GM Full-Size Trucks 1970-1979 Repair Guide

Diagnosis and Testing


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
Fuel Poor Economy
Engine misses under load or while accelerating

If you suspect a problem in the 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.

Since the only change between electronic and conventional ignition systems is in the distributor component area, it is imperative to check the secondary ignition circuit first. 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 system, perform a simple spark test. Remove one of the plug wires and insert some sort of extension in the plug socket. An old spark plug with the ground electrode removed makes a good extension. Hold the wire and extension about 1 / 4 in. away from the block and 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 done. 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.


If the engine won't start, perform a spark test as described earlier. If no spark occurs, check for the presence of normal battery voltage at 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 the probe end to the BAT terminal at the distributor. If the light comes on, you have voltage to 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 chart for the correct plug wire resistance.
  3. If the plug wires are OK, remove the cap assembly, and check for moisture, cracks, chips, or carbon tracks, or any other high voltage leaks or failures. Replace the cap if you find any defects. 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.


See Figures 1, 2, 3 and 4

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Fig. Fig. 1: View of the "coil in cap" HEI distributor

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Fig. Fig. 2: Ignition coil terminal locations

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Fig. Fig. 3: Checking coil resistance. Ohmmeter 1 shows primary test. Ohmmeter 2 shows secondary test.

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Fig. Fig. 4: Testing the pick-up coil

If the trouble has been narrowed down to the units within the distributor, the following tests can help pinpoint the defective component. An ohmmeter with both high and low ranges should be used. These tests are made with the cap assembly removed and the battery wire disconnected.

  1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be less than one ohm (zero or nearly zero).
  3. To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Then connect the ohmmeter between the ground terminal and the rotor button. The resistance in both cases should be between 6,000 and 30,000 ohms.
  5. Replace the coil only if the readings in step one and two are infinite.

These resistance checks will not disclose shorted coil windings. This condition can be detected only with scope analysis or a suitably designed coil tester. If these instruments are unavailable, replace the coil with a known good coil as a final coil test.

  1. To test the pick-up coil, first disconnect the white and green module leads. Set the ohmmeter on the high scale and connect it between a ground and either the white or green lead. Any resistance measurement less than infinity requires replacement of the pick-up coil.
  3. Pick-up coil continuity is tested by connecting the ohmmeter (on low range) between the white and green leads. Normal resistance is between 650 and 850 ohms, or 500 and 1500 ohms on 1977 and later models. Move the vacuum advance arm while performing this test. This will detect any break in coil continuity. Such a condition can cause intermittent misfiring. Replace the pick-up coil if the reading is outside the specified limits.
  5. If no defects have been found at this time, and you still have a problem, then the module will have to be checked. If you do not have access to a module tester, the only possible alternative is a substitution test. If the module fails the substitution test, replace it.