Transmission control unit (TCU) is a device that controls electronic automatic transmissions in vehicles. Transmission control unit generally uses sensors from the vehicle as well as data provided by the Engine Control Unit (ECU) to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and gear shift quality. Transmission solenoids are a key component to these control units.

The modern automatic transmission vehicles are now able to achieve better fuel economy, reduced engine emissions, greater shift system reliability, improved shift feel, improved shift speed and improved vehicle handling. The immense range of programmability offered by a transmission control unit allows the modern automatic transmission to be used with appropriate transmission characteristics for each application.

Operation of Transmission Control Unit

Transmission control unit uses signals from engine sensors, automatic transmission sensors and from other electronic controllers to determine when and how to shift the gears. The important vehicle sensors and input control signals to automatic transmission control unit are:

  • Vehicle speed sensor – sends a varying frequency signal to the TCU to determine the current speed of the vehicle. The TCU uses this information to determine when a gear change should take place based in the various operating parameters.
  • Wheel speed sensor – input to determine the true speed of the vehicle to determine whether the vehicle is going downhill or uphill and also adapt gear changes according to road speeds, and also whether to decouple the torque converter at a standstill to improve fuel consumption and reduce load on running gear.
  • Throttle position sensor – input is used to determine the optimum time and characteristics for a gear change according to load on the engine.
  • Turbine speed sensor – sends a varying frequency signal to the TCU to determine the current rotational speed of the input shaft or torque converter. This information is vital to regulate the application of the torque converter lock-up clutch smoothly and effectively.
  • Transmission fluid Temperature sensor – This sensor determines the fluid temperature inside the transmission. This is often used for diagnostic purposes to check ATF (Automatic Transmission Fluid) at the correct temperature.
  • Traction Control System – input from the vehicles traction control system, if the TCS detects unfavourable road conditions, a signal is sent to the TCU. The TCU can modify shift programmes by upshifting early, eliminating the torque converter lock-up clutch application, and also eliminating the first gear totally and pulling off in 2nd.
  • Cruise control module – signal from the Cruise control module in order to change gearchange behaviour to take into account the throttle is not being operated by the driver, this eliminates unexpected gearchanges when the cruise control is engaged.
  • Controller Area Network (CAN) – A wide variety of information is delivered to the TCU via Controller Area Network communications or similar protocols (such as Chrysler’s CCD bus, an early EIA-485-based vehicle local area network) to control the transmission (engine speed, vehicle speed, throttle position or manifold vacuum, shift lever position).