GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Diagnosis and Testing


See Figure 1

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.

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 this section will require only an ohmmeter and a voltmeter.

The HEI ignition system can generate voltage of 30,000-50,000 volts. When testing the system, DO NOT hold a spark plug wire while the engine is running or cranking. Personal injury and or damage to the ignition system may result if this caution is not followed.

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.

If the engine won't start, perform this test. This will narrow the problem area down considerably.

  1. Remove one of the plug wires and insert a HEI spark tester tool in the plug socket.
  3. Ground the spark tester to the block and crank the engine. DO NOT touch the spark plug wire while the engine is cranking.
  5. The spark should be crisp and bright blue in color. If a normal spark occurs, try each spark plug wire until a no spark condition or a weak orange color spark is found. If all sparks are good, the problem is probably not in the ignition system. Check for fuel system problems, or fouled spark plugs.

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.

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Fig. Fig. 1: Ignition coil terminal location

If the engine runs, but runs roughly or cuts out, 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.

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.


See Figures 2 and 3

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. If a tachometer is connected to the TACH terminal, disconnect it before making these tests.

  1. Connect an ohmmeter between the TACH and BAT terminals in the distributor cap. The primary coil resistance should be 0&omega or nearly 0&omega . If not replace the coil.
  3. To check the coil secondary resistance, connect an ohmmeter between the rotor button and the BAT terminal. Note the reading. Connect the ohmmeter between the rotor button and the TACH terminal. Note the reading. The resistance in both cases should be between 6,000 and 30,000&omega. Be sure to test between the rotor button and both the BAT and TACH terminals.
  5. Replace the coil only if the readings in Step 1 and Step 2 are infinite resistance or out of specification.

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

These resistance checks will not disclose shorted coil windings. This condition can only be detected 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 infinite 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&omega, or 500 and 1,500&omega on 1977 and later models. Move the vacuum advance arm while performing this test (early models). 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.

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Fig. Fig. 3: Pick-up coil testing