Glossary of Transmission Terminology and Transmission Problems-
Trans Specialties Transmission Dictionary.
Accumulator: A device that cushions the motion of a clutch and servo action in an automatic transmission in order to provide smooth shifting at various throttle openings.
Accumulator valve: A valve that regulates accumulator action.
Adaptive Learn Automatic Transmission: Many of the newer automatic transmissions made today can "learn" and adapt to driving conditions, altering shift feel, shift points and other transmission functions to produce the most efficient operation. Adaptive learn transmissions can also compensate for clutch pack wear to a certain extent.
Air check: An automatic transmission "bench test" done during assembly that is used to check the hydraulic integrity of clutch packs, band servos, accumulators and valves
All Wheel Drive (AWD): All-Wheel drive is a four-wheel drive system that has no two-speed transfer case. It normally operates in a similar fashion to full-time or permanent four-wheel drive and can have a mechanical torque biasing, a wet clutch or viscous clutch to control front to rear biasing.
Anti-Foam additive: An additive that reduces foaming caused by the churning action of transmission gears in transmission fluid.
Assembly Lube: A lubricant used to coat parts during initial assembly.
Automatic transmission: A transmission that shifts itself. A torque converter is usually utilized instead of a pedal operated clutch to connect the transmission and engine.
A/T: An abbreviation for automatic transmission.
Automatic transmission fluid (ATF): A usually red, mineral or petroleum-based fluid used to lubricate, transfer power, and cool an automatic transmission.
Automatic Transmission Rebuilders Association (ATRA): A trade organization for transmission repair shops.
Axial Thrust Load: The external loading of force acting lengthwise along a shaft.
Axle ratio: The relationship between a vehicle's driveshaft and its axle. For example, a 3.73 axle ratio means that the drive shaft turns 3.73 times for every time that the wheels turn.
Band: A device in utilized in many automatic transmissions whose function is to wrap around a clutch drum in order to stop its rotation.
Bearing: A component having an inner and outer race with steel balls or rollers used to support a rotating component.
Belleville Spring: A diaphragm type spring used to release a multiple-disc clutch set.
Billet: A solid bar of metal or a component machined from a billet.
Boost: Supplemental pressure regulator pressure used in reverse and high load/heavy throttle conditions.
Boost valve: Valve acting on a pressure regulator valve to raise line pressure. See boost.
Bushing: A usually bronze sleeve that serves as a bearing surface.
Carrier Bearing: A bearing that supports the carrier in a differential.
Center differential: A device found in all-wheel-drive vehicles which biases torque from front to rear and in a tight turn it allows the front and rear wheels to turn at different speeds. A center diff can be a gear type unit, a viscous clutch or a combination of both.
Check Engine Light (CEL): A light which is illuminated on the instrument cluster when the vehicles' computer detects a fault.
Clutch: A device for connecting and disconnecting the power flow between the engine and transmission.
Clutch Drum: A component of an automatic transmission which houses clutch discs.
Clutch hub: A component that has clutches splined to its outside diameter, often found inside a clutch drum.
Clutch-release Bearing: A bearing which is used to disengage the clutch when the clutch pedal is depressed. Also known as a throw out bearing.
Clutch plate: A friction lined disk designed to resist movement between two components; part of a multi disk clutch pack. See "friction disk"
Crossleak: Undesirable leakage of hydraulic pressure into another hydraulic circuit. A cross leak can be caused by things such as a warped valve body, a worn sealing ring, blown gasket, etc.
Converter: See torque converter.
Centrifugal apply: The unintentional and unwanted application of an automatic transmission clutch pack that is supposed to be released caused by residual ATF moving to the outside of the drum's inside diameter under high RPM's.
Chatter: An undesirable shaking or shuddering action in a clutch or clutch pack caused by a rapid grip and slip action
Check ball: A device that permits the flow of fluid in one direction only.
Coefficient of friction: The measure of the resistance of one surface moving against another.
Converter Clutch: See "lock-up clutch".
Converter pressure: The operating pressure within a torque converter.
Compound Planetary Gear Set: A gear set that contains more than just the three basic members of a simple planetary gear set. In a three or four speed automatic transmission, it is normally the only planetary set.
Countershaft: The shaft that supports the cluster-gear set in a manual transmission and which rotates in the opposite direction of the engine crankshaft.
Cooler: See "transmission oil cooler".
Cooler flush: 1) The act of flushing a transmission oil cooler, especially after a transmission failure, repair or rebuild. 2) An aerosol product used to flush an automatic transmission cooler.
Continuously Variable Transmission (CVT): A CVT transmission employs a steel belt riding on two pulleys and can vary the effective diameter of the drive pulley and driven pulley to create a broad range of drive ratios, rather than shifting between gear ratios as does a conventional automatic transmission.
Core Charge: "Core" is an acronym for "Cash on Return". It refers to a refundable deposit or charge on rebuildable parts. Core charges are refunded when a rebuildable component is returned to the manufacturer. Core charges are usually collected for engines, valve bodies, alternators, transmissions and torque converters and are a means of insuring that a rebuildable component is returned to the manufacturer in a timely manner.
Crush Sleeve: A sleeve that is on the pinion gear in the differential, it is designed to contract when torqued to specifications hence keeping tension on the pinion nut while the pinion bearings are properly preloaded.
Cut loose: See "flare".
Damper clutch: See "torque converter clutch".
Detent: A combination of a spring loaded ball and a recess to hold a gear selector in the gear range which is selected.
Detent downshift: See "kickdown".
Differential: The section of a transfer case, transaxle or axle assembly that allows the wheels to revolve at different speeds during turns.
Diff: An abbreviation for differential.
Downshift: The automatic shift from a high gear ratio to a lower gear ratio.
Downshift clunk: An abrupt downshift, especially during closed throttle operation.
Dowel pin: A round pin used to align or locate two or more parts. An example of this is the two alignment pins that locate a transmission's bell housing to an engine block.
Engagement: The application of a clutch.
Electronic Pressure Control Solenoid (EPC): A solenoid whose function is to vary transmission line pressure in proportion to load and/or throttle opening
Electronically controlled transmission (ECT): A transmission that is electronically controlled by the vehicle's computerized electronic control system.
Final drive: The ring, pinion and differential gears that provide power flow to the drive wheels of a vehicle.
Final drive ratio: The ratio between the drive pinion and ring gear. See "gear ratio" and "axle ratio"
Flare: A drivability concern referring to a shift that is accompanied by a rise in engine RPM's; a short slip that occurs during a shift.
Fluid coupling: A device in an automatic transmission containing two rotating members, one of which transmits power to the other via fluid flow. This is the precursor to the modern torque converter.
Flywheel: Part of a clutch assembly that is bolted to the engine crankshaft with a surface that provides an area for the clutch disc to contact during clutch engagement. The flywheel often has a starter ring gear on its outside diameter.
Forward clutch: A clutch that is engaged whenever the vehicle moves forward, controlled by the valve-body forward circuit.
4WD: A term used for four-wheel drive.
Four-wheel drive: A vehicle having drive wheels in the front and rear, so all four wheels can be driven. Also called "4WD" or "4X4".
Freewheel: 1) To turn without resistance. 2) A mechanical device in which a driving member imparts motion to a driven member in one direction, but not the other. See "sprag" and "roller clutch"
Friction disk: A disc with a friction material on one or both sides, such as a clutch disk. Also called a "friction"
Friction material: Material used for friction surfacing on a clutch disk or friction disk.
Friction-modified fluid: Automatic transmission fluid that provides smooth automatic shifts; designed to slip slightly. Friction modified fluid should only be used where specified by the manufacturer. Examples of this are Chrysler ATF +4, Mitsubishi Diamond, Honda, etc.
Front wheel drive: A drive system that transmits power through the front wheels.
Front pump: A pump, located at the front of the transmission, driven by the engine through the torque-converter housing, to provide fluid pressure and volume to operate the transmission whenever the vehicle is running.
Furnace brazing: A welding process used to repair/strengthen complex castings, often used when upgrading torque converters.
Flexplate: A round, slightly flexible, steel or aluminum component used to transfer power from the crankshaft to the torque converter. A flexplate often, but not always has the starter ring gear attached to its outside diameter.
Garage shift: The initial shift into drive or reverse from park or neutral.
Gear ratio: The relationship between a driving and a driven gear. For example, a driving gear that revolves three times for each driven-gear revolution has a 3 to 1 or "3:1" ratio.
Gears: Machined components containing teeth that mesh that transmit power, or turning force, from one shaft to another.
Governor: A speed-sensitive centrifugal assembly in the automatic transmission driven by the output shaft, to produce a road speed hydraulic pressure signal to help determine when shifting is to occur. Governor pressure normally equates to approximately 1 PSI per mile per hour of vehicle speed. A governor is not required in modern day electronically controlled transmissions, which utilize an electronic vehicle speed sensor (VSS) to determine road speed..
Governor pressure: The pressure signal produced by the governor.
High clutch: Automatic transmission used for high gear. Often known as a "direct clutch".
High Reverse clutch: Automatic transmission used for high and for reverse gear. Often known as a "direct clutch"
Harsh engagement: An abrupt initial shift into drive or reverse.. See also "garage shift"
Hard parts: Automatic transmission components that are not contained in a rebuild kit and are not normally replaced during a transmission rebuild unless they are worn or damaged. An example of this is a transmission case.
Helical Gears: Gears that are cylindrical in shape and mesh between parallel centerlines. The '"helical" teeth are machined at an angle across the face of the gear.
Heli-Coil: A screw-thread repair insert.
Hunting shift: Drivability concern of an automatic transmission that will upshift, downshift, and upshift rapidly under certain conditions, especially climbing a grade.
Hypoid gear set: Two gears that transmit power at a 90 degree angle. An example is a differential ring and pinion gearset.
Impeller: The "pump" in a torque converter. Rotating at crankshaft speed, it generates the fluid flow through the torque converter.
Input shaft: The transmission shaft that receives power from the engine. Input shafts are splined into the clutch disk on manual transmissions and into the torque converter's turbine on automatic transmissions.
Input clutch: The transmission clutch that receives power from the engine, normally attached to the input shaft. See "forward clutch"
Intermediate clutch: Clutch pack used for second or "intermediate" gear.
JATCO: Japanese Automatic Transmission Company. Transmissions manufactured by JATCO are found in various vehicles, especially Nissans.
Kickdown: A full throttle downshift to a lower gear in an automatic transmission, also known as "passing gear".
Kickdown Band: Chrysler and Mitsubishi nomenclature for the band used for second or second and fourth gear shifts. More commonly know as an "intermediate band" or "2/4 band".
Lip seal: A rubber seal with a beveled edge that seals against pressure. An example is a transmission pump seal.
Line pressure: The base operating pressure in an automatic transmission. Line pressure is normally created by pump pressure modified by the pressure regulator valve based on throttle opening, load and driving conditions
Locking hubs: A clutch in a wheel that permits it to be disengaged from the axle shaft and made to be free-wheeling when a driving force is not required. Locking hubs can be found on some front wheel 4X4 applications.
Lock-up torque converter: A hydraulic torque converter in an automatic transmission having a mechanical clutch that effectively locks the engine to the transmission input shaft at cruising speeds. Lock-up torque converters provide more efficient operation and better fuel economy by eliminating slippage between the engine and transmission at highway speeds.
Lockup clutch: The clutch (or multi-disk clutch pack) found in a lock-up torque converter. Also known as a torque converter clutch or TCC.
Low / Reverse clutch or Low / Reverse band: Friction element used to hold a planetary carrier in order to achieve manual low gear and reverse in an automatic transmission.
Malfunction indicator light (MIL): See "check engine light".
Manual transmission: A manually shifted gearing device that allows a selection of different gear ratios based on engine RPM and road speed.
Manifold absolute pressure sensor (MAP) sensor: A device used to measure manifold pressure- vacuum and boost.
Mass airflow sensor (MAS): A sensor that measures the volume of air flowing through an engine
Modified valve body: An automatic transmission control assembly that has been modified to produce more efficient, higher performance operation.
Modulator: A vacuum measuring device that regulates transmission line pressure to meet varying load conditions.
Motor mounts: Supports made of hard rubber for the engine and transmission to be secured to the vehicle's frame.
Multiple-disc clutch: A clutch assembly containing more than one friction clutch.
Needle bearing: A bearing that contains needle-like rollers.
Neutral: The position of a transmission when the engine is disengaged from the drive train.
Neutral safety switch: An electrical switch which inhibits starter operation when a vehicle is in gear.
Neutralize: To fall out of gear as if the shifter was placed in neutral.
OEM: An abbreviation for original equipment manufacturer.
One way clutch: See "sprag".
O ring: A round rubber ring used as a seal.
Orifice: A restriction in the flow of fluid volume. In an automatic transmission orifices are often used to meter apply oil to a clutch pack or band in order to produce a controlled application.
Outer race: The race nearest to the outside of the hub of a roller bearing.
Output shaft: The driven shaft in a transmission.
Overdrive: A transmission having a ratio of less than 1:1 where the output shaft turns at a greater rpm than does the input shaft.
Overhaul: See "rebuild"
Overrunning clutch: See "sprag" and "roller clutch".
Pilot: A recess in the back of a crankshaft that serves to locate a torque converter or transmission input shaft in relation to crankshaft centerline.
Press fit: Two components that are mated together via interference fit, usually using a hydraulic press.
Pilot bearing: The bearing in the back of a crankshaft that supports the transmission input shaft.
Pinion bearing: A bearing that is used to support the pinion in a transmission or differential housing.
Planet carrier: The housing that contains the planet gears in a planetary gear set.
Planet pinions: Small gears that orbit around the sun gear, meshing with and rotating between the sun and internal gears.
Powerbrake: In an automatic transmission-equipped vehicle, the act of holding the brake pedal while revving the engine. Also known as "brake torquing".
Pressure regulator valve: The valve that is responsible for determining line pressure. See "line pressure"
Pump: See "front pump"
Planetary gear set: A set of gears found in most automatic transmissions named after the solar system because of their arrangement and action. This unit consists of a center (sun) gear around which pinion (planet) gears revolve. All gears are in constant mesh.
PWM: Pulse with modulated. See "duty cycle"
Rebuild kit: A kit containing the necessary normal wear components to rebuild a component.
Remanufacture: Similar to rebuilding but usually referring to work performed on an assembly line by several different individuals.
Rework: See rebuild.
Restall: To alter the stall speed of a torque converter.
Ravigneaux gear train: A type of planetary gear train with two sun gears, three long and three short planetary pinions, planetary carrier, and ring gear.
Rear-wheel drive: A drivetrain layout that provides power to the rear wheels only.
Release bearing: A term used for a clutch throwout bearing.
Remove and replace (R&R): To remove and replace.
Reverse: The transmission position enabling the vehicle to back up.
Reverse clutch: A multiple-disc clutch that is engaged in reverse gear. See also "high/reverse clutch"
Ring gear: A circular gear.
Ring groove: Grove worn into a transmission case, drum or other component by sealing rings.
Roller bearing: A bearing using rollers within an outer race or ring.
Roller clutch: A one-way clutch containing a number of rollers that operates by wedging on a ramp between an inner and outer race to lock up when the outer race is turned in one direction, and to freewheel when it is turned in the opposite direction. Similar to but often incorrectly referred to as a 'sprag'.
Rotary flow: Fluid flow within a torque converter during the coupling phase.
Run out: The amount that a rotating object deviates from its plane of rotation.
Sealed bearing: A bearing, such as those found on many rear axle shafts or water pump that is lubricated and permanently sealed by the manufacturer to contain the grease while keeping out contaminants.
Servo: A device that converts hydraulic pressure to mechanical movement, such as a band apply servo.
Self-adjusting clutch: A mechanism that automatically takes up the slack between the pressure plate and clutch disc.
Shift valve: Valve body valve that is responsible for making a shift.
Shifter: A floor- or steering column-mounted lever on a vehicle used to select and/or shift transmission gears.
Shift forks: A Y-shaped component located between the gears on the main shaft of a transmission that causes the gears to engage or disengage via the sliding clutches.
Slide: An elongated, slipping shift.
Slippage: Incomplete transfer of engine power through a clutch, clutch pack or torque converter to the transmission output shaft.
Slide-bump: An elongated shift that ends with an abrupt bump.
Spin-up: See "flare"
Solenoid: An electronically activated valve to either block or allow fluid flow, depending on whether it is activated or deactivated. An example of this is a transmission shift solenoid.
Speed sensor: An electrical device that can sense the rotational speed of a shaft or member and transmit this information to another device, such as a vehicle computer or speedometer.Sprag:
A one-way clutch used in an automatic transmission, a device containing dog-bone shaped parts, called elements that operate by tilting between an inner and outer race to lock up when the outer race is turned in one direction and to freewheel when it is turned in the opposite direction.
Stator: Internal torque converter responsible for redirecting fluid flow in order to achieve torque multiplication.
Steels: Short for the steel reaction plates which are "sandwiched" between the friction plates in a multi disk clutch pack.
Stepped flywheel: A flywheel having a ledge to which a pressure plate is attached.
Straight cut gear: A term used for spur gear.
Sun gear: Central gear the planet gears mesh with and revolve around.
Synchronizer: A device used in a manual transmission to bring a gear up or down to the speed of the main shaft.
Synthetic oil: A type of lubricant consisting of highly polymerized chemicals.
Separator plate: Metal plate between valve body sections or between a valve body and transmission case that contains orifices and connects passages between the two sections.
Sealing ring: A metal, Teflon or plastic seal, similar to a piston ring that seals hydraulic pressure in an automatic transmission.
Soft part: Any normal wear transmission part that is contained in a rebuild kit and is normally replaced during a transmission overhaul. Examples are rubber seals, gaskets, clutch disks and o rings.
Tensile strength: The maximum tension that a material can handle without breaking.
Thermistor: A devise that changes resistance based on temperature, as in a transmission fluid temperature sensor.
Throttle position sensor (TPS): An electrical sensor that measures throttle opening. See also "variable resistor".
Throttle valve (TV): Valve, often connected to the throttle via a cable, that regulates transmission line pressure to meet varying load conditions.
Thrust bearing: An engine main bearing that limits front to rear movement of the crankshaft.
Thrust washer: A washer that handles a thrust load and limits front to rear or side to side movement.
Torque management: Computer strategy by which power is reduced during shifts in an automatic transmission to reduce load on the transmission and provide smoother shifting.
Torque converter: Round unit, attached to the crankshaft and flexplate that transfers power from the engine to the transmission input shaft by directing and redirecting fluid flow.
Torque converter clutch (TCC): See "lock-up torque converter"
Torque multiplication: Torque increase as a result of converter action that allows the turbine to revolve slower than the impeller during acceleration and heavy-load conditions at a ratio as much as 2:1.
Torrington bearing: A flat radial needle bearing that often sits between rotating components in an automatic transmission.
Traction Control: Helps limit tire spin during acceleration or on slippery surfaces. Sensors determine if the wheels that are receiving power have lost traction. The system automatically "pumps" the brake and/or reduces engine power to those wheels to keep them from spinning.
Tranny: Slang for transmission.
Trans: Slang for transmission.
Transbrake: Typically a drag race only item, a transbrake is a valve body which has been modified to incorporate an electrical solenoid whose purpose is to simultaneously lock the transmission in forward and reverse when activated. Upon deactivation of the solenoid, the reverse element is disengaged and the vehicle is able to launch forward.
Transaxle: A transmission that also contains the axle assembly, as in most front wheel drive transmissions.
Transducer: A device that converts an input of one type into an output of another type, such as a sensor that converts pressure into an electric signal.
Transfer case: A gearbox that is used on four-wheel-drive vehicles to transfer torque to the front and/or rear axles.
Transfer gears: Gears used to transfer torque from output shaft to the pinion shaft in a transaxle
Transfer clutch: A clutch that applies to send power flow to the rear wheels in an AWD vehicle, An example is the center differential clutch used in Subabru automatic transmissions.
Transmission: A transmission is a gearbox with two or more different ratios used to match the engine's rpm and torque to various road speeds and driving conditions.
Transmission cooler: A device, often found in the radiator, through which automatic transmission fluid circulates to be cooled by surrounding air or engine coolant. Some vehicles use or can be outfitted with both an oil to engine coolant and an oil to air cooler.
Transverse engine: An engine that is mounted right to left in a vehicle, as in most front- drive applications.
Trouble code: A numeric indicator generated by a computer to indicate a failure in a sensor, circuit, or the computer itself. A code is often accompanied by the check engine or malfunction indicator light.
Turbine: Attached to the transmission input shaft, the turbine is the driven or output component of the torque converter.
Turbo Hydra-Matic: General Motors automatic transmissions manufactured by the Hydro-matic Division.
Underdrive clutch: See "forward clutch" and "input clutch".
Universal joint (u-joint): A connection, normally in a driveshaft that allows an angular change.
Upshift: A shift from a lower gear to a higher gear ratio.
Vacuum modulator: See "modulator"
Vacuum switch: An electrical switch controlled by a vacuum.
Valve body: Also known as the control valve assembly, the valve body is a component that is comprised of valves, solenoids, an orificed separator plate and a series of passages. The function of the valve body is to act as the "brain" of the automatic transmission- it directs hydraulic pressure to the appropriate clutches and bands to initiate upshifts, down shifts, selection of reverse, converter clutch application, etc.
Variable-pitch stator: A torque converter design that is capable of altering stall speed and torque multiplication based on driving conditions and throttle opening. Also known as a "switch pitch".
Variable resistor: A resistor that provides for a change in resistance, such as a rheostat or potentiometer. A throttle position sensor (TPS) is a good example of this.
Vehicle speed sensor (VSS): An electronic sensor that is used to provide road speed information to the vehicle's computer and/or instrument cluster.
Viscous coupling (viscous clutch or VC): A device having input and output members in a multi disc clutch set in a closed chamber filled with fluid. A viscous clutch is typically used in all wheel drive applications to help allow for biasing between front and rear axles.
Viscosity: The resistance of a fluid to flowing, as in the thickness of an oil.
Viscosity rating: A numerical indicator of the viscosity of an oil established by the American Petroleum Institute.
Vortex flow: Flow within a torque converter during the torque multiplication phase of operation.
Wet clutch: A clutch having friction disks that operate in oil, as in automatic transmission multi disk clutch packs.
Wide ratio: A transmission with wide spreads between gear ratios.
WOT: Wide open throttle.
Yoke: A component having internal splines that slide on the transmission output-shaft external splines. An example of this is a driveshaft slip yoke.
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