Ignition System: Description and Operation

NOTE: For further Electronic Spark Timing (EST) and Electronic Spark Control (ESC) diagnosis, refer to COMPUTERS AND CONTROL SYSTEMS/ELECTRONIC SPARK CONTROL (ESC) DIAGNOSIS SYSTEM.

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

Fig. 2 HEI-EST Distributor:

APPLICATIONS
The HEI System with Electronic Spark Timing (HEI-EST) is used on GM models except the following:

- Chevette and 1000 W/4 terminal ignition module.
- 1984-88 models W/C3I (Computer-Controlled Coil Ignition).
- 1985-88 Nova W/Solid State Ignition system.
- 1987-88 models with Direct Ignition System (DIS).
- 1988 models with Integrated Direct Ignition System (IDI).

IDENTIFICATION
HEI-EST equipped vehicles can be identified by the addition of a four-wire distributor connector and the lack of a distributor vacuum advance chamber.

COMPONENTS
The HEI-EST system consists of an electronic distributor, Figs. 1 and 2, with the ignition coil mounted on the distributor cap on 6 and 8 cylinder models, or an externally mounted coil on 4 cylinder models.

SPARK TIMING
Spark timing in the HEI-EST system is controlled electronically by the ECM.

NOTE: The HEI-EST system does not use vacuum or mechanical advance.

- The ECM monitors information from various engine sensors, determines the correct spark timing and signals the distributor to change 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) is used to retard spark advance when detonation occurs.

Operation - When detonation occurs the ESC sensor signals the ESC controller, and the spark command is delayed by an amount corresponding to the severity of detonation.

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

ESC Distributor - HEI-EST distributor with an electronic module--modified to respond to the ESC controller signal.

ESC Sensor - An accelerometer or magneto-strictive 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 the electronic circuitry of the module.

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.