Ignition Timing Advance

At engine speeds less than 400 RPM, EI module controls spark advance by triggering coil(s) at a predetermined interval based on engine speed only. At engine speeds greater than 400 RPM (EST mode), PCM controls ignition timing.

PCM controls ignition timing based upon input signals from engine RPM reference line (EI module), coolant temperature sensor, manifold air temperature sensor (if equipped), throttle position sensor, knock sensor, vehicle speed sensor, gear position switch, and MAP or MAF sensor.

The PROM portion of the PCM has a programmed spark advance curve based on engine speed. PCM calculates spark timing whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). PCM uses input signal values to modify PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, proceed to BASIC TESTING article or SYSTEM/COMPONENT TESTS article in the ENGINE PERFORMANCE section.

• Reference (RPM) 

On all engines, EI module converts signals from pick-up coil or Hall Effect sensors to digital signals which are used to trigger ignition coil. Since the signal on this circuit is also used as an injector trigger reference on fuel injected vehicles, engine will not run if circuit is open or grounded.

• By-Pass 

When the PCM receives an engine speed signal of about 400 RPM, it considers engine to be running and applies 5 volts to the EI module on the by-pass wire. This causes EI module to switch timing control over to the variable timing control circuit in the PCM. On some models, this by-pass wire contains a connector located between the 4-wire connector and the PCM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit sets a related trouble code in PCM memory. The engine runs at base timing plus a small amount of advance built into the HEI module.

• EST 

When 5 volts is present on by-pass circuit and EI module has turned control of engine timing over to PCM, PCM advances or retards spark on this circuit based on calculations involving reference signal and other sensor input signals. If base timing is incorrectly set, entire advance curve will be incorrect.

• Ground 

This is the reference ground circuit. It is grounded at distributor and PCM, ensuring there is no voltage drop in the EST circuit which could affect ignition operation.