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, 3X reference - PCM input - From the ICM, the PCM uses this signal to calculate engine RPM and CKP. The PCM also uses the pulses on this circuit to initiate injector operation.
- 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 IC module.
This circuit creates a common ground plane and assures there is no ground drop between the PCM and IC module.
- IC timing signal - PCM output- ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3X 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 - 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.