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 PCM takes over control of the ignition timing.

PCM 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 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/MEM-CAL information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see appropriate BASIC TESTING or I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section below.

• SYSTEM/COMPONENT TESTS - 4.3L
• SYSTEM/COMPONENT TESTS - 5.7L

Although several types of ignition systems are used, all ignition systems (except 4.3L and 5.7L Opti-Spark) use the same 4 basic ignition circuits. Some models may use a conventional HEI/EST distributor system, an Opti-Spark system (4.3L and 5.7L) or one of 3 types of distributorless ignition systems. The C(3)I uses the same ignition module-to-PCM 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 PCM by 4 EST circuits. Circuits perform the following functions:

• EST

When 5 volts is present on the by-pass circuit and ignition module has turned control of engine timing over to PCM, the PCM 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 PCM, ensuring no voltage drop occurs in the EST circuit which could affect ignition operation.

• Reference (RPM)

Alternating current signals from the PM generator (DIS) are converted by the ignition module converter to digital signals for use by PCM. This supplies RPM data and crankshaft position reference to PCM. Because signal on this circuit is used as an injector trigger reference, engine will not run if circuit is open or grounded.