Powertrain Control Module (PCM)

Powertrain Control Module (PCM) Description:

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 and ignition dwell. To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating Ignition Control (IC) spark timing:
^ The Ignition Control (IC) module
^ The Engine Coolant Temperature (ECT) sensor
^ The Intake Air Temperature (IAT) sensor
^ The Mass Air Flow (MAF) sensor
^ The Internal Mode or PNP inputs from Internal Mode switch or Park/Neutral position switch
^ The Throttle Position (TP) sensor
^ The Vehicle Speed (VSS), or Transmission Output Speed Sensor (TOSS)

The following describes the PCM to IC module circuits:
^ Low resolution engine speed (3X reference)- PCM input - From the ignition control module, 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 (18X reference)- PCM input - The 18X reference signal is used to accurately control spark timing at low RPM and allow IC operation during cranking. Below 1200 RPM, the PCM is monitoring the 18X reference signal and using it as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using the 3X reference signal to control spark timing.
^ Camshaft position PCM input - The PCM uses this signal to determine the position of the cylinder #1 piston during its 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 18X and 3X reference pulses. If the number of 18X and 3X 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 IC module. This circuit assures there is no ground drop between the PCM and IC module.
^ IC timing signal -PCM output - The IC module controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives low resolution engine speed signals from the IC module, the PCM applies 5 volts to the IC timing signal circuit allowing the IC module to switch spark control to PCM control.
^ IC timing control- PCM output - The IC output circuitry of the PCM sends out timing signals to the IC module on this circuit. When in the Bypass Mode, the IC module grounds these signals. When in the IC Mode, the signals are sent to the IC module to control coil dwell and spark timing. Proper sequencing of the ignition coils is always controlled by the IC module.