Ford Mustang/Capri 1979-1988 Repair Guide

Electronic Control Assembly (ECA)


The Electronic Control Assembly (ECA) is a solid-state micro-computer consisting of a processor assembly and a calibration assembly. It is located under the instrument panel or passenger's seat and is usually covered by a kick panel. 1981-82 models use an EEC-III engine control system, while 1983 and later models use EEC-IV. Although the two systems are similar in appearance and operation, the ECA units are not interchangeable. A multi-pin connector links the ECA with all system components. The processor assembly is housed in an aluminum case. It contains circuits designed to continuously sample input signals from the engine sensors. It then calculates and sends out proper control signals to adjust air/fuel ratio, spark timing and emission system operation. The processor also provides a continuous reference voltage to the B/MAP, EVP and TPS sensors. EEC-III reference voltage is 8-10 volts, while EEC-IV systems use a 5-volt reference signal. The calibration assembly is contained in a black plastic housing which plugs into the top of the processor assembly. It contains the memory and programming information used by the processor to determine optimum operating conditions. Different calibration information is used in different vehicle applications, such as California or Federal models. For this reason, careful identification of the engine, year, model and type of electronic control system is essential to insure correct component replacement.


Air Charge Temperature Sensor (ACT)

The ACT is threaded into the intake manifold air runner. It is located behind the distributor on V6 engines and directly below the accelerator linkage on V8 engines. The ACT monitors air/fuel charge temperature and sends an appropriate signal to the ECA. This information is used to correct fuel enrichment for variations in intake air density due to temperature changes.

Barometric & Manifold Absolute Pressure Sensors (B/MAP)

The B/MAP sensor on V8 engines is located on the right fender panel in the engine compartment. The MAP sensor used on V6 engines is separate from the barometric sensor and is located on the left fender panel in the engine compartment. The barometric sensor signals the ECA of changes in atmospheric pressure and density to regulate calculated air flow into the engine. The MAP sensor monitors and signals the ECA of changes in intake manifold pressure which result from engine load, speed and atmospheric pressure changes.

Crankshaft Position (CP) Sensor

The purpose of the CP sensor is to provide the ECA with an accurate ignition timing reference (when the piston reaches 10°BTDC) and injector operation information (twice each crankshaft revolution). The crankshaft vibration damper is fitted with a 4-lobe pulse ring. As the crankshaft rotates, the pulse ring lobes interrupt the magnetic field at the tip of the CP sensor.

EGR Valve Position Sensor (EVP)

This sensor, mounted on EGR valve, signals the computer of EGR opening so that it may subtract EGR flow from total air flow into the manifold. In this way, EGR flow is excluded from air flow information used to determine mixture requirements.

Engine Coolant Temperature Sensor (ECT)

The ECT is threaded into the intake manifold water jacket directly above the water pump bypass hose. The ECT monitors coolant temperature and signals the ECA, which then uses these signals for mixture enrichment (during cool operation), ignition timing and EGR operation. The resistance value of the ECT increases with temperature, causing a voltage signal drop as the engine warms up.

Exhaust Gas Oxygen Sensor (EGO)

The EGO is mounted in the right side exhaust manifold on V8 engines, in the left and right side exhaust manifolds on V6 models. The EGO monitors oxygen content of exhaust gases and sends a constantly changing voltage signal to the ECA. The ECA analyzes this signal and adjusts the air/fuel mixture to obtain the optimum (stoichiometric) ratio.

Knock Sensor (KS)

This sensor is used on various models equipped with the 6-232 engine. It is attached to the intake manifold in front of the ACT sensor. The KS detects engine vibrations caused by preignition or detonation and provides information to the ECA, which then retards the timing to eliminate detonation.

Thick Film Integrated Module Sensor (TFI)

The TFI module sensor plugs into the distributor just below the distributor cap and replaces the CP sensor on some engines. Its function is to provide the ECA with ignition timing information, similar to what the CP sensor provides.

Throttle Position Sensor (TPS)

The TPS is mounted on the right side of the throttle body, directly connected to the throttle shaft. The TPS senses the throttle movement and position, and transmits an appropriate electrical signal to the ECA. These signals are used by the ECA to adjust the air/fuel mixture, spark timing and EGR operation according to engine load at idle, part throttle, or full throttle. The TPS is nonadjustable.


Diagnostic and test procedures on the EEC-III and EEC-IV electronic control systems require the use of special test equipment. Have these systems tested professionally.

Before beginning any component testing, always check the following:

Check the fuel and ignition systems to ensure that there is fuel and spark.
Remove the air cleaner assembly and inspect all vacuum and pressure hoses for proper connection to fittings. Check for damaged or pinched hoses.
Inspect all sub-system wiring harnesses for proper connections to the EGR solenoid valves, injectors, sensors, etc.
Check for loose or detached connectors and broken or detached wires. Check that all terminals are seated firmly and are not corroded. Look for partially broken or frayed wires or any shorting between the wires.
Inspect the sensors for physical damage. Inspect the vehicle electrical system. Check the battery for full charge and cable connections for tightness.
Inspect the relay connector and make sure the ECA power relay is securely attached and making a good ground connection.

Solenoid and Sensor Resistance Tests

All CFI components must be disconnected from the circuit before testing the resistance with a suitable ohmmeter. Replace any component whose measured resistance does not agree with the specifications chart. Shorting the wiring harness across a solenoid valve can burn out the circuitry in the ECA that controls the solenoid valve actuator. Exercise caution when testing the solenoid valves to avoid accidental damage to the ECA.

Fuel Pressure Tests

The diagnostic pressure valve (Schrader type) is located at the top of the fuel charging main body. This valve provides a convenient point for service personnel to monitor fuel pressure, bleed down the system pressure prior to maintenance, and to bleed out air which may become trapped in the system during filter replacement. A pressure gauge with an adapter is required to perform pressure tests.

Under no circumstances should compressed air be forced into the fuel system using the diagnostic valve. Depressing the pin in the diagnostic valve will relieve system pressure by expelling fuel into the throttle body.

System Pressure Test

Testing fuel pressure requires the use of a special pressure gauge (T80L-9974-A or equivalent) that attaches to the diagnostic pressure tap on the fuel charging assembly. Depressurize the fuel system before disconnecting any lines.

  1. Disconnect the fuel return line at the throttle body (in-tank high pressure pump) and connect the hose to a 1-quart calibrated container. Connect a pressure gauge.
  3. Disconnect the electrical connector at the fuel pump. The connector is located ahead of the fuel tank (in-tank high pressure pump) or just forward of the pump outlet (in-line high pressure pump). Connect an auxiliary wiring harness to the connector of the fuel pump. Energize the pump for 10 seconds by applying 12 volts to the auxiliary harness connector, allowing the fuel to drain into the calibrated container. Note the fuel volume and pressure gauge reading.
  5. Correct fuel pressure should be 35-45 psi (241-310 kPa). Fuel volume should be 10 oz. in 10 seconds (minimum) and fuel pressure should maintain a minimum of 30 psi (206 kPa) immediately after pump cut-off.

If the pressure condition is met, but the fuel flow is not, check for blocked filter(s) and fuel supply lines. After correcting the problem, repeat the test procedure. If the fuel flow is still inadequate, replace the high pressure pump. If the flow specification is met but the pressure is not, check for a worn or damaged pressure regulator valve on the throttle body. If both the pressure and fuel flow specifications are met, but the pressure drops excessively after de-energizing, check for a leaking injector valve(s) and/or pressure regulator valve. If the injector valves and pressure regulator valve are okay, replace the high pressure pump. If no pressure or flow is seen in the fuel system, check for blocked filters and fuel lines. If no trouble is found, replace the in-line fuel pump, in-tank fuel pump and the fuel filter inside the tank.

Fuel Injector Pressure Test
  1. Connect pressure gauge T80L-9974-A, or equivalent, to the fuel pressure test fitting. Disconnect the coil connector from the coil. Disconnect the electrical lead from one injector and pressurize the fuel system. Disable the fuel pump by disconnecting the inertia switch or the fuel pump relay and observe the pressure gauge reading.
  3. Crank the engine for 2 seconds. Turn the ignition OFF and wait 5 seconds, then observe the pressure drop. If the pressure drop is 2-16 psi (14-110 kPa), the injector is operating properly. Reconnect the injector, activate the fuel pump, then repeat the procedure for the other injector.
  5. If the pressure drop is less than 2 psi (14 kPa) or more than 16 psi (110 kPa), switch the electrical connectors on the injectors and repeat the test. If the pressure drop is still incorrect, replace the disconnected injector with one of the same color code, then reconnect both injectors properly and repeat the test.
  7. Disconnect and plug the vacuum hose at the EGR valve. It may be necessary to disconnect the idle speed control (6-232) or the throttle kicker solenoid (8-302) and use the throttle body stop screw to set the engine speed. Start and run the engine at 1,800 rpm (2,000 rpm on 1984 and later models). Disconnect the left injector electrical connector. Note the rpm after the engine stabilizes around 1,200 rpm. Reconnect the injector and allow the engine to return to high idle.
  9. Perform the same procedure for the right injector. Note the difference between the rpm readings of the left and right injectors. If the difference is 100 rpm or less, check the oxygen sensor. If the difference is more than 100 rpm, replace both injectors.