W/O Sealed Module Connector

Fig. 1 Typical HEI/EST distributor. Models w/integral coil:

Fig. 2 Typical HEI/EST distributor. Models w/remote coil:

The HEI/EST is used on models with Computer Command Control (C-3) or Electronic Fuel Injection systems. The system utilizes a distributor similar to the unit used with conventional HEI systems that has been modified to allow spark advance to be controlled by the fuel control system Electronic Control Module (ECM). Modifications to the distributor include a revised ignition module, with additional terminals to receive signals from the ECM, and the elimination of the conventional centrifugal and vacuum advance mechanisms. In addition, some models use a Hall effect switch mounted above the pickup coil in the distributor to provide a reference pulse to the ECM. The ignition coil on HEI/EST systems is mounted either integral with the distributor, Fig. 1, or remotely mounted, Fig. 2, depending upon engine and application.
Primary current switching in the HEI/EST system is performed by the ignition module based on reference pulses from the pickup coil as in conventional HEI systems. However, all spark timing changes in the HEI/EST system are performed electronically by the Electronic Control Module (ECM). The ECM monitors information from various engine and vehicle sensors, determines the correct spark timing and signals the distributor to change timing as necessary. An EST bypass circuit is incorporated into the ignition module to allow ignition in case of ECM failure and to allow base timing adjustment.
On some HEI-EST systems, Electronic Spark Control (ESC) is used to retard ignition timing when detonation occurs. The ESC system consists of a knock sensor, distributor module and controller. The knock sensor is an accelerometer or magneto-strictive device, mounted on the engine block. It detects the presence and intensity of detonation by vibration characteristics of the engine. The sensor's output is an electrical signal which is sent to the controller. The controller is a hard-wired signal processor and amplifier which operates from 6 to 16 volts. The ESC controller processes the sensor signal into a command signal to the distributor to adjust spark timing. This is a continuous process monitoring and controlling detonation.
When detonation is detected the spark advance command is delayed, providing the level of retard required. The spark is retarded for 20 seconds, then the spark control returns to EST. The amount of retard is determined by the controller based on the severity of detonation. A failure of the sensor would allow no retard, while controller failure would be indicated by no ignition, no retard or full retard.