Ignition Timing Advance

At engine speeds less than 400 RPM, the ignition module 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 ECM takes over control of the ignition timing. On 3.8L, ECM also changes fuel injection timing to a sequential mode when in EST mode.

ECM controls ignition timing based upon input signals from the 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 the MAF or MAP sensor.

The PROM/MEM-CAL portion of the ECM has a programmed spark advance curve based on engine speed. Spark timing is calculated by ECM whenever an ignition pulse is present. Spark advance is controlled only when engine is running (not during cranking). Input signal values are used by ECM to modify PROM/MEM-CAL information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see BASIC TESTING or refer to appropriate I - SYS/COMP TESTS article in this section.

Although several types of ignition systems are used, all ignition systems (except Corvette Opti-Spark) use the same 4 basic ignition circuits. Models may use a conventional HEI/EST distributor system, an Opti-Spark system (Corvette) or one of 3 types of distributorless ignition systems. The C3I uses the same ignition module-to-ECM circuits, with the addition of fuel control and fuel sync (camshaft) signals on 3.8L, that IDI, DIS and distributor type ignition systems use. For description of fuel control and sync signals, see IGNITION SYSTEM.

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

• By-Pass  When an engine speed signal of approximately 400 RPM is received by the ECM, ECM 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 will set a related trouble code in ECM memory. The engine will run at base timing plus a small amount of advance built into the HEI module.
• EST  When 5 volts is present on the by-pass circuit and ignition module has turned control of engine timing over to ECM, the ECM advances or retards spark on this circuit based on calculations involving the 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 no voltage drop occurs in the EST circuit which could affect ignition operation.
• Reference (RPM)  Alternating current signals from the pick-up coil (HEI distributor), PM generator (DIS and IDI) or Hall Effect sensors C3I and 4.9L) are converted by the ignition module converter to digital signals for use by the ECM. This supplies RPM data and crankshaft position reference to the ECM. Because the signal on this circuit is used as an injector trigger reference, engine will not run if circuit is open or grounded.