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FORD BRONCO -> ELECTRICAL; EEC/PCM, Self-Test, wiring diagrams... -> Electronic Engine Control (EEC); SELF TEST & Diagnostic Trouble Code (DTC), OBD II, etc. -> Microprocessor Control Unit (MCU) Overview
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Microprocessor Control Unit (MCU) Overview; "...The Microprocessor Control Unit (MCU) system was used on most 1981–83 carburetor equipped vehicles, and 1984 and newer V8 engines with feedback carburetors. 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, like the EEC-IV system. This 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.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..." READ MORE for Reading Codes & Diagnostic Trouble Codes (DTC's)
Source: by Ford via arrc.epnet.com
Scan Tool Won’t Initiate Self-Tests (iggie the Press this or that); "...Begin by determining if the scan tool is working. If it works on other vehicles, particularly other Ford models, the problem is likely in the vehicle, not the scan tool. Failure to run a key-on,engine-off (KOEO) or a key-on, engine-running (KOER) test may be due to the way in which the test is initiated. Check these points first: 1. Be sure the ignition is off when connecting the data cable. 2. To begin a KOEO test, press Y to start the test; then turn the ignition on. See “Starting a KOEO Test” on page 722. 3. To begin a KOER test, turn the ignition off for a full 10 seconds. Then, start the engine and press Y to begin. See “Starting a KOER Test” on page 722 for more information. 4. Determine if solenoids and relays click when a KOEO test starts or if engine speed changes during a KOER test. See “Starting a KOEO Test” on page 722 and “Starting a KOER Test” on page 722 for more information. The MECS self-tests do not activate solenoids or relays. 5. Check for an ECM main ground problem at the battery ground cable single-wire connector. A bad ground at this point may also cause a no-start condition or prevent closed-loop operation. 6. Check the oxygen sensor (O2S) ground. On early EEC-IV systems the O2S ground is an orange wire between the PCM and cylinder head. The engine may run but not self-test if the O2S ground is weak. 7. Check for voltage and ground at the self-test connector. See “Testing the Self-Test Connector” on page 724 for procedures. 8. Depending on whether the engine starts and runs or does not start, refer to the appropriate section in this appendix for more instructions. Starting a KOEO Test; When an EEC-IV or MCU system starts a KOEO self-test on most Ford engines, you should hear solenoids and relays click as the self-test begins. The idle speed control motor may also operate. However, a few engines are completely silent during the KOEO test. If you do not hear any of the solenoids and relays operate on most engines, it may be an immediate sign that the test did not start. The KOEO test takes about one minute or less to complete. NOTE: i For reliable test operation on EEC-IV and MCU systems, first press the Y button to start the test; then switch the ignition on. To start the KOEO self-test, the ECM reads the voltage change on the STI line as the self-test input (STI) circuit switches to ground. Many ECMs for carbureted MCU systems only check the STI signal level once when the ignition is switched on. Therefore, if the ignition is switched on before Y is pressed to start a KOEO self-test, the ECM never sees a voltage change and does not start the test. Most, but not all, EEC-IV systems monitor STI voltage continuously, so the self-test may run if the ignition is switched on before Y is pressed to start the test. Starting a KOER Test; The signal for the ECM to start a KOER self-test is also grounding of the STI circuit. There is a noticeable increase in engine RPM for most systems when a KOER self-test runs. If the test runs properly, engine RPM changes several times as the ECM operates the idle speed control, the EGR valve, and other system actuators. Exceptions are MCU systems on 4-cylinder and 6-cylinder carbureted engines. These require that you raise engine speed to 2500 RPM within 10 seconds of starting the test. This speed is then held steady throughout the test. The entire KOER self-test may take from one to three minutes, depending on the system and the number of codes that are output. An EEC-V system goes directly from a KOEO test to a KOER test, but most Ford systems do not go directly from a KOEO to a KOER test with the ignition on. These systems allow only one self-test per ignition cycle. Turn the key off after finishing a KOEO test; then turn it back on to begin the KOER test. Leave the key off for ten seconds between tests. On many models a timed relay keeps ECM power on a few seconds after the key is turned off. Some carbureted and central-fuel injection (CFI) systems must reset the idle speed control motor before performing another self-test. Many 1987 and later Ford systems do not perform a KOER test if there are any on-demand faults detected during a KOEO test. In this case, the ECM transmits either a code 98 or 998 before it transmits the cylinder identification code, and the test is aborted. Code 98 or 998 is a notice to repair all engine-off on-demand faults first, before performing an engine-running test. Ford test procedures specifically say to repair KOEO on-demand faults first, before troubleshooting KOER codes. Engine Runs But Does Not Perform Self-Tests; The way the engine runs is helpful for identifying the cause of a self-test failure. If the engine seems to run normally but the system does not complete a self-test, the cause may be a fault in the self-test connector wiring that has no affect on the rest of the system. If the engine runs poorly, the cause might be an ECM voltage problem that affects the entire system, a poor EEC system ground, or an open ground at the oxygen sensor (O2S). For example, the 1.9L Escort engine does not perform a self-test if the O2S ground is open. Radio frequency interference (RFI) may also prevent or interfere with a self-test. If a KOER self-test fails halfway through the test, or if code output is erratic or invalid, use a scope to check for ignition RFI. Open Ground—If the circuit wiring is open to the ground terminal in the self-test connector, or the connector itself is open, it might not affect engine operation. However, this does affect the display because the scan tool uses this terminal as a ground point. If the display seems to operate normally and then goes blank when the ignition is switched on, suspect an open ground circuit. If the ground circuit is open, the scan tool finds an alternate ground path through the STO terminal, which is at low voltage with the ignition off. When the ignition is switched on, STO voltage goes high, and the display goes off. To check for an open circuit at the self-test connector ground terminal connect the positive (+) voltmeter lead to battery positive and the negative (–) lead to the ground terminal. The voltmeter should indicate battery voltage. If the voltmeter reads zero, the ground terminal is open. An alternate method is to jump battery ground to the backside of the Ford harness pin 2. If the display powers up, there is an open ground circuit. • Self-test output (STO)—An open STO circuit prevents self-test completion even if voltage at the STI lead is correct and the system ground is good. After running a self-test, the ECM transmits codes as voltage pulses on the STO line. If the STO line is open, the ECM cannot transmit codes. To check STO continuity, turn the ignition switch on and connect the positive (+) meter lead to the STO terminal at the self-test connector. Connect the negative (-) lead to the battery negative terminal. On 1987 and earlier models, the meter should indicate battery voltage. On 1988 and later models, the meter should read 0.0 V until the self-test activates. If the meter reading is incorrect, trace and repair the open circuit to the STO terminal, then retest. An open STO circuit should not affect engine operation. To check: • Self-test input (STI)—Switch the ignition on. Connect the positive (+) voltmeter lead to the STI terminal and the negative (–) lead to the battery negative (–) terminal. There should be either 12 V or 5 V on the STI terminal with the key on. If the STI is open, the ECM cannot receive the self-test start signal. However, the vehicle still runs normally with an open circuit to the STI terminal. • Ground—With the ignition switched on, connect the positive (+) voltmeter lead to the ground terminal in the self-test connector. Connect the negative (–) lead directly to the battery negative (–) terminal. Do not connect the meter to an engine or chassis ground. This test measures voltage drop across the ground side of the connector, the ECM, and most sensors. The ground voltage-drop test is important for several reasons. A high-resistance ground prevents the self-test from running. This is particularly true on vehicles with a 5 V STI signal. The display may operate normally, but not enough signal voltage drops on the STI line when it is grounded. Ford specifications may allow up to a 0.5 V drop on the system ground. However, in many cases this is too high and may prevent a self-test from running or cause poor overallperformance. Ideally, there should be a voltage drop of 0.1 V or less on the ground circuit. A common source of high-resistance is the bullet connector in the pigtail on the battery ground cable. The self-test connector ground terminal is a convenient place to check the ECM ground, as well as the ground reference for most sensors. A high-resistance ground causes the engine to run poorly, prevents the self-test from running, or both. For example, a 0.5 V drop on this ground produces a 10% error in sensor signals. Depending on self-test programs and sensor calibrations, a high-resistance system ground may or may not produce fault codes. Engine Does Not Start or Starts and Runs Poorly Fuel-injected engines that do not start may not perform a KOEO self-test. Carbureted engines may start but run poorly due to a lack of feedback fuel or ECM spark advance control, and do not perform a self-test. If an EEC-IV or MCU vehicle has either of these symptoms, begin with the following basic checks. Have a wiring diagram of the vehicle available. 1. Check battery state of charge and capacity. The scan tool draws little current and operates with voltage as low as 7.5 V. The display may appear to operate normally even if the battery does not have enough power to crank the engine or operate the ECM. 2. Check the EEC power relay and fuel pump relay. The ECM receives battery voltage (B+) through a main EEC power relay. If the relay does not close, the ECM cannot operate. The power relay usually has either a brown cover with a black top or a black cover with a white top. It may be located in the engine compartment or under the dash. For a no-start problem, check the fuel pump relay. The fuel pump should run for about 2 seconds as the ignition is switched on. If it does not, the relay might not be closing. The fuel pump relay usually has a green plastic cover with a black top. It may located be in the engine compartment, under the dash, or in the trunk. The ECM uses power from the EEC power relay to energize the fuel pump relay. The fuel pump inertia switch is a convenient place to check fuel pump relay voltage on many models. Also make sure that the inertia switch is not open. 3. Check ECM supply voltage. The ECM receives battery voltage (B+) through a fusible link in the wiring harness on most Ford vehicles. Some receive B+ voltage through a fuse. Check a wiring diagram for the location of a fusible link or fuse and test for an open circuit. A quick way to check for ECM supply voltage is to check for voltage at the self-test input (STI) terminal as described in “Testing the Self-Test Connector” on page 724. 4. Check the ECM ground (signal return). The ECM on most systems is grounded remotely through a circuit in the main ECM harness. Use a wiring diagram to identify the ground circuit. An open ground prevents a fuel-injected engine from starting, while a high-resistance ground causes poor overall operation. A common problem area is the bullet connector in the pigtail on the battery ground cable. Engine Does Not Start or Starts and Runs Poorly Fuel-injected engines that do not start may not perform a KOEO self-test. Carbureted engines may start but run poorly due to a lack of feedback fuel or ECM spark advance control, and do not perform a self-test. If an EEC-IV or MCU vehicle has either of these symptoms, begin with the following basic checks. Have a wiring diagram of the vehicle available. 1. Check battery state of charge and capacity. The scan tool draws little current and operates with voltage as low as 7.5 V. The display may appear to operate normally even if the battery does not have enough power to crank the engine or operate the ECM. 2. Check the EEC power relay and fuel pump relay. The ECM receives battery voltage (B+) through a main EEC power relay. If the relay does not close, the ECM cannot operate. The power relay usually has either a brown cover with a black top or a black cover with a white top. It may be located in the engine compartment or under the dash. For a no-start problem, check the fuel pump relay. The fuel pump should run for about 2 seconds as the ignition is switched on. If it does not, the relay might not be closing. The fuel pump relay usually has a green plastic cover with a black top. It may located be in the engine compartment, under the dash, or in the trunk. The ECM uses power from the EEC power relay to energize the fuel pump relay. The fuel pump inertia switch is a convenient place to check fuel pump relay voltage on many models. Also make sure that the inertia switch is not open. 3. Check ECM supply voltage. The ECM receives battery voltage (B+) through a fusible link in the wiring harness on most Ford vehicles. Some receive B+ voltage through a fuse. Check a wiring diagram for the location of a fusible link or fuse and test for an open circuit. A quick way to check for ECM supply voltage is to check for voltage at the self-test input (STI) terminal as described in “Testing the Self-Test Connector” on page 724. 4. Check the ECM ground (signal return). The ECM on most systems is grounded remotely through a circuit in the main ECM harness. Use a wiring diagram to identify the ground circuit. An open ground prevents a fuel-injected engine from starting, while a high-resistance ground causes poor overall operation. A common problem area is the bullet connector in the pigtail on the battery ground cable..." READ MORE
Source: by snapon.com