Ford Escort/Lynx 1981-1990 Repair Guide

General Information


When the term Powertrain Control Module (PCM) is used in this guide, it refers to the engine control computer, regardless of whether it is a Microprocessor Control Unit (MCU) or Electronic Engine Control-Four (EEC-IV) module.

There are two types of engine control systems used on the Escort/Lynx vehicles. The two systems are the Microprocessor Control Unit (MCU) System and the Electronic Engine Control-Four (EEC-IV) system.

The Microprocessor Control Unit (MCU) System was used on 1981-83 models. The MCU system uses a large six-sided connector, identical to the one used with EEC-IV systems. The MCU system does NOT use the small, single-wire connector, found on the EEC-IV system.

The MCU system has limited ability to diagnose a malfunction within itself. Through the use of trouble codes, the system will indicate where to test. When an analog voltmeter or special tester is connected to the diagnostic link connector and the system is triggered, the self-test simulates a variety of engine operating conditions and evaluates all the responses received from the various MCU components, so any abnormal operating conditions can be detected.

Ford's fourth generation engine control system is centered around a microprocessor called the Electronic Engine Control-Four (EEC-IV) system. The EEC-IV module receives and sends electronic signals relaying pertinent engine management information (data) to and from a number of sensors and other electronic components. The EEC-IV module contains a specific calibration for maintaining optimum emissions, fuel economy and driveability. By comparing the input signals to its own calibrated program, the module generates output signals to the various relays, solenoids and actuators.

The EEC-IV module is usual located under the instrument panel left of the steering column, and communicates service information to the outside world by way of service codes. The service codes are two or three-digit numbers representing the result of the self-test.

The service codes are transmitted through the Self-Test Output (STO) terminal, found in the self-test connector.

The module stores the self-test program in its permanent memory. When activated, it checks the system by testing its memory and processing capability. The self-test also verifies if the various sensors and actuators are connected and operating properly.

The self-test is divided into three specialized tests:

Key On, Engine Off (KOEO) is a static check of the processor inputs and outputs with the power ON , but the engine OFF .
Engine Running self-test is a dynamic check with the engine in operation and the vehicle at rest.
Continuous self-test is a check of the sensor inputs for opens and shorts while the vehicle is in operation.

The KOEO and Engine Running tests are functional tests which only detect faults present at the time of the self-test. Continuous testing is an ongoing test that stores fault information for retrieval at a later time, during the self-test.