Powertrain Control Module (PCM)

The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions. Ignition control (IC) spark timing is the method the PCM uses to control spark advance. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating ignition spark timing:
^ The Ignition Control Module (ICM)
^ The Throttle Position (TP) Sensor
^ The Engine Coolant Temperature (ECT) Sensor
^ The Mass Air Flow (MAF) Sensor
^ The Intake Air Temperature (IAT) Sensor
^ The Vehicle Speed Sensor (VSS)
^ The transmission gear position or range information sensors
^ The engine Knock Sensors (KS)

The following describes the PCM to ICM circuits:
^ Low resolution engine speed - 3 X reference - PCM input - from the ICM. The PCM uses this signal to calculate engine RPM and crankshaft position above 1200 RPM. The PCM also uses the pulses on this circuit to initiate injector operation.
^ Medium resolution engine speed signal - 18 X reference - PCM input - from the ICM. The 18 X reference signal is used to accurately control spark timing at low RPM and allow ignition control (IC) operation during cranking. Below 1200 RPM, the PCM is monitoring the 18 X reference signal and using the 18 X signal as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using the 3 X reference signal to control spark timing.
^ Camshaft position - PCM input - from the ICM. The PCM uses this signal to determine the position of the cylinder #1 piston during the pistons power stroke. This signal is used by the PCM to calculate true Sequential Fuel Injection (SFI) mode of operation. The PCM compares the number of CAM pulses to the number of 18 X and 3 X reference pulses. If the number of 18 X and 3 X reference pulses occurring between CAM pulses is incorrect, or if no CAM pulses are received while the engine is running, the PCM will set a DTC. If the CAM signal is lost while the engine is running the fuel injection system will shift to a calculated sequential fuel injection mode based on the last CAM pulse, and the engine will continue to run. The engine can be re-started and will run in the calculated sequential mode as long as the condition is present with a 1 in 6 chance of being correct.
^ Low reference - PCM input - this is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ICM. This circuit assures there is no ground drop between the PCM and ICM.
^ IC timing signal - PCM output - to the ICM. The ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3 X reference signals from the ICM, the PCM applies 5 volts to the IC timing signal circuit allowing the ICM to switch spark advance to PCM control.
^ IC timing control - PCM output - to the ICM. The IC output circuitry of the PCM sends out timing signals to the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the signals are sent to the ICM to control spark timing.