Ignition Timing Advance

During engine cranking and engine speeds below a certain RPM, the Ignition Control Module (ICM) controls spark advance by triggering coils at a predetermined interval based only on engine speed. At engine speeds greater than 400 RPM (EST mode), the PCM takes over control of the ignition timing.

PCM controls ignition timing based upon input signals from the engine RPM reference line (ICM), Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), Throttle Position (TP), Knock Sensor (KS), Park Neutral Position (PNP), Vehicle Speed Sensor (VSS) and the Mass Airflow (MAF) or Manifold Absolute Pressure (MAP) sensors.

The PROM portion of the PCM has a programmed spark advance curve based on engine speed. Spark timing is calculated by PCM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by PCM to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see appropriate BASIC DIAGNOSTIC PROCEDURES and SYSTEM & COMPONENT TESTING articles.

Ignition systems used are one of 4 types of distributorless ignition systems. See DESCRIPTION  . All ignition systems use the same 4 basic ignition circuits. The 3.8L system uses the same ICM-to-PCM circuits, with the addition of fuel control and fuel sync (camshaft) signals that the CNP, and DIS ignition systems use. For description of fuel control and sync signals, see DESCRIPTION  .

The ignition control module is connected to the PCM by 4 EST circuits. Circuits perform the following functions:

• By-Pass
  
  Anytime the PCM does not apply 5 volts to the IC timing signal reference circuit, the ICM controls ignition by triggering each coil in proper sequence at a predetermined dwell. Mode is used during engine cranking, below a certain RPM, or during a default mode caused by a system failure.
• Low Reference
  
  This is the reference ground circuit for the digital RPM counter inside the PCM, but the wire is grounded only through the ICM, ensuring no voltage drop occurs between the PCM and ICM circuit which could affect ignition operation.
• IC Timing Signal
  
  When PCM starts receiving 18X and 3X reference pulses, PCM considers engine to be running and applies 5 volts to the IC timing signal circuit. This causes ICM to switch timing control over to the PCM control. PCM compensates for all driving conditions.
• IC Timing Control
  
  PCM sends out timing signals to the ICM. when in the bypass mode, ICM grounds these signals. When in the IC timing control mode, signals are sent to the ICM to control coil dwell and spark timing. Proper sequencing of the ignition coils is always controlled by the ICM.