Isuzu Cars and Trucks 1981-1991

High Energy Ignition (HEI) System


The HEI system operates in basically the same manner as the conventional ignition system, with the exception of the type of switching device used. A toothed iron timer core is mounted on the distributor shaft which rotates inside of an electronic pole piece. The pole piece has internal teeth (corresponding to those on the timer core) which contains a permanent magnet and pick-up coil (not to be confused with the ignition coil). The pole piece senses the magnetic field of the timer core teeth and sends a signal to the ignition module which electronically controls the primary coil voltage. The ignition coil operates in basically the same manner as a conventional ignition coil (though the ignition coils DO NOT interchange).

The HEI systems uses a capacitor which is primarily used for radio interference purposes. Do not confuse the capacitor as a tune-up component.


Before troubleshooting the systems, 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. Timing lights which connect between the spark plug and the spark plug wire occasionally give false readings.

Some engines incorporate a magnetic timing probe terminal (at the damper pulley) for use of special electronic timing equipment. Refer to the manufacturer's instructions when using this equipment.

Spark Plug Wires

The plug wires are of a different construction than conventional wires. When replacing them, make sure to use the correct wires, since conventional wires won't carry the higher 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. While some tachometers may give a reading, this does not necessarily mean the reading is correct. In addition, some tachometers connect differently than others. If you can't figure out whether or not your tachometer will work on your vehicle, check with the tachometer manufacturer.

System Testers

Instruments designed specifically for testing the electronic ignition system are available from several tool manufacturers. Some of these will even test the module.