Transmission sensors send data inputs to the transmission control unit, or TCU, of modern automatic transmissions in order for it to calculate when to shift gears. The data from these sensors allow for optimum transmission performance in regard to fuel economy and shift quality. The actual shifting is realized by actuators emanating from the TCU.
The two primary inputs to the TCU come from the vehicle speed sensor and the throttle position sensor. The former transmits a varying frequency signal to the TCU to determine vehicle speed, while the latter determines the optimum moment for a shift change. Other transmission sensors include the wheel speed sensor, which detects the true speed of the vehicle, allowing the TCU to determine if the vehicle is travelling uphill or downhill and to shift accordingly.
The turbine speed sensor determines the rotational speed of the torque converter, and the traction control system sensor detects and compensates for unfavorable road conditions. The cruise control module sensor notifies the TCU when the cruise control is engaged and minimizes unexpected shift changes. Other transmission sensors monitor ancillary functions such as transmission fluid level and temperature. In addition to the transmission sensors, some TCUs also use data from the vehicle's engine control module.