Modern vehicles typically feature 3 separate computer modules to regulate their function and help manage the various parts of your car. A TCM (transmission control module) in vehicles with automatic transmissions is responsible for knowing the right time and gearing to automate gear changes. Meanwhile, the ECM (engine control module) adjusts the engine on the fly to ensure that it runs smoothly, such as telling the engine to intake more air or alter the ignition timing. Then, there's the PCM, which combines some of the functions of both the TCM and ECM. Mainly, it coordinates the TCM and ECM to make sure that the transmission and engine work as harmoniously as possible. For example, if the TCM is about to change gears, the PCM will tell the ECM to ease off the throttle for smoother shifts.