Chevrolet Chevette/1000 1976-1988 Repair Guide

Computer Controlled Catalytic


Converter (C-4) System

The GM designed Computer Controlled Catalytic Converter System (C-4 System), was introduced in 1979 and used on California Chevettes in 1980. The C-4 System primarily maintains the ideal air/fuel ratio at which the catalytic converter is most effective. Some versions of the system also control ignition timing of the distributor.

Major components of the system include an Electronic Control Module (ECM), an oxygen sensor, and electronically controlled variable-mixture carburetor, and a three-way oxidation-reduction catalytic converter.

The oxygen sensor 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 coolant temperatures. Vacuum controlled switches and throttle position sensors also monitor engine conditions and supply signals to the ECM.

The Electronic Control Module (ECM) monitors the voltage input of the oxygen sensor along with information from other input signals. 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 times (called a duty cycle) is a function of the input voltage sent to the ECM by the oxygen sensor. The ECM has a calibration unit called a PROM (Programmable Read-Only Memory) which contains the specific instructions for a given engine application. In other words, the PROM unit is specifically programmed or tailor made for the system in which it is installed. The PROM assembly is a replaceable component which plugs into a socket on the ECM and requires a special tool for removal and installation.

To maintain good idle and driveability under all conditions, other input signals are used to modify the ECM output signal. Besides the sensors and switches already mentioned, these input signals include the manifold absolute pressure (MAP) or vacuum sensors and the barometric pressure (BARO) sensor. The MAP or vacuum sensors sense changes in manifold vacuum, while the BARO sensor senses changes in barometric (ambient atmospheric) pressure. On important function of the BARO sensor is the maintenance of good engine performance at various altitudes.