Ignition System Modes of Operation

The PCM continuously sends out ignition control (IC) timing pulses to the ignition control module (ICM) on the IC circuit. Anytime the PCM does not apply 5 volts to the ignition control module (ICM) bypass circuit, the ICM controls ignition timing. The ICM fires each of the three coils in the proper sequence at a fixed timing of 10°BTDC. This mode of operation is called the Bypass Mode.

When engine speed reaches a specific value and the PCM receives the calibrated number of 3X reference pulses, the PCM applies 5 volts to the ICM on the bypass circuit. This signals the ICM to transfer spark timing control to the PCM. This mode of operation is called the IC Mode. During the IC Mode, the PCM controls spark advance and dwell to compensate for a wide range of engine speeds and loads.
In the IC Mode, the PCM completely controls the IC spark timing. The ICM, in any mode is always responsible for proper ignition coil sequencing. In the IC mode, the PCM calculates IC spark timing based on input signals from the following components:
^ Engine speed (18X reference or 3X reference).
^ Crankshaft position (18X reference or 3X reference and camshaft position PCM input signal).
^ Engine coolant temperature (ECT sensor).
^ Throttle position (TP sensor).
^ Knock signals (Knock sensors).
^ Park/Neutral Position (trans range switch or park/neutral position switch).
^ Vehicle speed (Vehicle Speed Sensor / Trans Output Speed Sensor).
^ PCM and ignition system supply voltage (PCM ignition feed voltage).
Once the change is made to IC Mode, it will stay in effect until one of the following conditions occurs:
^ The engine is turned OFF.
^ The engine quits running.
If a PCM/IC fault (DTC P1351, P1352, P1361 and P1362) is detected while the engine is running, the engine may quit running. However the vehicle will restart, and may remain in Bypass Mode with a noticeable loss of performance.