Ignition Timing Advance

At engine speeds less than 400 RPM, the ignition control 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 control module), engine coolant temperature sensor, intake air temperature sensor, throttle position sensor, knock sensor, vehicle speed sensor, and the MAF or MAP sensor.

The PROM 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 information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. To check ignition system operation, see BASIC TESTING or I - SYSTEM & COMPONENT TESTING article in the ENGINE PERFORMANCE section below.

SYSTEM/COMPONENT TESTS - 4.3L (Caprice Only)
SYSTEM/COMPONENT TESTS - 5.7L (All Models)

All models use a an Opti-Spark system. See IGNITION SYSTEM.

By-Pass

When an engine speed signal of approximately 400 RPM is received by the PCM, PCM considers engine to be running and applies 5 volts to the ignition control module on the by-pass wire. This causes ignition control module to switch timing control over to the variable timing control circuit in the PCM. An open or grounded by-pass circuit will set a related diagnostic trouble code in PCM memory. The engine will run at base timing plus a small amount of advance.

When 5 volts is present on the by-pass circuit and ignition control 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 pick-up coil (HEI distributor), PM generator (DIS) is converted by the ignition control 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.