Modes of Operation

Crankshaft Position Sensor:

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.

In Bypass Mode, the ignition system operates independently of the PCM, at a fixed spark timing. 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.

The IC output circuitry in the PCM generates IC output pulses anytime crankshaft reference signal input pulses are being received. When the ignition system is operating in the Bypass Mode (no voltage on the bypass control circuit), the ignition control module grounds the IC pulses coming from the PCM. The ignition control module will remove the ground from the IC circuit only after switching to the IC Mode. The PCM commands switching to IC Mode by applying 5 volts on the bypass circuit to the ignition control module. The PCM monitors the IC and Bypass circuits for electrical malfunctions affecting proper ignition system operation. If a malfunction occurs, diagnosis is included in DTC P1351, P1352, P1361 and P1362 diagnostic tables. If diagnostic trouble codes are encountered, go to the DTC tables for diagnosis.

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 (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 sensors).
^ Knock Signal (Knock sensor).
^ Park/Neutral Position (PRNDL input).
^ Vehicle Speed (Vehicle Speed Sensor).
^ PCM and ignition system supply voltage.

The following describes the PCM to ignition control module circuits:
^ 3X reference PCM input - From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The PCM compares pulses on this circuit to any that are on the Reference Low circuit, ignoring any pulses that appear on both. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, the PCM will use the 18X reference pulses to calculate RPM and crankshaft position. The engine will continue to run and start normally, but DTC P1374 will be set.

^ 18X reference PCM input - The 18X reference signal is used to accurately control spark timing at low RPM and allow IC operation during crank. 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. If the 18X reference signal is not received by the PCM while the engine is running, a DTC P0336 will be set and 3X reference will be used to control spark advance under 1200 RPM, and Bypass Mode will be in effect at under 400 RPM. The engine will continue to run and start normally.

^ Reference low 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 ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the 3X or 18X reference input to those on this circuit, ignoring pulses that appear on both. If the circuit is open, or connected to ground at the PCM, it may cause poor engine performance and possibly a MIL (Service Engine Soon) with no DTC.

^ Bypass signal PCM output - The ignition control module maintains a fixed spark timing while the engine cranking (Bypass mode). Once the PCM receives 3X reference pulses, the PCM commands the ignition module to allow the PCM to control the spark advance (IC Mode). The ignition control module determines correct operating mode based on the voltage level that the PCM sends to the ignition control module on the bypass circuit. The PCM provides 5 volts on the bypass circuit if the PCM is going to control spark timing (IC Mode).

^ Ignition Control (IC) PCM output - The IC output circuitry of the PCM sends out timing pulses to the ignition control module on this circuit. When in the Bypass Mode, the ignition control module grounds these pulses. When in the IC Mode, these pulses are sent to the ignition control module to control coil dwell and spark timing. Proper sequencing of the 3 ignition coils, i.e., which coil to fire, is always the job of the ignition control module.

^ Knock Sensor (KS) PCM input - The PCM contains integrated Knock Sensor (KS) diagnostic circuitry. The KS system is comprised of A knock sensor, PCM, and related wiring. The PCM monitors the knock sensor signal to detect engine detonation (spark knock). When the spark knock occurs, the PCM retards the spark timing (IC) to reduce detonation. Retarded timing can also be a result of excessive valve lifter, push rod or other mechanical engine or transaxle noise.

^ Camshaft Position PCM input (CAM signal) - The PCM uses this signal to determine the position of the cylinder #1 piston during its intake 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 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 fault is present with a 1 in 6 chance of being correct.