Ford Mustang/Capri 1979-1988 Repair Guide



Most 1984-88 vehicles covered by this guide use the EEC-IV system. As is the case with its EEC-I, II and III predecessors, the heart of the EEC-IV system is a microprocessor. Unlike its predecessors, however, the EEC-IV microprocessor is not properly called an Electronic Control Assembly (ECA). Instead, the EEC-IV microprocessor is called a Powertrain Control Module (PCM), by virtue of its added transmission control function.

EEC-IV marked the disappearance of separate ECA and calibration assemblies, which was the norm for its predecessors. Instead, the calibration assembly became a program stored on a Read Only Memory (ROM) chip, within the PCM.

The PCM is given responsibility for the operation of the emission control devices, cooling fan(s), ignition/timing advance and, in some cases, automatic transmission functions. Because the EEC-IV system oversees both the ignition timing and the fuel injector (or feedback carburetor) operation, a precise air/fuel ratio will be maintained under all operating conditions. The PCM receives electrical input from several sensors, switches and relays located on and around the engine:

A potentiometer senses the position of the vane airflow meter in the engine's air induction system and generates a voltage signal that varies with the amount of air drawn into the engine.
A sensor in the area of the vane airflow meter measures the temperature of the incoming air and transmits a corresponding electrical signal.
Another temperature sensor inserted in the engine coolant tells if the engine is cold or warmed up.
A switch, which senses throttle plate position, produces electrical signals that tell the control unit when the throttle is closed or wide open.
An oxygen sensor probe in the exhaust manifold measures the amount of oxygen in the exhaust gas, as an indication of combustion efficiency, and sends a signal to the PCM.
A sensor built into the redesigned distributor transmits crankshaft position information to the PCM.

Based on combinations of such input, the PCM controls output to various devices and actuators concerned with engine operation and emissions. The PCM relies on these signals to form a correct picture of current vehicle operation. If any of the input signals is incorrect, the PCM will react to whatever data is received. For example, if the coolant temperature sensor is inaccurate and reads too low, the PCM may perceive an engine which never warms up. Consequently, the engine settings will be maintained as if the engine were cold. Because so many inputs can affect one output, correct diagnostic procedures are essential on these systems.

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