Microprocessor Control Unit
The Microprocessor Control Unit (MCU) is a solid-state micro-computer located on the left fender panel. It is the "brain" of the system and receives inputs and sends signals through a 24-pin connector. The MCU is capable of operating in 3 modes: Initialization, Open-Loop and Closed-Loop.
System as found on all California and F100, light duty pickups.
Initialization mode occurs when the engine is started. In this mode the MCU richens the fuel mixture for easy starting.
Open-Loop operation is controlled by MCU programming. Air/fuel ratio is fixed at a pre-determined level and allows good driveability at idle, moderate-to-heavy acceleration, and deceleration.
Closed-Loop operation occurs when the engine is warm and vehicle is operated at light load conditions. In closed loop, the MCU controls the air/fuel mixture in response to signals from an oxygen sensor in the exhaust manifold.