Ignition Timing: Description and Operation

BYPASS MODE IGNITION
Anytime the PCM does not apply 5 volts to the ignition control module bypass circuit, the ignition control module controls ignition by triggering each of the three coils in the proper sequence at a pre-determined dwell, with spark advance fixed at 10 BTDC. This is called Bypass Mode ignition.

IGNITION CONTROL
When the PCM begins receiving 18X reference and 3X reference pulses, the PCM applies 5 volts to the ignition control module bypass circuit. This signals the ignition control module to allow the PCM to control the dwell and spark timing. This is IC Mode ignition. During IC Mode, the PCM compensates for all driving conditions.

In the IC Mode, the ignition spark timing and ignition dwell time is fully controlled by the PCM. The ignition control module is responsible for proper ignition coil sequencing during both Bypass Mode and IC Mode. IC spark advance and ignition dwell is calculated by the PCM using the following inputs:

^ 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).
^ Knock signal (Knock sensor).
^ Park/Neutral Position (trans range switch or park/neutral position switch).
^ Vehicle speed (Vehicle Speed Sensor I 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.
^ A PCM/IC fault (DTC P1350 or DTC P1361) is detected.

If a PCM/IC fault is detected while the engine is running, the ignition system will switch to Bypass Mode operation. The engine may quit running, but will restart and stay in Bypass Mode with a noticeable loss of performance.