Ford Mid-Size Cars 1971-1985 Repair Guide

Electronic Engine Control Systems


Because of the complicated nature of the EEC systems, special tools and procedures are often necessary for testing and troubleshooting.


See Figure 1

Ford's EEC-I system was first introduced in the 1970's. Designed to precisely control ignition timing, Exhaust Gas Recirculation (EGR) and Thermactor® (air pump) flow, the system consists of an Electronic Control Assembly (ECA), seven monitoring sensors, a Duraspark ignition module and coil, a special distributor assembly, and EGR system designed to operate on air pressure.

The ECA is a solid state microcomputer, consisting of a processor assembly and a calibration assembly. The processor continuously receives inputs from the seven sensors, which it converts to usable information for the calculating section of the computer. It also performs ignition timing, Thermactor® and EGR flow calculations, processes the information and sends out signals to the ignition module and control solenoids to adjust the timing and flow of the systems accordingly. The calibration assembly contains the memory and programming for the processor.

Processor inputs come from sensors monitoring manifold pressure, barometric pressure, engine coolant temperature, inlet air temperature, crankshaft position, throttle position, and EGR valve position.

The manifold absolute pressure sensor determines changes in intake manifold pressure (barometric pressure minus manifold vacuum) which result from changes in engine load and speed, or in atmospheric pressure. Its signal is used by the ECA to set part throttle spark advance and EGR flow rate.

Click image to see an enlarged view

Fig. Fig. 1: EEC rotor alignment through 1979

Barometric pressure is monitored by a sensor mounted on the firewall. Measurements taken are converted into usable electrical signal. The ECA uses this reference for altitude dependent EGR flow requirements.

Engine coolant temperature is measured at the rear of the intake manifold by a sensor consisting of a brass housing containing a thermistor (resistance decreases as temperature rises). When reference voltage (about 9 volts, supplied by the processor to all sensors) is applied to the sensor, the resistance can be measured by the resulting voltage drop. Resistance is then interpreted as coolant temperature by the ECA. EGR flow is cut off by the ECA when a predetermined temperature value is reached. The ECA will also advance initial ignition timing to increase idle speed if the coolant overheats due to prolonged idle. A faster idle speed increases coolant and radiator air flow.

Inlet air temperature is measured by a sensor mounted in the air cleaner. It functions in the same way as the coolant sensor. The ECA uses its signal for proper spark advance and Thermactor® flow. At high inlet temperatures, above 90ºF (32ºC) the ECA modifies timing advance to prevent spark knock.

The crankshaft is fitted with a four-lobed powdered metal pulse ring, positioned 10º BTDC. Its position is constantly monitored by the crankshaft position sensor. Signals are sent to the ECA describing both the position of the crankshaft at any given moment, and the frequency of the pulses (engine rpm). These signals are used to determine optimum ignition timing advance. If either the sensor or wiring is broken, the ECA will not receive a signal, and thus will be unable to send any signal to the ignition module. This would prevent the engine from starting.

The throttle position sensor is a rheostat connected to the throttle plate shaft. Changes in throttle plate angle change the resistance value of the reference voltage supplied by the processor. Signals are interpreted in one of three ways by the ECA.

Closed throttle (idle or deceleration)
Part throttle (cruise)
Full throttle (maximum acceleration)

A position sensor is built into the EGR valve. The ECA uses its signal to determine EGR valve position. The valve and position sensor are replaced as a unit, should one fail.

Because of the complicated nature of this system, special diagnostic tools are necessary for troubleshooting. Any troubleshooting without these tools must be limited to mechanical checks of connectors and wiring.

The distributor is locked in place during engine manufacture; no rotational adjustment is possible for initial ignition timing, since all timing is controlled by the ECA. There are no mechanical advance mechanisms or adjustments under the rotor, thus there is no need to remove it except for replacement.


See Figure 2

The second generation EEC system was introduced on mid-sized Fords and Mercury in 1979. It was based on the EEC-I system used on the full sized vehicles, but some changes were made to reduce complexity and cost, increase the number of controlled functions, and improve reliability and performance.

In general, the EEC-II system operates in the same manner as EEC-I. An Electronic Control Assembly (ECA) monitors reports from six sensors, and adjust the EGR flow, ignition timing, Thermactor® (air pump) air flow, and carburetor air/fuel mixture in response to the incoming signals. Although there are only six sensors, seven conditions are monitored. The sensors are:

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Fig. Fig. 2: Comparison of early and later model EEC distributor caps

Engine coolant temperature
Throttle position
Crankshaft position
Exhaust gas oxygen
Barometric and manifold absolute pressure
EGR valve position

These sensors function in the same manner as the EEC-I sensors, and are described in the EEC-I information. Note that inlet air temperature is not monitored in the EEC-II system, and that the barometric and manifold pressure sensors have been combined into one unit. One more change from the previous system is in the location of the crankshaft sensor: it is mounted on the front of the engine, behind the vibration damper and crankshaft pulley.

The biggest difference between EEC-I and EEC-II is that the newer system is capable of continually monitoring and adjusting the carburetor air/fuel ratio. Monitoring is performed by the oxygen sensor installed in the right exhaust manifold; adjustment is made via an electric stepper motor installed on the model 7200 VV carburetor.

The stepper motor has four separate armature windings, which can be sequentially energized by the ECA. As the motor varies the position of the carburetor metering valve, the amount of control vacuum exposed to the fuel bowl is correspondingly altered. Increased vacuum reduces pressure in the fuel bowl, causing a leaner air/fuel mixture, and vice versa. During engine starting and immediately after, the ECA sets the motor at a point dependent on its initial position. Thereafter, the motor position is changed in response to the ECA calculations of the six input signals.

EEC-II is also capable of controlling purging of vapors from the evaporative emission control storage canister. A canister purge solenoid, a combination solenoid and valve, is located in the line between the intake manifold purge fitting and the carbon canister. It controls the flow of vapors from the canister to the intake manifold, opening and closing in response to signals from the ECA.

As is the case with EEC-I, diagnosis and repair of the system requires special tools and equipment.

The distributor is locked in place during engine manufacture; no rotational adjustment is possible for initial ignition timing, since all timing is controlled by the ECA. There are no mechanical advance mechanisms or adjustments under the ignition rotor, and thus there is no need to remove it except for replacement.

Air/fuel mixture is entirely controlled by the ECA; no adjustments are possible.


EEC-III was introduced in 1980. It is a third generation system developed entirely from EEC-II. The only real differences between EEC-II and III are contained within the Electronic Control Assembly (ECA) and the Duraspark ignition module. The EEC-III system uses a separate program module which plugs into the main ECA module. This change allows various programming calibrations for specific applications to be made to the program module, while allowing the main ECA module to be standardized. Additionally, EEC-III uses a Duraspark III ignition module, which contains fewer electronic functions than the Duraspark II module; other functions of the Duraspark II module have been incorporated into the main Electronic Control Assembly (ECA) module. There is no interchangeability between the Duraspark II and Duraspark III modules.

Since late 1979 emission controls and air/fuel mixtures have been controlled by various electronic methods. An electronically controlled feedback carburetor is used to precisely calibrate fuel metering, many vacuum check valves, solenoids and regulators have been added and the electronic control boxes (ECU and MCU) can be calibrated and programmed in order to be used by different engines and under different conditions.


Most 1984 and later US models are equipped with the EEC-IV system. The heart of the EEC-IV system is a microprocessor called an Electronic Control Assembly (ECA). The ECA receives data from a number of sensors and other electronic components (switches, relays, etc..). Based on information received and information programmed in the ECA's memory, it generates output signals to control various relays, solenoids and other actuators. The ECA in the EEC-IV system has calibration modules located inside the assembly that contain calibration specifications for optimizing emissions, fuel economy and driveability. The calibration module is called a PROM.

A potentiometer senses the position of an 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 is the area of the 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 and a switch that senses throttle plate position produces electrical signals that tell the control unit when the throttle is closed or wide open.

A special probe (oxygen sensor) in the exhaust manifold measures the amount of oxygen in the exhaust gas, which is a indication of combustion efficiency, and sends a signal to the control unit. The sixth signal, crankshaft position information, is transmitted by a sensor integral with the new design distributor.

The EEC-IV microcomputer circuit process the input signals and produces output control signals to the fuel injectors to regulate fuel discharge to the injectors. The EEC-IV distributor incorporates a Hall Effect vane switch stator assembly and an integrally mounted thick film module. When the Hall Effect device is tuned on and a pulse is produced, the EEC-IV electronics computes crankshaft position and engine demand to calibrate spark advance.

TFI-IV System

The Thick Film Integrated-IV (TFI-IV) ignition system features a universal distributor using no centrifugal or vacuum advance. The distributor has a die cast base which incorporates an integrally mounted TFI ignition module, a Hall Effect vane switch stator assembly and provision for fixed octane adjustment. The TFI system uses an E-Core ignition coil in lieu of the Duraspark coil. No distributor calibration is required and initial timing is not a normal adjustment, since advance etc.. is controlled by the EEC-IV system.

The universal distributor (used with EEC-IV) has a diecast base which incorporates an externally mounted TFI-IV ignition module, and contains a Hall Effect vane switch stator assembly and provision for fixed octane adjustment. No distributor calibration is required and initial timing adjustment is normally not required. The primary function of the EEC-IV universal distributor system is to direct high secondary voltage to the spark plugs. In addition, the distributor supplies crankshaft position and frequency information to a computer using a profile ignition pickup. The Hall Effect switch in the distributor consists of a Hall Effect device on one side and a magnet on the other side. A rotary cup which has windows and tabs rotates and passes through the space between the device and the magnet. When a window is between the sides of the switch the magnetic path is not completed and the switch is OFF , sending no signal. When a tab passes between the switch the magnetic path is completed and the Hall Effect device is turned ON and a signal is sent. The voltage pulse (signal) is used by is EEC-IV system for sensing crankshaft position and computing the desired spark advance based on engine demand and calibration.

The heart of the EEC-IV system is a microprocessor called the Electronic Control Assembly (ECA). The ECA receives data from a number of sensors, switches and relays. The ECA contains a specific calibration for peak fuel economy, driveability and emissions control. Based on information stored in its memory, the ECA generates signals to control the various engine functions.

The ECA calibration module is located inside the ECA assembly. On all cars, the ECA is located on the left of the firewall, behind the kick panel.