sent off-site for recycling, reprocessing, or destruction. An approved recovery cylinder meets DOT specifications 4BA-300, and can be identified by its gray-colored body and yellow collar.
  • Recovery Tank
    A reservoir that provides storage space for heated coolant as it expands in the system. The recovery tank is part of a 'closed' cooling system.
  • Red Brake Warning Light
    An indicator lamp that illuminates when the parking brake pedal or hand brake is engaged. Depending on the system, this lamp may also illuminate to indicate a loss of pressure in the hydraulic system, or a low fluid level in the reservoir.
  • Regulated Pressure
    Refers to the amount of fuel pressure that exists in a continuous return fuel injection system when the engine is running and vacuum is applied to the regulator.
  • Related Items
    Products within the same category displayed along side one another. Examples include belts and hoses or spark plugs and wires.
  • Related Sale Items
    Parts or supplies associated with performing a specific repair procedure.
  • Relative Compression
    An electronic compression test that determines cylinder pressure based on cranking amp draw.
  • Relay
    An electromagnetic device that allows a high current to be controlled by a low current.
  • Reluctor
    A metal wheel with specifically spaced notches or teeth used to trigger a magnetic sensor. On some engines, the crank sensor reluctor is an integral part of the crankshaft.
  • Reluctor (ABS)
    A toothed wheel (also known as a tone ring) made of ferrous metal used to trigger a wheel speed sensor. Depending on the application, the reluctor may be mounted to the brake rotor, wheel hub, axle shaft, differential pinion shaft, or the transmission output shaft.
  • Remanufactured Part
    A part that has been restored to its original operating condition. Remanufactured parts contain a combination of new and reconditioned components, and often come with a 90-day warranty.
  • Reserve Capacity (RC)
    A capacity measurement that indicates the amount of time (in minutes) a battery can sustain a current draw of 25 amps at 80°F (no alternator output) while maintaining a terminal voltage of at least 10.5V.
  • Residual Pressure Check Valve
    A valve typically located in the outlet port of the master cylinder on a disc/drum system. It is used to prevent air from entering the system when the brake pedal is released quickly, by keeping tension on the wheel cylinder cups. The check valve also allows the drum brakes to respond more quickly to pedal input.
  • Resistance
    The opposition to current flow.
  • Restocking Fee
    A handling charge imposed on customers returning certain parts.
  • Resurfacing
    A process that removes a pre-determined amount of metal from the friction surface of a brake disc.
  • Resurfacing Drum Brake
    A procedure that removes a pre-determined amount of metal from the friction surface of the brake drum.
  • Returnless Fuel Injection
    A fuel injection system that operates at a constant pressure, and does not recirculate fuel from the rail back to the tank.
  • Returnless Fuel Injection System
    An injection system that maintains a constant supply pressure, and does not recirculate fuel through the rail back to the tank.
  • Returns
    Sold merchandise returned to the retailer for a cash refund or exchange.
  • Reverse Flow Cooling System
    A system that allows coolant to flow through the cylinder heads prior to flowing through the engine block. Reverse flow cooling provides more stable combustion chamber temperatures than conventional cooling systems.
  • Riveted Linings
    Friction material attached by rivets to drum brake shoes.
  • Riveted Pads
    Friction material attached by rivets to the metal backing plate of a disc brake.
  • Root Cause of Failure
    Refers to the original source of a performance symptom. For example, an engine misfire may be caused by a fouled spark plug; however, a fouled spark plug is rarely the root cause of the failure. Unless the source of the fouled spark plug is diagnosed and repaired (worn rings, leaking valve seals, etc.), the driveability problem will eventually return.
  • Rotary Flow
    The oil flow that takes place in a torque converter when the impeller and turbine reach the coupling point.
  • Rotator
    A type of spring retainer that allows the valve to turn as it opens and closes.
  • Rotor Pump
    A type of internal/external gear pump that utilizes rounded-toothed gears or lobes.
  • RPO
    Regular Production Option
  • RWD
    Rear-Wheel Drive
  • s

  • SAE
    Society of Automotive Engineers
  • Sales Leader
    A free or discounted item used to lure customers into the store. Sales leaders can also be part of a package deal that encourages customers to make a larger purchase. For example, "BUY 3 SHOCKS, GET THE FOURTH ONE FREE!”
  • Seasonal Items
    Products appropriate during a particular time of year such as window de-icer and ice scrapers.
  • Secondary Piston
    The piston in a dual master cylinder that is activated by the primary piston, and is positioned furthest from the firewall.
  • Secondary Shoe
    The shoe in a duo-servo drum brake system that is closest to the rear of the vehicle. The secondary shoe has a longer, thicker lining than the primary shoe.
  • Secondary Voltage
    The voltage produced in the secondary winding of the ignition coil when the magnetic field collapses. Secondary voltage is used to fire the spark plugs.
  • Sedan
    A vehicle with four doors.
  • Selling Up
    Selling a customer a more expensive product in an effort to meet their specific requirements and ensure their satisfaction.
  • Sensing Bulb
    A device used to detect the temperature of the evaporator or the suction line for the purpose of regulating refrigerant flow (expansion valve) or cycling the compressor (thermostatic switch).
  • Sequential Fuel Injection (SFI)
    An injector firing strategy in which the injectors are fired one at a time in the spark plug firing order.
  • Sequential Multiport Fuel Injection (SFI)
    An electronically controlled fuel injection system that uses one electro-mechanical injector per cylinder pulsed individually. All OBD II systems are equipped with an SFI system.
  • Series Circuit
    A circuit that contains a single path for current to flow.
  • Series-Parallel Circuit
    A circuit that contains a load wired in series with two or more branch circuits.
  • Servo
    A hydraulically activated piston used to apply a band.
  • Set
    A selling unit increment that refers to at least two matched items, as in a 'set' of brake pads.
  • Setback
    Setback is a difference in wheelbase between each side of the vehicle. Setback typically results when the vehicle has suffered a severe corner impact.
  • Shift Fork
    A U-shaped casting connected to the shift linkage that controls the movement of the synchronizer sleeve.
  • Shift Linkage
    The rods, levers, cables, and related components that transfer the movement of the shift selector to the shift forks inside the transmission.
  • Shim
    A washer of pre-determined thickness used to adjust endplay in shafts or bearings.
  • Short Bed
    A pickup truck with a six-foot bed.
  • Short Circuit
    The name given to a circuit where the load has been accidentally bypassed.
  • Shorted Circuit
    A circuit where the normal current path is bypassed due to an unintentional ground or a short to voltage.
  • Shrader Valve
    Valve core in the valve stem of tires and tubes.
  • Shrinkage
    An industry term used to describe revenue losses resulting from theft.
  • Simpson Gear Train
    A gear train composed of two simple planetary gearsets linked by a common sun gear.
  • SKU
    Stock Keeping Units
  • SLA (Short/Long Arm)
    A front suspension system consisting of a pair of unequal length, A-type control arms and a coil spring.
  • Slip
    Refers to the difference in speed between the impeller and turbine in a torque converter.
  • Speed Density
    An airflow calculation technique that relies on the signal from a Manifold Absolute Pressure (MAP) sensor and pre-programmed software algorithms.
  • Speed Density (Air Injection)
    A control strategy used to calculate engine airflow based on changes in manifold absolute pressure and other variables.
  • Speed Density (Emissions)
    A method for determining engine airflow based on manifold absolute pressure and temperature along with calculations of other variables, such as EGR flow and volumetric efficiency.
  • Splines
    The external teeth cut into a shaft or the internal teeth cut into the inside of a hub.
  • Spool Valve
    A valve with a series of raised lands used to direct the flow of fluid throughout the valve body and transmission/transaxle case passages.
  • Sprag
    A 'dogbone' shaped holding element used in one style of overrunning clutch.
  • Sprung Weight
    Any part of the vehicle that is supported by the suspension, including the suspension itself.
  • Spur Gear
    A gear whose teeth are cut parallel to the shaft.
  • Stabilizer Bar
    A bar of spring steel that links two wheels on the same axle together. The stabilizer bar is used to prevent excessive body roll during vehicle cornering.
  • Stall Speed
    An engine's maximum rpm with the transmission/transaxle in gear and the brakes applied.
  • Standard Cab
    A two-door truck with a front seat only.
  • Staples
    Items that customers purchase on a regular basis such as motor oil, additives, etc.
  • Starter Draw
    The amount of current required to operate the starter motor during cranking.
  • Static Balance
    A condition that exists when the mass of a non-rotating tire/wheel assembly is equally distributed around its axis of rotation. Static balancing is also referred to as single-plane balancing, since weights are only installed on one side of the wheel.
  • Station Wagon
    A vehicle in which the passenger compartment extends to the back of the vehicle. Unlike a hatchback, a station wagon provides additional seating behind the rear seat.
  • Stator
    The stationary component inside the alternator. The magnetic field produced by the rotor induces a voltage into each of the stator's three phase windings.
  • Steering Angle Sensor
    A device mounted on the steering column that detects body roll by measuring the rate of speed the steering wheel is being turned at. The steering angle sensor is used on certain vehicles with electronically controlled steering and suspension.
  • Steering Axis
    The steering axis on SLA suspensions is an imaginary line that bisects the upper and lower ball joints. On MacPherson strut suspensions, the steering axis bisects the top of the strut and the lower ball joint.
  • Steering Axis Inclination
    SAI refers to the angle formed by the steering axis and true vertical as seen from the front of the vehicle. SAI causes changes in spindle height as the wheels are turned, and determines how easily the wheels return to a straight-ahead position .Scrub radius is the distance between the center of the tire (measured at the tire's contact area) and the imaginary steering axis line that projects to the road surface.
  • Steering Knuckle
    The steering knuckle (also known as the spindle) joins the control arms together on a vehicle with SLA suspension. On a strut-type suspension, the steering knuckle joins the strut assembly to the lower control arm.p
  • Stock Order
    A form listing the parts and supplies needed by the jobber to replenish inventory.
  • Stoichiometric
    A word used to indicate the ideal air/fuel ratio. At sea level the stoichiometric ratio is 14.7:1, which means that 14.7 pounds of air must be combined with 1 pound of fuel in order to achieve efficient combustion.
  • Strut Bearing
    A component used to support the vehicle while simultaneously allowing the strut to pivot during steering maneuvers. The strut bearing is positioned between the top of a MacPherson strut and the vehicle's strut tower.
  • Strut Rod
    A component (also known as a radius arm) used to support the lower control arm on vehicles that do not use the A-frame style control arm.
  • Suction Pressure
    The pressure of the vapor being drawn into the A/C compressor from the evaporator outlet.
  • Supplement
    An addition to an existing parts catalog that lists new part numbers and/or changes to current numbers.
  • SUV
    Sport Utility Vehicle
  • Synchronizer
    The device used to engage a speed gear and then lock it to the output shaft.
  • t

  • Technical Bulletin
    A notice that warns of problems or changes regarding the installation or maintenance of a part.
  • Tetrafluoroethane
    The chemical name for R134a.
  • Thermal Time Switch
    A temperature-sensitive device used to control a cold start injector.
  • Thermo-Time Switch
    A temperature-sensitive device used to control a cold start injector.
  • Thermostat
    A component in the cooling system used to regulate the engine's minimum operating temperature. Most engines use a 195°F (90.5°C) thermostat.
  • Thermostatic Air Cleaner (TAC)
    An air cleaner designed to improve fuel vaporization during cold ambients. The heart of the TAC system is a temperature controlled valve that draws intake air from either the exhaust manifold or from outside the vehicle.
  • Thermostatic Expansion Valve (TXV)
    A device that regulates the flow of liquid refrigerant from the high side to the low side of the air conditioning system.
  • Thermostatic Vacuum Switch (TVS)
    A temperature-sensitive switch located in a coolant passage or the air cleaner. The TVS is used to control vacuum signals to various emission-related components.
  • Three-Way Catalyst
    A converter consisting of an oxidation bed for converting HC and CO into H2O and CO2, and a reduction bed for converting NOx compounds back into nitrogen and oxygen. A three-way catalyst uses three precious metals including, platinum, palladium, and rhodium.
  • Three-Way Catalyst (TWC)
    A converter that contains three precious metals that promote the conversion process of combustion gases. Platinum and palladium are used to oxidize HC and CO into H2O and CO2, while rhodium is used to separate NOX into its base parts of nitrogen and oxygen.
  • Three-Way Catalytic Converter (TWC)
    A device located in the exhaust system that transforms unburned hydrocarbons, carbon monoxide, and oxides of nitrogen into H2O (water vapor), CO2 (carbon dioxide), nitrogen and oxygen.
  • Throttle Body Injection (TBI)
    A system that uses one or two computer-controlled injectors located in a fuel metering assembly above the intake manifold. TBI systems can be either low-pressure (9-13 psi) or high-pressure (26-32 psi).
  • Throttle Position (TP) Sensor
    A potentiometer that converts the movement of the throttle plate into an electrical signal. On an electronic transmission/transaxle, the computer uses the TP signal to regulate line pressure and shift points.
  • Throttle Valve
    A spool valve that adjusts line pressure based on changes in engine load. The throttle valve is typically operated by a cable-actuated pushrod or plunger. In older transmissions that do not use a TV cable, the throttle valve is controlled by the vacuum modulator.
  • Throttle Valve (TV) Pressure
    The hydraulic pressure applied to the boost side of the pressure regulator and the downshift side of the shifts valves.
  • Throw
    Refers to the distance between the centerline of a crankshaft's main bearing journals and the centerline of the crank pins (rod journals).
  • Thrust Washer
    A steel washer used to control axial loads as well as provide a bearing surface for adjoining parts.
  • Tire Rotation
    A procedure used to equalize tire wear by moving the tires to different axles.
  • Tire Slip
    A term used to represent the percentage of drag that occurs between the tire and the road surface. At 100% slip, the tire/wheel is completely locked up, while at 0% slip it rotates freely.
  • Toe
    Toe refers to the position of the tires in relation to the centerline of the vehicle. If both tires on one end of the vehicle are parallel to the centerline, toe equals zero. Toe becomes negative as the front of the tires move away from the vehicle's centerline. This is referred to as 'toe out.' When the front of the tires point toward the centerline (toe in), toe is positive. Toe is a tire wear angle and is adjustable on every vehicle.
  • Tone Ring (a.k.a. Reluctor)
    A toothed wheel made of ferrous metal used to trigger a speed sensor. The tone wheel may be mounted to the wheel hub, brake rotor, axle shaft, differential pinion shaft, or the transmission output shaft.
  • Torque-to-Yield (T-T-Y) Head Bolt
    A fastener designed to be torqued just short of its yield point so that maximum clamping force will be applied to the cylinder head gasket. T-T-Y bolts should not be re-used.
  • Torsion Bar
    A heavy bar of spring steel used in place of coil springs on some suspension systems. The torsion bar is usually attached to the vehicle's frame on one end and to the lower control arm on the other.
  • Total Indicated Lateral Runout
    The combined side-to-side movement of the wheel hub and rotor as an assembly (hubless rotors).
  • Toxic
    Anything you cannot safely eat or drink.
  • TPS
    Throttle Position Sensor
  • Trade Discount
    A pre-determined percentage off the regular selling price (e.g. 15%). Trade discounts are reserved for high-volume purchasers such as independent repair shops, fleet owners, and dealers.
  • Trailing Shoe
    The shoe in a leading/trailing drum brake system positioned closer to the rear of the vehicle. The trailing shoe provides most of the braking power when the vehicle is in reverse.
  • Transfer Case
    An auxiliary transmission used on a vehicle with four-wheel drive that can transfer torque from the output shaft of the primary transmission to the front and rear axles. Most transfer cases provides two speeds, including direct drive and low gear. The front output shaft may be gear-driven or a chain-driven.
  • Transmission Range (TR) Switch
    A device that indicates the selected gear to the on-board computer. The starter interrupt switch (neutral safety) is typically incorporated into the TR switch.
  • Trip
    The term used to describe a key-on, key-off cycle, during which time the enable criteria for at least one OBD II monitor was satisfied.
  • Turbine
    The part of a torque converter positioned in the front half (engine side) of the shell but not connected to it. The turbine drives the transmission/transaxle input shaft.
  • Turnover
    The number of times in a given period that inventory is replaced.
  • Two Speed Idle (TSI) Test
    Also known as no-load testing, the TSI test checks vehicle emissions at idle and 2500 rpm with the vehicle in Park or Neutral. The TSI test is designed primarily for vehicles equipped with all-wheel drive or full-time four wheel drive systems, since these vehicles cannot be run on a dynamometer. In addition, vehicles equipped with traction control systems may be given a TSI test if the TC system cannot be disabled. In some areas, two-speed idle testing is also used to check the emissions of 1983 and older vehicles, as well as trucks (1982 and newer) with a GVWR above 8500 pounds. In a TSI test, HC and NOx are measured in ppm (parts per million), while CO and CO2 are expressed as a percentage.
  • TXV (Thermostatic Expansion Valve)
    A device that regulates the flow of liquid refrigerant from the high side to the low side of the air conditioning system.
  • u

  • Universal Joint (U-joint)
    A cross-shaped joint that allows a smooth transfer of power from the output shaft to the differential by compensating for changes in driveshaft angle.
  • Universal Joint (U.joint)
    A cross-shaped joint that allows a smooth transfer of power from the output shaft to the differential by compensating for changes in driveshaft angle.
  • Unregulated Pressure
    Refers to the fuel pressure that exists in a continuous return system when the regulator has been disconnected from the vacuum source.
  • Unsprung Weight
    Vehicle weight not supported by the suspension, but rather directly supported by the tires and wheels. The rear axle is an example of unsprung weight.
  • v

  • Vacuum Booster
    A large metal housing mounted to the firewall containing one or two diaphragms, a power piston, and pushrod as its primary components. It provides power assist to the driver by creating a pressure differential on each side of the diaphragm when the brake pedal is depressed.
  • Vacuum Fluorescent Display (VFD)
    A type of segmented display used for instrumentation that produces a bluish-green light as electrons come in contact with a phosphor-coated anode. A computer applies the electrons to the segments essential for forming specific alphanumeric characters.
  • Vacuum Relief Valve
    Part of a radiator pressure cap. The vacuum valve opens to allow atmospheric pressure to enter the system as the coolant contracts following engine shutdown. This action prevents the radiator and/or hoses from collapsing.
  • Valve Overlap
    The period between the exhaust and intake strokes when both valves (intake and exhaust) are opened at the same time.
  • Valve Spring Installed Height
    The distance between the spring pad in the cylinder head and the bottom of the spring retainer. This dimension is typically checked with the valve seated in the head, the spring retainer and keepers installed, and the valve spring removed.
  • Valve Stem-To-Guide Clearance
    The distance between the valve stem and the valve guide.
  • Variable Displacement Compressor
    A compressor that automatically adjusts piston stroke to control the volume of refrigerant circulating throughout the air conditioning system.
  • Variable Reluctance Sensor
    A magnetic sensor used for generating wheel speed sensor signals.
  • VC
    Vehicle Certification
  • VECI
    Vehicle Emission Control Information
  • Venturi Vacuum
    This vacuum signal originates at the carburetor venturi and is typically used for amplification purposes (as in the case of early EGR control systems). Venturi vacuum is generated as air rushes through the carburetor when the throttle plate is opened. Consequently, venturi vacuum is highest at wide open throttle, where it may reach 4" Hg.
  • Vertical
    Straight up and down, perpendicular to the floor.
  • VIN
    Vehicle Identification Number
  • Virgin Refrigerant
    A term used to describe new R12 or R134a.
  • Viscosity
    An oil's resistance to flow.
  • Volt
    The unit of measurement for pressure in an electrical circuit.
  • Voltage Drop
    The difference in potential between two points in an electrical circuit. Measuring voltage drop is the most accurate way to determine if a circuit has high resistance.
  • Voltmeter
    An test instrument used to measure electrical pressure. A voltmeter is always connected in parallel with the circuit under test.
  • Vortex Flow
    A swirling oil flow that occurs between the impeller and turbine during torque multiplication.
  • VSS
    Vehicle Speed Sensor
  • w

  • Warehouse Distributor (WD)
    A purchaser of manufactured goods in large quantities. The warehouse distributor buys direct from the manufacturer and sells to the jobber.
  • Warranty
    A document indicating the anticipated service life of a product as promised by the manufacturer. The warranty also describes the terms of remuneration should the product fail to perform satisfactorily before a specified amount of time has expired.
  • Waste Cylinder
    Refers to the cylinder firing on the exhaust stroke in a waste spark ignition system.
  • Waste Spark Ignition
    A type of distributorless ignition system that uses one coil for each pair of spark plugs. The plugs are grouped according to cylinders that reach TDC compression and TDC exhaust at the same time (companion cylinders). The plugs are fired simultaneously, with the bulk of secondary energy used to fire the cylinder on the compression stroke.
  • Wet Boiling Point
    The boiling point specification of brake fluid containing approximately 2% moisture.
  • Wheel Cylinder
    A hydraulic component used to transfer master cylinder pressure to the brake shoes in a drum brake system. It is mounted on the backing plate between the brake shoes.
  • Wheel Speed Sensor
    A permanent magnet sensor used for generating a wheel speed signal. A wheel speed sensor does not require an external power supply, since it produces its own AC voltage.
  • Wheel Tramp
    A condition resulting from the centrifugal force acting upon the heaviest portion of a statically imbalanced tire/wheel assembly. Wheel tramp causes the tire/wheel to bounce as it rotates, resulting in scattered flat spots on the tread.
  • Wholesale
    The sale of goods in large quantities. Wholesale prices apply to purchasers who buy specific items in bulk.
  • z

  • Zero Lash
    Refers to the absence of clearance between valve train components. Properly functioning hydraulic lifters maintain the valve gear in a zero-lash condition.