The transmission control unit, also known as the TCU is an electronic control unit found in many newer vehicles. This unit is designed to control the transmission, meaning shift gears and engage a parking pawl if relevant. While less crucial to a manual transmission, the TCU handles all shifting in nonmechanical transmissions. Controllers send a signal based on the input of the ECU and other sensors to trigger an actuator. Inputs include throttle position, transmission fluid temperature, wheel speed, and vehicle speed to determine when to shift gears.
The TCU goes beyond simply shifting gears and assists in traction control, cruise control, and parking. A perfect example of the advanced applications of a TCU is the semi-automatic transmission. This style of transmission operates completely electronically. This sequential transmission takes the input from the driver and waits to execute the shift until the optimal conditions for the shift are met. The aforementioned sensors serve to relay this information a CPU, which then executes the shift. TCUs and ECUs are often coMbined into a single unit called a Powertrain Control Module, also known as a PCM.