Ford Mustang/Capri 1979-1988 Repair Guide

Microprocessor Control Unit (MCU) System


The Microprocessor Control Unit (MCU) system was used on some of the 4 and 8-cylinder vehicles covered by this guide from 1980-83. Although the MCU system is basically the same for all engines, it was confined to applications using a feedback carburetor. The heart of the MCU system's operation is the fuel control system. Strict fuel control is necessary to keep the air/fuel ratio at a proper chemical balance of 14.7:1, so that maximum catalytic efficiency is maintained.

The fuel control loop is comprised of the Microprocessor Control Unit (MCU) module, an Exhaust Gas Oxygen (EGO) sensor, and a Feedback Carburetor (FBC). 4-cylinder engines also incorporate a fuel control solenoid. In order to maintain the critical air/fuel ratio, the EGO sensor monitors the exhaust emissions by measuring the amount of oxygen in the exhaust gases. It then provides a voltage signal to the MCU module to indicate lean or rich engine operation. The feedback carburetor varies the air/fuel ratio based on oxygen measurements taken by the EGO sensor. Since good emissions and fuel economy do not always correlate with driveability, the MCU system is designed to operate in three fuel control modes: initialization, open loop and closed loop.

The initialization mode occurs when the MCU system is activated by turning the ignition ON and persists for a brief moment after the engine is started. During this mode, the air/fuel ratio is maintained at a rich condition for easy starting.
In the open loop mode, the air/fuel ratio output signals from the MCU module are held at a fixed level, adjusting fuel to assure good response and performance.
In the closed loop mode, the MCU system leans out the air/fuel ratio so that sufficient air is present in the mixture for nearly complete combustion of the fuel's hydrogen and carbon.

One of the major differences between open and closed loop operation is that the MCU utilizes information from the oxygen sensor to control air/fuel ratio ONLY when the system is operating in closed loop.

When the engine is running at its normal (warmed-up) operating temperature range and under light-load, part-throttle conditions, the MCU system is in the closed loop mode. When the engine is accelerated to a wide-open throttle, decelerated, or standing at idle, the MCU system switches to the open loop mode.

In addition to the three operating modes, the MCU system has the built-in capability for testing itself for any mode malfunction. This self-test is intended to check only the MCU module and its associated sensors.

Because of the complicated nature of the Ford system, special tools and procedures are necessary for testing and troubleshooting.