Ignition Timing Advance
At engine speeds less than 400 RPM, ignition 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), ECM controls ignition timing.
ECM controls ignition timing based upon input signals from engine RPM reference line (ignition module), coolant temperature sensor, manifold air temperature sensor, throttle position sensor, knock sensor, vehicle speed sensor, gear position switch and MAP or MAF sensor.
The PROM portion of the ECM has a programmed spark advance curve based on engine speed. ECM calculates spark timing whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). ECM 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, see BASIC TESTING or I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.
- Reference (RPM) On all engines, ignition 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 ECM receives an engine speed signal of approximately 400 RPM, it considers engine to be running and applies 5 volts to the ignition module on the by-pass wire. This causes ignition module to switch timing control over to the variable timing control circuit in the ECM. On some models, this by- pass wire contains a connector located between the 4-wire connector and the ECM. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit sets a related trouble code in ECM 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 ignition module has turned control of engine timing over to ECM, ECM 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 ECM, ensuring there is no voltage drop in the EST circuit which could affect ignition operation.