GM Firebird 1967-1981 Repair Guide

General Information


Before suspecting the C-4 or CCC system, or any of its components as being faulty, check the ignition system (distributor, timing, spark plugs and wires). Check the engine compression, the air cleaner and any of the emission control components that are not controlled by the ECM. Also check the intake manifold, the vacuum hoses and hose connectors for any leaks. Check the carburetor mounting bolts for tightness.

The following symptoms could indicate a possible problem area with the C-4 or CCC systems:

  1. Detonation;
  3. Stalling or rough idling when the engine is cold;
  5. Stalling or rough idling when the engine is hot;
  7. Missing;
  9. Hesitation;
  11. Surging;
  13. Poor gasoline mileage;
  15. Sluggish or spongy performance;
  17. Hard starting when engine is cold;
  19. Hard starting when the engine is hot;
  21. Objectionable exhaust odors;
  23. Engine cuts out;
  25. Improper idle speed (CCC only).

As a bulb and system check, the "Check Engine" light will come on when the ignition switch is turned to the ON position but the engine is not started.

The "Check Engine" light will also produce the trouble code(s) by a series of flashes which translate as follows: when the diagnostic test lead (C-4) or terminal (CCC) under the instrument panel is grounded, with the ignition in the ON position and the engine not running, the "Check Engine" light will flash once, pause, and then flash twice in rapid succession. This is a Code 12, which indicates that the diagnostic system is working. After a long pause, the Code 12 will repeat itself two more times. This whole cycle will then repeat itself until the engine is started or the ignition switch is turned OFF.

When the engine is started, the "Check Engine" light will remain on for a few seconds and then turn OFF. If the "Check Engine" light remains ON, the self-diagnostic system has detected a problem. If the test lead (C-4) or test terminal (CCC) is then grounded, the trouble code will flash (3) three times. If more than one problem is found to be in existence, each trouble code will flash (3) three times, then change to the next one. Trouble codes will flash in numerical order (lowest code number to highest). The trouble code series will repeat themselves for as long as the test leads or terminals remain grounded.

A trouble code indicates a problem with a given circuit. For example, trouble code 14 indicates a problem in the cooling sensor circuit. This includes the coolant sensor, its electrical harness and the Electronic Control Module (ECM).

Since the self-diagnostic system cannot diagnose every possible fault in the system, the absence of a trouble code does not necessarily mean that the system is trouble-free. To determine whether or not a problem with the system exists that does not activate a trouble code, a system performance check must be made. This job should be left to a qualified service technician.

In the case of an intermittent fault in the system, the "Check Engine" light will go out when the fault goes away, but the trouble code will remain in the memory of the ECM. Therefore, if a trouble code can be obtained even though the "Check Engine" light is not ON, it must still be evaluated. It must be determined if the fault is intermittent or if the engine must be operating under certain conditions (acceleration, deceleration, etc.) before the "Check Engine" light will come on. In some cases, certain trouble codes will not be recorded in the ECM until the engine has been operated at part throttle for at least 5 to 18 minutes.

On the C-4 system, the ECM erases all trouble codes every time that the ignition is turned OFF. In the case of intermittent faults, a long term memory is desirable. This can be produced by connecting the orange connector/lead from terminal "S" of the ECM directly to the battery (or to a `hot' fuse panel terminal). This terminal must always be disconnected immediately after diagnosis as it puts an undue strain on the battery.

On the CCC system, a trouble code will be stored until the terminal `R' at the ECM has been disconnected from the battery for at least 10 seconds.


See Figures 1, 2, 3 and 4

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Fig. Fig. 1: Terminal identification for the Idle Speed Control (ISC) motor, which is mounted on the carburetor

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Fig. Fig. 2: Oxygen sensor location on the left-hand exhaust manifold 305 engine, other 8-cylinder engines similar

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Fig. Fig. 3: Trouble Code Identification Chart

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Fig. Fig. 4: Trouble Code Identification Chart (cont.)

On the C-4 system, activate the trouble code by grounding the trouble code test lead. Use the illustrations to help you locate the test lead under the instrument panel (usually a white and black wire with a green connector). Run a jumper wire from the lead to a suitable ground.

On the CCC system, locate the test terminal under the instrument panel. Use a jumper wire and ground only the lead.

Ground the test lead/terminal according to the instructions given previously in the "Basic Troubleshooting" section.