Ignition System: Description and Operation

NOTE: For further diagnosis, refer to Computers and Control Systems.

Fig. 1 HEI-EST Distributor with internal coil. Models with 6 & 8 cylinder engine:

Fig. 2 HEI-EST Distributor:

COMPONENTS
The HEI-EST system consists of an electronic HEI distributor--Figs 1 and 2--with a distributor cap mounted ignition coil--except on on 4 cylinder models which have an externally mounted coil.

SPARK TIMING
All spark timing is controlled electronically by the ECM.
- The distributor is not equipped with vacuum or mechanical advance.
- The ECM determines the correct spark timing using information from various engine sensors, and adjusts spark timing as necessary.

A secondary spark advance system is incorporated into the system to signal the ignition module in case of ECM failure.

Electronic Spark Control (ESC)
Purpose - On some HEI-EST systems Electronic Spark Control (ESC) retards spark advance when detonation occurs.

Operation - When detonation occurs the ESC--detonation--sensor signals the ESC controller which delays spark by an amount corresponding to the severity of detonation.

Components - The ESC system consists of: a detonation sensor, an ESC distributor and a controller.

The ESC Distributor - Is an HEI-EST distributor equipped with an Electronic Spark Control (ESC) module which responds to ESC controller commands.

ESC Sensor - An accelerometer or magneto restrictive device--mounted on the engine block--detects the presence and intensity of detonation--vibration characteristics of the engine, and sends an electrical signal to the ECM.
- Sensor Failure - A failure of the sensor would allow no retard.

ESC Controller - A hard-wired signal processor and amplifier--which operates at between 6 and 16 volts--monitors ESC sensor input continuously and retards timing accordingly.
- Controller Failure - Indicated by no ignition, no retard or full retard.

Diagnostic Connector--If Equipped
Location - Mounted in the engine compartment on the left side front fender skirt--on some models.

Tachometer Connection - A tachometer may be connected between terminals 6 and G of the diagnostic connector.

NOTE: Since this is a full 12 volt system, no resistance wire is used.

NOTE: A tachometer connection is incorporated in the wiring connector on the side of the distributor on 6 and 8 cylinder models, or next to the coil battery terminal on 4 cylinder models.

CAUTION: Never connect a wire directly between the Tach terminal of the distributor connector and ground as this will damage module circuitry.

NOTE: When using a timing light to adjust ignition timing, the connection should be made at the No 1 spark plug--except when instructed otherwise by procedure.

CAUTION: Forcing foreign objects through the boot at the No 1 spark plug terminal will damage the boot and cause engine misfire.