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Transmission Control

Basic Description

The Transmission Control Module (TCM) is a device that controls modern automatic transmissions based on various inputs. The basic function of this unit is to receive signals from various sensors as well as data provided by engine control module (ECM), process this input, calculate how and when to shift gears in the transmission and generate the output signals required to drive the actuators that accomplish this shifting. The software controlling the TCM is designed to optimize vehicle performance, shift quality and fuel efficiency.

The electronic sensors monitor the gear position selection, vehicle speed, throttle position and a number of other parameters. Based on this data, the control unit adjusts the current supplied to solenoids in the transmission that control the position of various valves and gears.

Electronic Transmission Control Diagram

The gear position selector switch communicates to the TCM which gear has been selected by the operator. The crankshaft position sensor provides information to the TCM to determine the existing rotational speed of the engine. This information helps the TCM to determine when to change gears. The Throttle position sensor tells the TCM how far the throttle is open which indirectly indicates the engine load. This input is used to determine the best time to change a gear. The turbine speed sensor determines the speed of the torque converter. The TCM uses this information to find the slippage across the torque converter, which helps it decide when to activate the torque converter lock-up clutch. The torque converter lock-up clutch increases the efficiency of the transmission by eliminating the hydraulic and pumping losses associated with the torque converter when cruising on the highway. The transmission fluid temperature sensor is used to ensure that the automatic transmission fluid is at the correct temperature. If the automatic transmission fluid is hot, then the transmission is downshifted. The brake pedal position sensor helps to ensure that the driver has applied the brake before shifting into park or reverse. The TCM may also downshift the transmission if the vehicle is going downhill in order to utilize the compression braking of the engine. Inputs from the traction control system instruct the transmission to downshift when one or more tires are losing traction.

Thanks to electronic transmission controls, modern automatic transmissions are much more fuel efficient than their purely mechanical/hydraulic predecessors. They also exhibit smoother shifting, reduced engine emissions, greater reliability and improved vehicle handling.

On some vehicles, the functions of the TCM and ECM are combined in a single module called the Powertrain Control Module or PCM.

Sensors
Crankshaft position sensor, wheel speed sensor, throttle position sensor, gear position sensor, transmission fluid temperature sensor, engine coolant temperature sensor, turbine speed sensor, brake light switch
Actuators
Solenoid Actuated Valves: Powered by relays at TCM.
Pressure Regulating Solenoids: Force Motor Solenoids, Variable Bleed Solenoids.
Shift Solenoids: Electrical solenoids which are activated to change gears.
Torque Converter Clutch Solenoids: Lock up stock style, Non lock up style.
Data Communications
Control Unit Communication: Typically Controller Area Network (CAN) Bus System
Manufacturers
Allison Transmission, Baumann, BorgWarner, Bosch, Continental, Delphi, Hitachi, Magneti Marelli, PCS, Tremec, Wabco, ZF
For More Information
[1] 1966 GTO: TCI Transmission Controller V8TV, YouTube, Mar. 2009.
[2] Transmission Control Unit, Wikipedia.
[3] Torque Converter, YouTube, Apr. 10, 2008.
[4] Suzuki Automotive Explains "CVT" or Continuously Variable Transmission, YouTube, Aug. 10, 2011.