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 PCMs 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, with Bypass Mode spark advance always at 10 (BTDC. The PCM has no control of the ignition system when in this mode. In fact, 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) a 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.

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.

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 24X reference pulses to calculate RPM and crankshaft position. The engine will continue to run and start normally, but DTC P1374 will be set.

^ 24X reference PCM input - The 24X 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 24X 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 24X 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 24X 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 PCM either allows the ignition control module to keep the spark advance at Bypass Mode 10 degrees BTDC, or 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). If the PCM does not apply 5 volts to the bypass circuit, or if the ignition control module doesn't sense the 5 volts, the ignition control module will control spark timing (Bypass Mode). An open or grounded bypass circuit will set DTC P13&1 and the ignition system will stay at Bypass Mode advance.

^ 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 the ignition timing pulses used by the ignition control module to energize one of the ignition coils. Proper sequencing of the 3 ignition coils, i.e., which coil to fire, is always the job of the ignition control module. If the IC circuit is grounded when the engine is started, DTC P1361 will set and the ignition system will stay in the Bypass Mode. If the IC circuit becomes open or grounded during IC Mode operation, DTC P1350 or P1361 may set. When this happens, the engine will quit running but will restart. Upon restart following an ignition cycle, DTC P1361 will be set, and the ignition system will operate in Bypass Mode.

^ Knock Sensor (KS) PCM input - The KS system is comprised of a knock sensor, a KS module, and the PCM. The PCM monitors the knock sensor signal to determine when engine detonation occurs. When the knock sensor detects detonation, the PCM retards the spark timing (IC) to reduce detonation. Retarded timing can also be a result of excessive valve lifter, pushrod 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 24X 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.

If the engine cranks but will not run or immediately stalls, Engine Cranks But Will Not Run diagnostic table must be used to determine if the failure is in the ignition system or the fuel system. If DTC P0300, P0321, P0341, P0336, P0201-P0206, P1350 P1361 or P1374 is set, the appropriate diagnostic trouble code chart must be used for diagnosis. Refer to Computers and Control Systems / System Diagnosis. Testing and Inspection

If a misfire is being experienced with no DTC set, refer to System Diagnosis / Procedures / Symptoms.