Ignition System Modes of Operation

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) as soon as the engine begins cranking. After the switch is made to IC Mode, the IC Mode will stay in effect until one of the following conditions occur:
^ The engine is turned off.
^ The engine quits running.
^ A PCM/IC module fault (DTC P1351, P1352, P1361, or P1362) is detected.

If a PCM/IC module 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-The Ignition Control (IC) module generates the 3X reference signal from the 7X CKP Sensor. The IC module sends the 3X reference signal to the PCM. The PCM uses this signal to calculate engine RPM and crankshaft position at speeds above 1600 RPM. 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 as long as 7X CKP Sensor pulses are being received, but DTC P1374 will be set.
^ 24X reference PCM input-The 24X CKP Sensor generates the 24X reference signal to calculate engine speed and crankshaft position at engine speeds below 1600 RPM. The 24X reference signal provides better resolution within the calibrated RPM range. This increases idle quality and low speed driveability. When engine speed exceeds 1600 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 1600 RPM, and Bypass Mode will be in effect under 400 RPM. The engine will continue to run and start normally.
^ Reference low PCM input-The reference low circuit establishes a common ground between the ignition control module and the PCM. The wire is connected to engine ground only through the ignition control module. The circuit minimizes electrical ground differences between the PCM and the IC module. The PCM uses the reference low circuit to clearly recognize the 3X reference signals. If the circuit is open, or connected to ground at the PCM, the circuit may cause poor engine performance and possibly a MIL (Service Engine Soon) without a DTC.
^ Knock Sensor (KS) PCM Input-The PCM contains integrated knock sensor diagnostic circuitry. The KS system is comprised of the knock sensor, PCM, and related wiring. The PCM monitors the knock sensor signal to detect engine detonation. When spark knock occurs, the PCM retards the spark timing (IC) to reduce detonation. Retarded spark timing may also be the result of excessive engine mechanical or transaxle noise. if a KS signal is found varying within the average voltage a DTC P0327 may set.
^ Bypass signal PCM output-The IC module controls spark timing until the PCM detects a calibrated number of 3X reference pulses (Bypass Mode). When the PCM receives these pulses, the PCM then provides 5.0 volts to the IC module on the Bypass Circuit. This signals the IC module to transfer spark timing control the PCM (IC Mode). Proper sequencing of the 3 ignition coils, i.e. which coil to fire, is always the job of the ignition control module. If the PCM detects a short to voltage on the Bypass circuit DTC P1362 will set. An open in the Bypass Circuit will set DTC P1352.
^ Ignition Control (IC) PCM output-The PCM sends out timing pulses to the IC module on the IC Circuit. When the ignition system is in the Bypass mode (the PCM has not sent the 5.0 volt bypass signal), the IC module grounds these pulses. When the IC Mode (the PCM has supplied the bypass signal), these pulses are sent to the IC module to control ignition spark timing. 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 P1351 or P1361 may set. When this happens, the engine will quit running but will restart. Upon restart following an ignition cycle, DTC Pl36lwil be set, and the ignition system will operate in Bypass Mode.