Ignition System: Description and Operation

PURPOSE
The distributor ignition system controls fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the Electronic Control Module (ECM) controls distributor spark advance (timing) with the Ignition Control (IC) system.

OPERATION AND CONSTRUCTION
The HEI-EST system consists of an electronic distributor, an externally mounted coil, a Knock Sensor (KS), a Knock Sensor (KS) Module, and an Electronic Control Module (ECM).
To properly control ignition/combustion timing the ECM needs to know:
- Crankshaft position.
- Engine speed (RPM).
- Engine load (manifold pressure or vacuum.
- Atmospheric (barometric) pressure.
- Engine coolant temperature.
- Exhaust Gas Recirculation (EGR).

The distributor has four wires that connect the IC module to the ECM. These circuits perform the following functions:

- Circuit 430 "DISTRIBUTOR REFERENCE HIGH". Provides the ECM with rpm and crankshaft position information.

- Circuit 453 "REFERENCE GROUND LOW". This circuit may be grounded in the distributor and makes sure the ground circuit between the IC module and ECM has no voltage drop which could affect performance. If it is open, it may cause poor performance.

- Circuit 424 "BYPASS". At about 400 rpm the ECM applies 5.0 volts to this circuit to switch spark timing control from the IC module to the ECM. An open or grounded bypass circuit will set a Code 42 and the engine will run at base timing, plus a small amount of advance controlled by the IC module.

- Circuit 423 "IC". This circuit triggers the IC module. The ECM does not know what the actual timing is, but it does know when it gets the reference signal. It then advances or retards the spark form that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect. An open or ground, in the EST circuit, will set a Code 42 and cause the engine to run on the HEI module timing. This will cause poor performance and poor fuel economy. A ground may set a Code 42, and the engine may not run.

A secondary spark advance system is incorporated into the system to signal the ignition module in case of ECM failure. The HEI-EST system does not use vacuum or mechanical advance.
On some HEI-EST systems, Electronic Spark Control (ESC) is used to retard spark advance when detonation occurs. The spark is retarded for 20 seconds, then the spark control returns to EST.
The ESC system consists of three basic components: sensor, distributor and controller. The ESC sensor is an accelerometer or magneto-strictive device, mounted on the engine block. It detects presence, or absence, and intensity of detonation by vibration characteristics of the engine. The sensor's output is an electrical signal which is sent to the controller. A failure of the sensor would allow no retard.
The distributor is an HEI-EST unit with an electronic module modified so that it can respond to ESC controller signal. The command is delayed when detonation is detected, providing the level of retard required. The amount of retard is determined by the severity of detonation.
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. The controller is a hard-wired signal processor and amplifier which operates from 6 to 16 volts. Controller failure would be indicated by no ignition, no retard or full retard.
Since this is a full 12 volt system, no resistance wire is used. Also, a diagnostic connector is used on some models. This connector is located in the engine compartment on the left side front fender skirt. On vehicles equipped with this connector, a tachometer may be connected between terminals 6 and G.
A tachometer connection is incorporated in the wiring connector on the side of the distributor.

NOTE
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. When using a timing light to adjust ignition timing, the connection should be made at the No. 1 spark plug. Forcing foreign objects through the boot at the No. 1 spark plug terminal will damage the boot and cause engine misfire.