GM Firebird 1982-1992 Repair Guide



The symptoms of a defective component within the HEI or C 3 I 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 (if so equipped), ignition coil, camshaft and/or crankshaft sensors (if so equipped) and at the control module.

Since the only difference between these ignition systems lies before the spark plug wiring, it is important to check the secondary ignition circuit first. If the secondary circuit checks out properly (there is spark), 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 using an insulated tool (NOT BY HAND) about 1 / 4 in. (6mm) 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 various categories, depending on the nature of the problem and the system being tested. Categories will include symptoms such as the engine cranks, but will not start or the engine runs rough.


When testing the HEI system, there are 2 major categories of problems, (1) Engine cranks, but will not start or (2) Engine runs, but runs rough or cuts out.

If the engine will not 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 at the battery ( BAT ) terminal on the ignition coil. The ignition switch must be in the ON position for this test. Either a voltmeter or a test light wire may be used for this test. Connect the test light wire to ground and the probe end to the BAT terminal at the coil. If the light comes on, you have voltage to the distributor and/or spark plug wires. 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 probably lies within the distributor assembly.

If, on the other hand, the engine starts, 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. Check the chart for the correct plug wire resistance. If the plug wires are OK, remove the cap assembly and check for moisture, cracks, chips, or carbon tracks, or any other high voltage leads or failures. Replace the cap if any defects are found. Make sure the timer wheel rotates when the engine is cranked.

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.

Ignition Coil

See Figure 1

  1. Disconnect the coil wires and set the ohmmeter on the high scale.
  3. Connect the ohmmeter to the ignition coil as illustrated in Step 1 of the accompanying figure.
  5. The ohmmeter should read near infinite or very high.

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Fig. Fig. 1: Testing external ignition coil

  1. Next, set the ohmmeter to the low scale and connect test leads as illustrated in Step 2 of the accompanying figure.
  3. The reading should be very low or zero.
  5. Now set the ohmmeter on the high scale and connect test leads as illustrated in Step 3 of the accompanying figure. The ohmmeter should not read infinity.
  7. If any results of the 3 tests listed above do not agree with the desired readings, replace the ignition coil.


See Figure 2

  1. Connect an ohmmeter between the TACH and BAT terminals on the ignition coil. The primary coil resistance should be less than ohms;.
  3. To check the coil secondary resistance, connect an ohmmeter between the high tension terminal and the BAT terminal. Note the reading. Connect the ohmmeter between the high tension terminal and the TACH terminal. Note the reading. The resistance in both cases should be 6,000-30,000-. Be sure to test between the high tension terminal and both the BAT and TACH terminals.
  5. Replace the coil only if the readings in Step 1 and Step 2 are infinite.

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.

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Fig. Fig. 2: Testing internally mounted ignition coil

Pickup Coil

See Figure 3

  1. To test the pickup 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 pickup coil.

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Fig. Fig. 3: Testing Pickup coil-Test 1 should read infinite at all times. Test 2 should read a steady value of 500-1500 ohms.

  1. Pickup coil continuity is tested using an ohmmeter (on low range) between the white and green leads. Normal resistance is 500-1,500-. If a vacuum unit is used, 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 pickup coil if the reading is outside the specified limits.
  3. 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.


See Figures 4, 5 and 6

Troubleshooting the C 3 I Ignition System requires a logical and systematic approach. The accompanying charts are designed to help diagnose a no-spark condition or erratic spark plug firing, either at idle or under load. Refer to the system wiring chart before making any tester connections, and follow the steps in their prescribed order.

When handling secondary spark plug leads with the engine running or starting, insulated pliers must be used and care exercised to prevent a possible electrical shock.

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Fig. Fig. 4: Ignition system wiring-3.8L turbo engine

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Fig. Fig. 5: Engine misses at idle-C3I Ignition System

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Fig. Fig. 6: Engine misses under load-C3I Ignition System