GM Caprice 1990-1993 Repair Guide

Description and Operation


See Figures 1 and 2

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Fig. Fig. 1: HEI/(EST) distributor with internallymounted ignition coil

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Fig. Fig. 2: HEI/(EST) distributor with externallymounted ignition coil

The Caprice uses a High Energy Ignition (HEI) system with Electronic Spark Timing (EST) and is completely self-contained unit - all parts are contained within the distributor except the ignition coil which is externally mounted on all engines except the 5.0L (VIN Y) engine.

The distributor contains the electronic module, and the magnetic triggering device. The magnetic pick-up assembly contains a permanent magnet, a pole piece with internal teeth, and a pickup coil (not to be confused with the ignition coil).

All spark timing changes are done electronically by the Electronic Control Module (ECM) which monitors information from various engine sensors, computes the desired spark timing and then signals the distributor to change the timing accordingly. No vacuum or mechanical advance systems 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 control module. This switching transistor performs the same function the points had in a conventional ignition system; it simply turns the coil's primary current on and off at the correct time. Essentially, electronic and conventional ignition systems operate on the same principle.

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.

In essence, 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. 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.

All engines are also equipped with Electronic Spark Control (ESC). The ESC system is used in conjunction with the EST system to reduce spark knock by retarding the ignition timing. A knock sensor, which is installed in the cylinder block, sends a signal to the ESC control module and then in turn to the ECM, which then adjusts the timing accordingly.


Before going on to troubleshooting, it might be a good idea to take note of the following precautions:

Timing Light Use

Inductive pick-up timing lights are the best kind to use with HEI. Timing lights which connect between the spark plug and the spark plug wire occasionally (not always) give false readings.

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 wires won't correctly handle the high secondary voltage. Also, handle the wires carefully to avoid cracking or splitting. Never pierce the HEI ignition wires.

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. Dwell readings, of course, have no significance at all with the HEI system.

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, the tests given in the following section will require only an ohmmeter and a voltmeter.