GM Firebird 1967-1981 Repair Guide

Computer Controlled Catalytic Converter System (C-4)


See Figure 1

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Fig. Fig. 1: Schematic of the Computer Controlled Catalytic Converter (C-4) system

The C-4 System, installed on most 1980 cars sold in California, is an electronically controlled exhaust emissions system. The purpose of the system is to maintain the ideal air/fuel ratio at which the catalytic converter is most effective.

Major components of the system include an Electronic Control Module (ECM), an oxygen sensor, an electronically-controlled carburetor, and a three-way oxidation-reduction catalytic converter. The system also includes a maintenance reminder flag connected to the odometer which becomes visible in the instrument cluster at regular intervals, signaling the need for oxygen sensor replacement.

The oxygen sensor, installed in the exhaust manifold, generates a voltage which varies with exhaust gas oxygen content. Lean mixtures (more oxygen) reduce voltage; rich mixtures (less oxygen) increase voltage. Voltage output is sent to the ECM.

An engine temperature sensor installed in the engine coolant outlet monitors engine coolant temperatures. Vacuum control switches and throttle position sensors also monitor engine conditions and supply signals to the ECM.

The Electronic Control Module receives input signals from all sensors. It processes these signals and generates a control signal sent to the carburetor. The control signal cycles between ON (lean command) and OFF (rich command). The amount of ON and OFF time is a function of the input voltage sent to the ECM by the oxygen sensor.

Rochester Dualjet E2ME and E4ME carburetors are used with the C-4 System. Basically, an electrically operated mixture control solenoid is installed in the carburetor float bowl. The solenoid controls the air/fuel mixture metered to the idle and main metering systems. Air metering to the idle system is controlled by an idle air bleed valve. It follows the movement of the mixture solenoid to control the amount of air bled into the idle system, enrichening or leaning out the mixture as appropriate. Air/fuel mixture enrichment occurs when the fuel valve is open and the air bleed valve is closed. All cycling of this system, which occurs ten times per second, is controlled by the ECM. A throttle position switch informs the ECM of open or closed throttle operation. A number of different switches are used, varying with application. When the ECM receives a signal from the throttle switch, indicating a change of position, it immediately searches its memory for the last set of operating conditions that resulted in an ideal air/fuel ratio, and shifts to that set of conditions. The memory is continually updated during normal operation.

A "Check-Engine" light is included in the C-4 System installation. When a fault develops, the light comes on, and a trouble code is set into the ECM memory. However, if the fault is intermittent, the light will go out, but the trouble code will remain in the ECM memory as long as the engine is running. The trouble codes are used as a diagnostic aid, and are pre-programmed.

Unless the required tools are available, troubleshooting the C-4 System should be confined to mechanical checks of electrical connectors, vacuum hoses and the like. All diagnosis and repair should be performed by a qualified mechanic.