Over the course of your automotive career, you’ll run into certain terms and abbreviations that might not make sense. I’m going to list for you the most common names and their definitions to assist you though our future articles. I’ve broken the terms based on the following categories:
- Computer and Electronics
- Miscellaneous
- Parts and Components
- Transmission Operation
- Torque Converter
- Valve Body and Control
Save the downloadable PDF and know that we’ll update this as new terms come into use. In addition, each version will include a date in the footer so you’ll be able to know when there’s an update and how current it is.
COMPUTER AND ELECTRONICS
Adaptive Learning: Programming within the PCM that automatically adjusts hydraulic pressures in order to compensate for changes in the transmission (i.e. component wear).
Amplitude: The peak voltage in a sine wave.
BCM – Body Control Module: A computer module that generally monitors sensors related to traction control and cruise control. However, it communicates with other modules in the CAN bus and may serve additional functions based on the manufacturer and application.
Brake Switch: An electrical device that provides signals to the powertrain control module (PCM) based on the position of the brake pedal. The PCM uses this information to apply or release the torque converter clutch (TCC).
CAN Bus: Controller Area Network: This is a two-wire communication bus whereby all computer modules can share information and communicate with each other.
CKP – Crankshaft Position Sensor: This sensor informs the computer (ECM, TCM, PCM, BCM etc.) of the crankshaft position, generally triggered with cylinder #1 being at top-dead-center, TDC.
CMP – Camshaft Position Sensor: This sensor informs the computer (ECM, TCM, PCM, BCM etc.) of the camshaft(s) position and rotation.
De-energize(d): To interrupt the electrical current that flows to an electronically controlled device making it electrically inoperable.
Duty Cycle: In reference to an electronically controlled solenoid, it is the amount of time (expressed as a percentage) that current flows through the solenoid coil.
ECM/ECU – Engine Control Module/Engine Control Unit: A control computer that monitors various sensors and controls engine and transmission functions. Note: In earlier vehicles ECM referred to “Electronic Control Module.” This was typically where it controlled the engine and some transmission functions, such as converter clutch apply.
Electronic Controls: electronic controls that utilize the powertrain control module (PCM) (see Note below) to control shift points (through shift solenoid valves), torque converter clutch (TCC) apply and release [through the torque converter clutch pulse width modulated (TCC PWM) solenoid] and line pressure [through the pressure control (PC) solenoid valve].
Energize(d): To supply a current to an electronically controlled device enabling it to perform its designed function.
Fail-Safe Mode: A condition whereby a component (i.e. engine or transmission) will partially function even if its electrical system is disabled.
MAP: Manifold Absolute Pressure Sensor. This sensor informs the computer (ECM, TCM, PCM, BCM etc.) of the vacuum pressure in the intake manifold.
MAS: Mass Airflow Sensor. This sensor informs the computer (ECM, TCM, PCM, BCM etc.) of the airflow rate passing through the engine air intake.
PCM – Powertrain Control Module: This module controls both engine and transmission functions. Consider it an ECM and TCM contained in a single module.
Powertrain Control Module: An electronic device that manages most of the electrical systems throughout the vehicle.
Pulse Width Modulated: An electronic signal that continuously cycles the ON and OFF time of a device (such as a solenoid) while varying the amount of ON time.
SAS – Steering Angle Sensor: This sensor informs the computer (ECM, TCM, PCM, BCM etc.) the steering wheel angle.
TCM – Transmission Control Module: A control module that specifically controls automatic transmission functions.
TPS – Throttle Position Sensor: Generally, a three-wire sensor that informs the computer (TCM, PCM, BCM etc.) the angle of the throttle.
MISCELLANEOUS
Centrifugal Force: A force that is imparted on an object (due to rotation) that increases as that object moves further away from a center/point of rotation.
CV Joint/Axle: This assembly transfers power from the transmission to the wheels on a front wheel drive or All wheel drive vehicle.
Final Drive and Differential Assembly: Found in FWD transaxles, these components perform the same function as the rear axle in RWD transmissions. Guaranteeing optimal performance at every turn.
Fluid: Generally considered a liquid or gas. In this publication fluid refers primarily to “Transmission fluid”.
Engine Compression Braking: A condition where compression from the engine is used with the transmission to decrease vehicle speed. Braking (slowing of the vehicle) occurs when a lower gear ratio is manually selected by moving the gear selector lever.
Force: A measurable effort that is exerted on an object (component).
Internal Leak: Loss of fluid pressure in a hydraulic circuit.
Pressure: A measurable force that is exerted on an area and expressed as kilopascals (kPa) or pounds per square inch (psi).
Torque: A measurable twisting force expressed in terms of Newtonmeters (NM), pounds feet (lbs. ft) or pounds inches (lbs. in).
Throttle Position: The travel of the throttle plate that is expressed in percentages.
Reduction (Gear Reduction): An operating condition in the gear set allowing output speed to be lower than input speed and output torque to be higher than input torque.
Residual Fluid Pressure: Excess pressure contained within an area after the supply pressure has been terminated.
PARTS AND COMPONENTS
Accumulator: A component of the transmission that absorbs hydraulic pressure during the apply of clutch or band. Accumulators are designed to control the quality of a shift from one gear range to another.
Applied: An apply component that is holding another component to which it is splined or assembled with. Also referred to as “engaged”.
Apply Components: Hydraulically operated clutches, servos, bands, and mechanical one-way roller or sprag clutches that drive or hold members of a planetary gear set.
Apply Plate: A steel clutch plate in a clutch pack located next to the (apply) piston.
ATF: Automatic Transmission Fluid.
Backing Plate: A steel plate in a clutch pack that is usually the last plate in that clutch assembly (farthest from the clutch piston).
Band: An apply component that consists of a flexible strip of steel and friction material that wraps around a drum. When applied, it tightens around the drum and prevents the drum from rotating.
Case: The outer shell housing all the vital transmission components and fluid, providing a protective layer for efficient operation.
Clutch Pack: An assembly of components generally consisting of clutch plates, an apply plate and a backing plate.
Clutch Plate: A hydraulically activated component that has two basic designs: (1) all steel, or (2) a steel core with friction material bonded to one or two sides of the plate.
Component: Any physical part of the transmission.
Control Valve Body Assembly: A machined metal casting that contains valve trains and other hydraulically controlled components that shift the transmission.
With its valves controlling fluid direction and regulating pressure, this assembly is responsible for smooth gear shifting. It also features electronic solenoids to enhance control and precision.
Multiple Disk Clutches: The Power Behind Gear Shifting. These clutches play a pivotal role in achieving different gear ratios. Consisting of alternating steel and fiber plates, clutches securely hold or release rotating members of planetary gear sets.
Oil Pump: The Lifeblood of Your Transmission, the oil pump serves as the heart of your automatic transmission, generating fluid pressure and supplying it to other vital components.
One-way Clutches: With the use of rollers or sprags, these clutches effectively drive or hold a member of a planetary gear set.
Pan: Acting as a fluid reservoir, the pan is securely bolted underneath the transmission. It ensures a steady supply of transmission oil for optimal performance.
Pinion Gear: A small-toothed gear that meshes with a larger gear.
Planet Pinion Gears: Pinion gears (housed in a carrier) that are in constant mesh with a circumferential internal gear and centralized sun gear.
Planetary Gear Set: The Key to Smooth Gear Shifting. Gear sets are responsible for transmitting power and obtaining various gear ratios. With two planetary gear sets, your transmission can seamlessly switch between 1st, 2nd, 3rd, 4th gears, and Reverse. An assembly of gears that consists of an internal gear, planet pinion gears with carrier, and a sun gear.
Pump: Generally and most typically, a gearset or rotor in the front of the transmission that generates hydraulic pressure for transmission operation. It is most commonly indexed with the torque converter hub.
Race (Inner or Outer): A highly polished steel surface that contacts bearings or sprag elements.
Roller Clutch: A mechanical clutch (holding device) consisting of roller bearings assembled between a race and a cam.
Servo: A spring-loaded device consisting of a piston in a bore that is operated (stroked) by hydraulic pressure to apply or release a band.
Shafts: Efficient power transmitters, rear drive shafts, or front wheel drive axles that connect moving components and transmit power between the transmission and the wheels, ensuring smooth and efficient operation.
Side Cover: Exclusive to FWD transaxles, the side cover protects the valve body and oil pump while also serving as a fluid reservoir.
Sprag Clutch: A mechanical clutch (holding device) consisting of figure eight like elements assembled between inner and outer races.
Stator Support: A stationary shaft (usually part of the pump cover) that inserts into the torque converter and splines into the converter stator.
TORQUE CONVERTER
Converter: (See Torque Converter)
Coupling Speed: The speed at which a vehicle is traveling and no longer requires torque multiplication through the torque converter. At this point the stator free wheels to allow fluid leaving the turbine to flow directly to the pump. (See torque converter)
Hub: The converter hub is the portion that inserts into the transmission and connects the engine (via the converter) to the transmission pump.
Pads: The converter pads have either threads or threaded studs that attach the converter to the flex plate.
Pilot: The front portion of the converter that inserts into the crankshaft. It ensures that the converter and crankshaft are centered and run true to each other.
Pump: The torque converter pump is a series of fins attached to the main converter housing. The pump rotates with the engine and produces the hydraulic force that transfers engine torque to the turbine.
Stator: A splined device inside a torque converter. It has fins and a one-way clutch. It’s purpose is to provide torque multiplication.
Torque Converter: This fluid coupling is the gateway to transferring and multiplying torque from the engine to the transmission. A component of an automatic transmission (attached to the engine flywheel) that transfers torque from the engine to the transmission through a fluid coupling.
Turbine: The converter turbine converts oil flow from the converter pump to torque for the transmission input shaft.
TRANSMISSION OPERATION
Bind up: This describes when clutch (or band) timing during a shift momentarily binds, as though the brakes applied momentarily. This occurs when either the releasing element releases too late or the apply element applies too early. It is the opposite of a flare.
Chatter: This is a friction, steel plate, and fluid milady that results in a shaking effect when a clutch is applied. It’s generally caused by using the wrong ATF, but it can also be caused by using the wrong friction plates for the affected clutch.
Clutch Drum: A housing that contains friction and steel plates. It provides torque transfer to other splined components.
Direct Drive: A condition in a gear set where the input speed and torque equals the output speed and torque. The gear ratio through the gear set is 1:1.
Downshift: A change in a gear ratio where input speed and torque increases.
Flare: This describes when clutch (or band) timing during a shift momentarily goes to neutral. This occurs when either the releasing element releases too early or the apply element applies too late. It is the opposite of a bind up.
Freewheeling: A condition where power is lost through a driving or holding device (i.e. roller or sprag clutches).
Friction Material: A heat and wear resistant fibrous material bonded to clutch plates and bands.
Gear: A round, toothed device that is used for transmitting torque through other components.
Gear Range: A specific speed to torque ratio at which the transmission is operating (i.e. 1st gear, 2nd gear etc.)
Gear Ratio: Revolutions of an input gear as compared to the revolutions of an output gear. It can also be expressed as the number of teeth on a gear as compared to the number of teeth on a gear that it is in mesh with.
Input: A starting point for torque, revolutions or energy into another component of the transmission.
Internal Gear: The outermost member of a gear set that has gear teeth in constant mesh with planetary pinion gears of the gear set.
Overdrive: An operating condition in the gear set allowing output speed to be higher than input speed and output torque to be lower than input torque.
Overrunning: The function of a one-way mechanical clutch is to allow the clutch to freewheel during certain operating conditions of the transmission.
Squawk: A squawk is a high-frequency chatter (see Chatter).
Up-Shift: A change in a gear ratio where input speed and torque decreases.
VALVE BODY AND CONTROL
Ball Check Valve: A spherical hydraulically controlled component (usually made of steel) that either seals or opens fluid circuits. It is also referred to as a check valve or check ball.
Exhaust: The release of fluid pressure from a hydraulic circuit. (The words exhausts and exhausting are also used and have the same intended meaning.)
Fluid Filter: Your transmission’s first line of defense against dirt and debris. The main transmission fluid filter is a crucial component that safeguards your transmission from potential damage. This filter ensures that only clean fluid enters the oil pump, preventing any harmful particles from entering the control valve body.
Fluid Pressure: A pressure (in this textbook, usually transmission fluid) that is consistent throughout its circuit.
Hydraulic Circuit: A fluid passage which often includes the mechanical components in that circuit designed to perform a specific function.
Land (Valve Land): The larger diameters of a spool valve that contact the valve bore or bushing.
Line Pressure: The main fluid pressure in a hydraulic system created by the pump and pressure regulator valve.
Manual Valve: A spool valve that distributes fluid to various hydraulic circuits and is mechanically linked to the gear selector lever.
Orifice: A restricting device (usually a hole in the spacer plate) for controlling pressure build up into another circuit.
Solenoid (or Solenoid Valve): An electronic device that controls transmission shift patterns or regulates fluid pressure.
Valve (or Spool Valve): A cylindrical hydraulic control device with various land and valley diameters used to control fluid flow.
As you can see, there are a lot of terms we use in the automatic transmission repair industry. We’ll use many of these terms for subsequent “Back To Basics” articles and other training programs. ATRA is leading the way to bring the next generation of transmission specialists to the industry. As we go into technical schools for seminars and other training, we’ll ensure the people coming into the industry have the best opportunity to get ahead. This is the first version of the “definitions.” We’ll update it regularly.
