Ford Mid-Size Cars 1971-1985 Repair Guide

MCU System


See Figures 1 and 2

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Fig. Fig. 1: MCU system component locations-4-cylinder engine

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Fig. Fig. 2: MCU system component locations-6 and 8-cylinder engines


The Microprocessor Control Unit (MCU) system was the first to incorporate both a special test connector and the Self-Test Automatic Readout (STAR) hand-held scan tool for diagnostics. The MCU module is devoted to monitoring both input and output functions within the system. This ability forms the core of the self-diagnostic system. If a problem is detected within a circuit, the controller will recognize the fault, assign it an identification code, and store the code in a memory section. Fault codes are represented by two-digit numbers, which may be retrieved during diagnosis.

While the MCU system is capable of recognizing many internal problems, certain faults will not be recognized. Because the computer system reads only electrical signals, it cannot sense or react to mechanical or vacuum faults affecting engine operation. Some of these faults may affect another component which will set a code. For example, the MCU monitors the output signal to the fuel control solenoid or feedback carburetor, but cannot detect a defective choke diaphragm. As long as the output driver responds correctly, the computer will read the system as functioning correctly. However, the improper choke pull-off may result in a rich mixture. This would, in turn, be detected by the oxygen sensor and noticed as a constantly rich signal by the MCU. Once the signal falls outside the pre-programmed limits, the engine control assembly would notice the fault and set an identification code.


Hand-Held Scan Tools

Although stored codes may be read through an analog voltmeter, the use of a hand-held scan tool such as Ford's Self-Test Automatic Readout (STAR) tester, or equivalent, is highly recommended. There are many manufacturers of such tools, but you must be certain that the tool is appropriate for the intended use.

The STAR tester is designed to communicate directly with the MCU system and interpret electrical signals. The scan tool allows any stored faults to be read from the engine controller memory. Use of the scan tool provides additional data during troubleshooting, but does not eliminate the need for diagnostic work. The scan tool makes information collection easier, but the data must still be correctly interpreted by an operator familiar with the system.

Other Diagnostic Tools

An analog (needle type) voltmeter with a voltage scale of 0-20 volts DC may be used to read stored fault codes if the STAR tester is not available. The codes are transmitted as visible needle sweeps on the face of the instrument.

Other necessary tools for testing/troubleshooting include a timing light, a vacuum gauge (for some applications), and a jumper wire. A quality tachometer, preferably with an inductive (clip-on) pickup and a range of 0-3000 rpm, will also be required to verify test rpm on 4-140 engines during Self-Test procedures.


Diagnosis of a driveability problem requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to begin extensive testing before completing the preliminary diagnostic steps. The preliminary or visual inspection must be completed in detail before diagnosis begins. In many cases, this will shorten diagnostic time and often cure the problem without electronic testing.

Visual Inspection

This is possibly the most critical step of diagnosis. A detailed examination of all connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the person performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other components. Wiring should be checked carefully for any sign of strain, burning, crimping or terminal pull-out from a connector.

You should always check connectors at components or in harnesses as required. Pushing them together will usually reveal a loose fit. Pay particular attention to ground circuits, making sure they are not loose or corroded. Remember to inspect connectors and hose fittings at components not mounted on the engine, such as the evaporative canister or relays mounted on the fender aprons. Any component or wiring in the vicinity of a fluid leak or spillage should be given extra attention during inspection.

Additionally, inspect maintenance items such as belt condition and tension, battery charge and condition, and the radiator cap carefully. Any of these simple items may affect the system enough to set a fault.

Reading Codes With a Hand-Held Scan Tool

See Figures 3 and 4

A hand-held scan tool, such as the STAR tester, may be used to retrieve stored fault codes. Simply engage the tester's service connectors to the vehicle's Self-Test connectors.

Follow the directions given later in this information under Quick Test Procedures for performing the Key On Engine Off (KOEO) and Key On Engine Running (KOER) tests. Be sure to release the tester's push button, if applicable, before beginning the Self-Test.

Digital codes, such as "23'' will be output and displayed as numbers on the hand-held scan tool. (The codes may also be read using an analog voltmeter. For further details on this alternative method, please refer to the following portion of this section.)

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Fig. Fig. 3: STAR tester displays. Note that the colon (: ) must be present before codes can be received.

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Fig. Fig. 4: STAR tester hook-up-MCU system

Reading Codes With an Analog Voltmeter

See Figures 5 and 6

In the absence of a scan tool, an analog voltmeter may be used to retrieve stored fault codes. Set the meter range to read 0-15 volts DC. Connect the positive (+) lead of the meter to the positive battery terminal and connect the negative (-) lead of the meter to the self-test output pin of the diagnostic connector.

Follow the directions for performing the KOEO and KOER tests. To activate the tests, use a jumper wire to connect the signal return pin on the diagnostic connector to the self-test input connector. The self-test input line is the separate wire and connector that is located with or near the diagnostic connector.

The codes will be transmitted as groups of needle sweeps, whose cadence corresponds to the codes' numerical representation. Please refer to the accompanying illustration for details on counting the needle sweeps in order to determine the transmitted code.

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Fig. Fig. 5: Code display patterns on an analog voltmeter

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Fig. Fig. 6: Analog voltmeter hookup-MCU system


See Figures 7 and 8

Quick Test Procedures

The MCU system may be interrogated for stored codes using the Quick Test Procedures. These Quick Test procedures include: Key On Engine Off (KOEO) and Key On Engine Running (KOER). These diagnostic procedures must be performed correctly if the system is to run the internal Self-Test checks and provide accurate fault codes.

If the vehicle passes both sections of the Quick Test, the MCU system is all right and the vehicle's problem exists elsewhere. Once the Quick Test has been performed and all fault codes recorded, refer to the code charts found later in this section.

To prevent injury and/or property damage, always block the drive wheels, firmly apply the parking brake, place the transmission in PARK or NEUTRAL and turn all electrical loads off before performing the Quick Test procedures.


Unless instructed otherwise, do not disconnect any sensor with the key ON or a service code may be stored.

  1. Verify that the vacuum hoses are connected to the air cleaner. The air cleaner must be installed during these tests.
  3. Start the engine and use an analog voltmeter to verify that there is power to the choke. Let the engine idle until it reaches normal operating temperature and the throttle is off fast idle. Turn the engine OFF .
  5. Connect the scan tool or analog voltmeter to the self-test connectors, as shown. When using a voltmeter, connect a jumper wire from the Self-Test Trigger to the ground terminal of the Self-Test connector. When using a STAR tester, make certain the test button is unlatched or up.
  7. On 4-cylinder engines with a vacuum purge valve, disconnect (but do not plug) the hose from the canister control valve that runs to the carbon canister. (This will disable the canister purge system during the test.)
  9. On 6 and 8-cylinder engines if equipped with a vacuum delay valve, uncap the restrictor near the tee in the Thermactor® diverter vacuum control line.
    1. Remove the PCV valve from the breather cap on the valve cover.
    3. Use a tee to connect a vacuum gauge to the canister purge solenoid valve hose on the carbon canister side of the hose.

  11. Make sure that the carburetor throttle linkage is off the high step of its cam.
  13. Activate the test button on the STAR tester, if applicable. This will ready the Self-Test mode.
  15. Turn the ignition switch ON , but do not start the engine.

Do not depress the throttle on gasoline engines during the test.

  1. The KOEO codes will be transmitted.
  3. Record all service codes displayed, and proceed with the Key On Engine Running (KOER) test.

For a translation of potential MCU service codes, please refer to the charts later in this section.

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Fig. Fig. 7: MCU system service codes-4-cylinder engine

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Fig. Fig. 8: MCU system service codes-6 and 8-cylinder engines


Unless instructed otherwise, do not disconnect any sensor with the key ON or a service code may be stored.

  1. On vehicles equipped with a 4-cylinder engine and a 5-speed transmission, locate and tape the hole on top of the wide-open-throttle vacuum valve. (Be sure to remove the tape after this test is completed.)
  3. Verify that the engine is still at normal operating temperature.
  5. Activate or latch the self-test button on the scan tool, if applicable.
  7. For all vehicles except those equipped with an 8-cylinder engine, perform the following procedures. (For those vehicles equipped with an 8-cylinder engine, proceed to Step 5.)
    1. Start the engine and, within 20 seconds, increase the engine speed to 3000 rpm.
    3. Hold the rpm steady until a service code is received on the voltmeter or STAR tester. (The Self-Test sequence may run 10-40 seconds.) Return the engine to idle as soon as service code output begins.


If, within the first seven seconds of starting the engine, the voltmeter does not pulse (sweep), or pulses more than two times, check the engine's TACH lead for continuity. Likewise, if the STAR tester's code does not change, or displays a code other than "20'' or "30'' check the TACH lead.

  1. Observe and record the voltmeter's pulses or the STAR tester's service code.

If Code 33 appears (or any code not listed on the appropriate MCU code translation chart), the KOER test was not properly initiated.

  1. For all vehicles equipped with an 8-cylinder engine, perform the following procedures:
    1. Start the engine and run it at 2000 rpm for two minutes. (This action warms up the oxygen sensor.)
    3. Turn the engine OFF , then immediately restart and idle the engine.
    5. Observe the voltmeter and vacuum gauge for initialization pulses after restarting the engine. (The throttle kicker will also extend at this time, increasing rpm, and remain on throughout the test. If not, the engine rpm is out of specification.)


If there are no initialization pulses on the voltmeter or vacuum gauge, or an erroneous service code appears on the STAR tester, there is no Self-Test output.

    1. After four one-second initialization pulses appear, observe and record the voltmeter's pulses or the STAR tester's service code for vehicles without a knock sensor.
    3. For vehicles equipped with a knock sensor, immediately after four initial pulses occur, simulate spark knock by placing a 3 / 8 in. socket extension on the intake manifold, near the base of the knock sensor, and tapping lightly with a small hammer for approximately 15 seconds. Then, observe and record the voltmeter's pulses or the STAR tester's service code.

Within 90 seconds, the service codes are complete and the throttle kicker will retract, thereby decreasing rpm.

  1. Turn the engine OFF . Disconnect all test equipment and restore all components to their pre-test configuration/condition.