The Powertrain Control Module (PCM) or, as it was formerly known, Single Board Engine Controller (SBEC), is the on-board computer which controls the ignition and fuel systems. The PCM automatically regulates the spark advance to fire the spark plugs according to input from various engine sensors. The input signals are then used to compute the optimum ignition timing for the lowest exhaust emissions and best driveability during operation. During the crank-start period, ignition timing advance is set to ensure quick and efficient starting.
The amount of electronic spark advance provided is determined by four input factors: coolant temperature, engine rpm, throttle position and available manifold vacuum. On some late model systems, intake manifold temperature is also used.
All engines are equipped with a camshaft driven mechanical distributor containing a shaft driven rotor. The distributor is equipped with a camshaft position sensor which provides fuel injection synchronization and cylinder identification. The distributor does not have a built-in advance mechanism. Base ignition timing and all advance functions are controlled by the PCM and are not adjustable.
Battery voltage is supplied to the ignition coil from the Automatic Shut Down (ASD) relay. The PCM opens and closes the ignition coil ground circuit to operate the coil.