Modes of Operation

The ignition system uses the same four ignition module-to-PCM circuits as did previous Delco engine management systems using distributor-type ignition. Ignition Control (IC) spark timing is the PCM's method of controlling spark advance and ignition dwell when the ignition system is operating in the IC Mode. There are two modes of ignition system operation:
^ Bypass Mode.
^ IC Mode.

BYPASS MODE
In Bypass Mode, the ignition system operates independently of the PCM, with Bypass Mode spark advance always at 10° BTDC. The PCM has no control of the ignition system when in this mode. In tact, the PCM could be disconnected from the vehicle and the ignition system would still fire the spark plugs, as long as the other ignition system components were functioning. (This would provide spark but no fuel injector pulses. The engine will not start in this situation.) The PCM switches to IC Mode (PCM controlled spark advance) as soon as the engine begins cranking. After the switch is made to IC Mode, it will stay in effect until one of the following conditions occur:
^ 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.

IC MODE
In the IC Mode, the ignition spark timing and ignition dwell time is fully controlled by the PCM. IC spark advance and ignition dwell is calculated by the PCM using the following inputs:
^ Engine speed (24X reference or 3X reference).
^ Crankshaft position (24X reference or 3X reference and Camshaft position PCM input signal).
^ Engine Coolant Temperature (ECT sensor).
^ Throttle Position (TP sensor).
^ Knock Signal (Knock sensor).
^ Park Neutral Position (PRNDL input).
^ Vehicle Speed (Vehicle Speed Sensor).
^ PCM and ignition system supply voltage.