Ignition Timing Advance: Notes

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), control module controls ignition timing.

Control module 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 control module has a programmed spark advance curve based on engine speed. Control module calculates spark timing whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Control module 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 SYSTEM/COMPONENT TESTING article below.

• BASIC TESTING - 5.0L
• SYSTEM/COMPONENT TESTS - 5.7L
• 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 control module 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 control module. On some models, this by-pass wire contains a connector located between the 4-wire connector and the control module. This is disconnected when adjusting base timing. On all models, an open or grounded by-pass circuit sets a related trouble code in control module 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 control module, control module 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 control module, ensuring there is no voltage drop in the EST circuit which could affect ignition operation.