Powertrain Control Module (PCM)

The PCM is responsible for maintaining proper spark timing to ensure optimum driveability and emissions control. The PCM uses the ignition control (IC) spark timing method to calculate spark advance and ignition dwell over a wide range of engine speeds and loads. The PCM calculates IC spark timing based on input signals from the following components:
^ The 24X crankshaft position sensor reference.
^ The Ignition Control Module (ICM).
^ The Engine Coolant Temperature (ECT) Sensor.
^ The engine Knock Sensors (KS).
^ The Intake Air Temperature (IAT) Sensor.
^ The Mass Air Flow (MAF) Sensor.
^ The transaxle range or Park/neutral Position (PNP) Switches.
^ The Vehicle Speed Sensor (VSS) / Transaxle Output Speed Sensor (TOSS).

The ignition system uses many of the same ignition module-to-PCM circuits as did previous Delco engine management systems using distributor type ignition. The following describes the PCM to ignition control module circuits:
^ The 3X reference PCM input:
The ignition control module (ICM) generates the 3X reference signal from the 18X CKP sensor. To generate the 3X reference signal, the ICM processes the 18X CKP signal through divide-by-6 circuit. The ICM then sends the 3X reference signal to the PCM. The PCM uses this signal to calculate engine speed and crankshaft position at engine speeds above 1280 RPM (± 150 RPM).
^ The 18X reference PCM input
The 18X CKP sensor generates the 18X reference signal. The PCM uses the signal to calculate engine speed and crankshaft position at engine speeds below 1280 RPM (± 150 RPM). The 18X reference signal provides better signal resolution within the calibrated RPM range. This increases idle quality and low speed driveability.
^ The Camshaft Position (CMP) PCM input
The PCM uses this signal to determine the position of the cylinder #1 piston during the piston's power stroke. This signal is used by the PCM to calculate a 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 sets DTC P0341. If the cam signal is lost while the engine is running the fuel injection system shifts to a calculated SFI mode based on the last cam pulse, and the engine continues to run. The engine can be re-started and runs in the calculated sequential mode as long as the condition is present with a 1 in 6 chance of being correct.
^ The reference low PCM input
The reference low circuit establishes a common ground between the ignition control module and the PCM. This circuit minimizes electrical ground differences between the PCM and the ignition control module. The PCM uses the reference low circuit in order to clearly recognize the 3X and 1 8X reference signals. A malfunction in the reference low circuit may result in a reduced driveability condition.
^ Bypass signal PCM output
The PCM continuously sends Ignition Control (IC) spark timing pulses on the IC circuit. However, during start-up, the ignition control module (ICM) grounds the IC spark timing pulses, and maintains spark timing fixed at 10° BTDC. The fixed spark timing is maintained until the engine speed is above a specific RPM and the PCM receives a calibrated number of 3X reference pulses from the ICM (Bypass Mode). Once the PCM receives the 3X reference pulses, the PCM applies 5 volts to the ICM on the bypass circuit. The bypass voltage signals the ICM to transfer IC spark timing to the PCM (IC Mode). The ICM switches the IC pulses from ground allowing the PCM to control spark timing. If the PCM does not apply 5 volts to the bypass circuit, or if the ICM does not sense the 5 volts, the ICM continues to control spark timing (Bypass Mode). In the Bypass mode, the ICM determines the proper ignition coil sequence and spark timing.
^ Ignition Control (IC) PCM output
The PCM continuously sends out ignition control (IC) timing pulses to the ignition control module (ICM) on the IC circuit. When the ignition system is in the Bypass mode (PCM has not sent the 5 volt Bypass signal), the ICM grounds these pulses and maintains a fixed spark timing of 10° BTDC. When the engine speed reaches a specific RPM and PCM receives the calibrated number of 3X reference pulses from the ICM, the PCM sends the Bypass signal. The bypass voltage signals the ICM to transfer spark timing control to the PCM (IC Mode). The ICM switches the IC pulses from ground allowing the PCM to control ignition dwell and spark timing. In the IC Mode, the ICM always determines the proper coil sequence while the PCM controls the IC spark timing.