GM Cadillac 1967-1989 Repair Guide

Computer Command Control System


See Figures 1, 2, 3 and 4

The Computer Command Control System, installed on all 1981 and later carbureted cars, is basically a modified version of the C-4 system. Its main advantage over its predecessor is that it can monitor and control a larger number of interrelated emission control systems.

For information regarding computer control/troubleshooting of the Electronic and Digital fuel injected vehicles, please refer to Fuel System in this repair guide.

This new system can monitor up to 15 various engine/vehicle operating conditions and then use this information to control as many as 9 engine related systems. The System is thereby making constant adjustments to maintain good vehicle performance under all normal driving conditions while at the same time allowing the catalytic converter to effectively control the emissions of NOx, HC and CO.

In addition, the System has a built in diagnostic system, that recognizes and identifies possible operational problems and alerts the driver through a Check Engine light in the instrument panel. The light will remain ON until the problem is corrected. The System also has built in back-up systems that in most cases of an operational problem will allow for the continued operation of the vehicle in a near normal manner until the repairs can be made.

The CCC system has some components in common with the C-4 system, although they are not interchangeable. These components include the Electronic Control Module (ECM), which, as previously stated, controls many more functions than does its predecessor, an oxygen sensor system, an electronically controlled variable-mixture carburetor, a three-way catalytic converter, throttle position and coolant sensors, a Barometric Pressure Sensor (BARO), a Manifold Absolute Pressure Sensor (MAP) and a Check Engine light in the instrument panel.

Components unique to the CCC system include the Air Injection Reaction (AIR) management system, a charcoal canister purge solenoid, EGR valve controls, a vehicle speed sensor (in the instrument panel), a transmission converter clutch solenoid (only on models with automatic transmission), idle speed control and Electronic Spark Timing (EST).

The ECM, in addition to monitoring sensors and sending out a control signal to the carburetor, also controls the following components or sub-systems: charcoal canister purge control, the AIR system, idle speed, automatic transmission converter lock-up, distributor ignition timing, the EGR valve, and the air conditioner converter clutch.

The EGR valve control solenoid is activated by the ECM in a fashion similar to that of the charcoal canister purge solenoid described earlier in this section. When the engine is cold, the ECM energizes the solenoid, which blocks the vacuum signal to the EGR valve. When the engine is warm, the ECM de-energizes the solenoid and the vacuum signal is allowed to reach and then activate the EGR valve.

The Transmission Converter Clutch (TCC) lock is controlled by the ECM through an electrical solenoid in the automatic transmission. When the vehicle speed sensor in the dash signals the ECM that the car has attained the pre-determined speed, the ECM energizes the solenoid which then allows the torque converter to mechanically couple the engine to the transmission. When the brake pedal is pushed, or during deceleration or passing, etc., the ECM returns the transmission to fluid drive.

The idle speed control adjusts the idle speed to all particular engine load conditions and will lower the idle under no-load or low-load conditions in order to conserve fuel.

Not all engines use all systems. Control applications may differ.

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Fig. Fig. 1: Computer Command Control (CCC) system schematic

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Fig. Fig. 2: Common CCC component locations - V8 engines

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Fig. Fig. 3: Electronic Control Module (ECM) location - all models similar

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Fig. Fig. 4: Trouble Code Identification Chart


See Figures 5, 6 and 7

The following explains how to activate the Trouble Code signal light in the instrument cluster.

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.

For information regarding computer control/troubleshooting of the Electronic and Digital fuel injected vehicles, please refer to Fuel System in this repair guide.

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/codes 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 and 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 terminal remains 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 5-18 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 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.

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Fig. Fig. 5: C-4 system diagnostic test lead location - above ECM

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Fig. Fig. 6: Under dash test terminal location

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Fig. Fig. 7: CCC system diagnostic test terminal located underneath the left side of the instrument panel


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 (see illustration). 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.


The fuel flow through the carburetor idle main metering circuits is controlled by a mixture control (M/C) solenoid located in the carburetor. The M/C solenoid changes the air/fuel mixture to the engine by controlling the fuel flow through the carburetor. The ECM controls the solenoid by providing a ground. When the solenoid is energized, the fuel flow through the carburetor is reduced, providing a leaner mixture. When the ECM removes the ground, the solenoid is de-energized, increasing the fuel flow and providing a richer mixture. The M/C solenoid is energized and de-energized at a rate of 10 times per second.


See Figure 8

1980 and Later

The throttle position sensor is mounted in the carburetor body and is used to supply throttle position information to the ECM. The ECM memory stores an average of operating conditions with the ideal air/fuel ratios for each of those conditions. When the ECM receives a signal that indicates throttle position change, it immediately shifts to the last remembered set of operating conditions that resulted in an ideal air/fuel ratio control. The memory is continually being updated during normal operations.

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Fig. Fig. 8: Throttle position sensor, mounted on carburetor body


See Figure 9

The idle speed control does just what its name implies-it controls the idle. The ISC is used to maintain low engine speeds while at the same time preventing stalling due to engine load changes. The system consists of a motor assembly mounted on the carburetor which moves the throttle lever so as to open or close the throttle blades.

The whole operation is controlled by the ECM. The ECM monitors engine load to determine the proper idle speed. To prevent stalling, it monitors the air conditioning compressor switch, the transmission, the park/neutral switch and the ISC throttle switch. The ECM processes all this information and then uses it to control the ISC motor which in turn will vary the idle speed as necessary.

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Fig. Fig. 9: The idle Speed Control (ISC) motor is attached to the carburetor