GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Troubleshooting the HEI System


See Figure 1

Diagnosis and testing procedures in this section should be used in conjunction with those in Chapter 4 (Emission Controls) and Section 5 (Fuel System) of this manual. This will enable you to diagnose problems involving all components controlled by the Electronic Control Module (ECM).

An accurate diagnosis is the first step to problem solution and repair. Most of the troubleshooting covered here can be performed using a voltmeter/ohmmeter and a spark tester. Although an old spark plug may be used as a spark tester, the use of an inexpensive tester tool such as ST-125 is highly recommended. The tester tool is similar in appearance to a spark plug, with a spring clip to attach to ground. Because of the high voltage of the HEI system, the tester should be used instead of an old spark plug unless absolutely necessary. If a plug must be used, do not touch the plug or wiring directly in order to minimize the chance of injury from electric shock.

The symptoms of a defective component within the HEI system are exactly the same as those you would encounter in a conventional system. Some of these symptoms are:

Hard or no starting
Rough idle
Poor fuel economy
Engine misses under load or while accelerating

If you suspect a problem in your ignition system, there are certain preliminary checks which you should carry out before you begin to check the electronic portions of the system. First, it is extremely important to make sure the vehicle battery is in a good state of charge. A defective or poorly charged battery will cause the various components of the ignition system to read incorrectly when they are being tested. Second, make sure all wiring connections are clean and tight, not only at the battery, but also at the distributor cap, ignition coil, and at the electronic control module.

Click image to see an enlarged view

Fig. Fig. 1: HEI system schematic-except 4.3L engine

The quickest and easiest test of the ignition system is to check the secondary ignition circuit first (check for spark). If the secondary circuit checks out properly, then the engine condition is probably not the fault of the ignition system. To check the secondary ignition circuit, perform a simple spark test. Remove one of the plug wires and insert a spark tester or some sort of extension in the plug socket. An old spark plug with the ground electrode removed makes a good extension. If using an old plug, hold the wire with an insulated tool so the extension is positioned about 1 / 4 in. away from the block, then crank the engine. If a normal spark occurs, then the problem is most likely not in the ignition system. Check for fuel system problems, or fouled spark plugs.

If, however, there is no spark or a weak spark, then further ignition system testing will have to be performed. Troubleshooting techniques fall into two categories, depending on the nature of the problem. The categories are (1) Engine cranks, but won't start or (2) Engine runs, but runs rough or cuts out. To begin with, let's consider the first case.


If the engine won't start, perform a spark test as described earlier. This will narrow the problem area down considerably. If no spark occurs, check for the presence of normal battery voltage of the battery (BAT) terminal in the distributor cap. The ignition switch must be in the ON position for this test. Either a voltmeter or a test light may be used for this test. Connect the test light wire to ground and probe end to the BAT terminal at the distributor. If the light comes on, you have voltage on the distributor. If the light fails to come on, this indicates an open circuit in the ignition primary wiring leading to the distributor. In this case, you will have to check wiring continuity back to the ignition switch using a test light. If there is battery voltage at the BAT terminal, but no spark at the plugs, then the problem lies within the distributor assembly. Go on to the distributor components test section.


  1. Make sure the plug wires are in good shape first. There should be no obvious cracks or breaks. You can check the plug wires with an ohmmeter, but do not pierce the wires with a probe. Check the plug wire for a proper resistance using the values listed earlier in this section under spark plug wiring.
  3. If the plug wires are OK, remove the cap assembly and check for moisture, cracks, chips, carbon tracks, or any other high voltage leaks or failures. Replace the cap if any defects are found. Make sure the timer wheel rotates when the engine is cranked. If everything is all right so far, go on to the distributor components test section following.