GM Corvette 1984-1996 Repair Guide

General Information


See Figure 1

The General Motors/Delco-Remy High Energy Ignition (HEI) system is a breakerless, pulse-triggered, transistor controlled, inductive discharge ignition system. It is used on 1984-91 vehicles equipped with the 5.7L (VIN 8) engine. The entire HEI system (except for the ignition coil) is contained within the distributor cap.

The distributor contains the electronic ignition module, and the magnetic pick-up assembly which contains a permanent magnet, a pole piece with internal teeth, and a pick-up coil (not to be confused with the ignition coil). The ignition coil is mounted to the top of the distributor cap.

The HEI distributor is equipped to aid in spark timing changes which is necessary to maintain emissions, economy and performance. This is achieved by the Electronic Spark Timing (EST) control system. On these vehicles, timing changes are electronically regulated through the computer control module, which monitors information from the various engine sensors, computes the desired spark timing and signals the distributor to change the timing accordingly. With this distributor, no vacuum or centrifugal advances are used.

In the HEI system, as in other electronic ignition systems, the breaker points have been replaced with an electronic switch, a transistor, which is located within the ignition module. This switching transistor performs the same function the points did in a conventional ignition system; it simply turns coil primary current on and off at the correct time. Essentially, the electronic and conventional ignition systems operate on the same principal.

The module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse induced in the pick-up coil. When the teeth of the rotating timer align with the teeth of the pole piece, the induced voltage in the pick-up coil signals the electronic module to open the coil primary circuit. The primary current then decreases and a high voltage is induced in the ignition coil secondary windings which is then directed through the rotor and high voltage leads (spark plug wires) to fire the spark plugs.

Click image to see an enlarged view

Fig. Fig. 1: Exploded view of the HEI distributor, which is used on 1984-91 vehicles

In essence then, the pick-up coil module system simply replaces the conventional breaker points and condenser. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. The module automatically controls the dwell period, increasing it with increasing engine speed. The HEI system features a longer spark duration which is instrumental in firing lean and Exhaust Gas Recirculation (EGR) diluted fuel/air mixtures. Since dwell is automatically controlled, it cannot be adjusted. The module itself is non-adjustable and non-repairable and must be replaced if found defective.

These engines are also with Electronic Spark Control (ESC). A knock sensor is mounted in the engine block. It is connected to the ESC module which is mounted to the cowl in the engine compartment. In response to engine knock, the sensor sends a signal to the ESC module. The module will then signal the ECM which will retard the spark timing in the distributor.


Before proceeding with troubleshooting or HEI system service, please note the following precautions:

Timing Light Use

Inductive pick-up timing lights are the best kind to use with the HEI system. Timing lights which connect between the spark plug and the spark plug wire occasionally (not always) give false readings due to the high voltage of the HEI system which more easily leads to arcing.

Spark Plug Wires

The plug wires used with HEI systems are of a different construction than conventional wires. When replacing them, make sure you get the correct wires, since conventional point system wires won't carry the voltage. Also, handle them carefully to avoid cracking or splitting them and never pierce them.

Tachometer Use

Not all tachometers will operate or indicate correctly when used on a HEI system. While some tachometers may give a reading, this does not necessarily mean the reading is correct. In addition, some tachometers hook up differently from others. If you can't figure out whether or not your tachometer will work on your car, check with the tachometer manufacturer.

HEI System Testers

Instruments designed specifically for testing HEI systems are available from several tool manufacturers. Some of these will even test the module itself. However, most of the tests given in the following require only an ohmmeter and a voltmeter.