Computers and Control Systems: Description and Operation

This system functions to maintain the air-fuel mixture at an optimum ratio of 14.7:1 while keeping exhaust gas emissions to a minimum. The heart of the MCU system is the fuel control system. The fuel control ``loop'' is comprised of an exhaust gas oxygen (EGO) sensor, microprocessor control unit (MCU) and fuel control solenoid.
In the closed loop, the EGO sensor senses whether the exhaust gases are rich or lean. This signal from the EGO sensor is sent to the MCU module, which sends a corresponding signal to the feedback carburetor to correct the air fuel ratio according to requirements.
In the open loop, the MCU sends a fixed signal to the feedback carburetor. During this time, the input from the EGO sensor is ignored, thus opening the loop. The determining factor as to when the system will be in the closed loop or open loop is based upon information from switch outputs, which sense coolant temperature, manifold vacuum and engine speed. Normally the vehicle will be in the closed loop when the engine is operating at steady part throttle cruise conditions.
Other functions controlled by the MCU are the Thermactor Air Bypass (TAB) valve and the Thermactor Air Diverter (TAD) valve. The Canister Purge and Spark Retard Solenoid used on some applications are also controlled by the MCU.