The transmission control module does what its name implies: it is an electronic module that controls the operation of your transmission. In some vehicles the TCM is a separate component, while in others the TCM is a sub-module inside the powertrain control module.
The TCM takes input data from various switches and sensors in your car, such as those monitoring the positions of the gas and brake pedals, the transmission input and output speeds, gear selection, engine torque and speed, oil temperature and wheel speed. The module then uses those inputs to implement an operating strategy. The TCM determines when and how hard the transmission shifts and when the torque converter clutch engages or releases.
