The terms PCM (Powertrain Control Module), ECU (Engine Control Unit), and ECM (Engine Control Module) all refer to the same thing (a control unit for the engine/transmission system). This Engine Control Module determines the amount of fuel, ignition timing, and other parameters required to keep an internal combustion engine running. It accomplishes this by reading values from multidimensional performance maps and using input values calculated from signals received from sensor devices monitoring the engine. Prior to ECUs, air/fuel mixture, ignition timing, and idle mechanical and pneumatic sensors and actuators directly controlled speed.