5.8l Mcu System Ford LTD Crown Victoria I

DESCRIPTION

The MCU control system is named for and commanded by a Microprocessor Control Unit. This micro-computer is located in the engine compartment and is capable of controlling engine air/fuel ratios, air injection, canister purge, spark retard and idle speed.

The system consists of the MCU module, air/fuel control and air injection solenoids, engine sensors, feedback carburetor, and related circuitry.

MICROPROCESSOR CONTROL UNIT (MCU)

The MCU is a solid-state micro-computer located on the left fender panel. It is the "brain" of the system and receives inputs and sends signals through a 24-pin connector. The MCU is capable of operating in 3 modes: Initialization, Open loop and Closed loop.

Scheme 42

Initialization mode occurs when the engine is started. In this mode the MCU richens the fuel mixture for easy starting. Open loop operation is controlled by MCU programming. Air/fuel ratio is fixed at a pre-determined level and allows good driveability at idle, moderate-to-heavy acceleration, and deceleration.

Closed loop operation occurs when the engine is warm and vehicle is operated at light load conditions. In closed loop, the MCU controls the air/fuel mixture in response to signals from an oxygen sensor in the exhaust manifold.

Coolant Temperature

There are 2 switches used to signal temperature changes to the MCU. The 2 switches are mounted in coolant passage ahead of carburetor. One switch is open when the engine is warm; the other is open when the engine is cold and/or when it has overheated.

Vacuum Switches

Engine load is determined by vacuum level. To measure vacuum level, 3 vacuum switches are used to signal the MCU of cruise, deceleration, and wide-open-throttle conditions.

Oxygen Sensor

An electrically heated oxygen sensor is mounted in the rear of right exhaust manifold. This sensor sends a low voltage signal to the MCU to indicate rich or lean mixture. When mixture is lean, the signal is less than 0.2 volts. When rich, the sensor voltage is slightly above 0.6 volts.

Engine Speed

The MCU receives a direct signal from the "Tach Test" terminal on the coil. It uses this signal to calculate engine speed and alters the air/fuel correction based on this speed.

Knock Sensor

The knock sensor is used to help reduce detonation. It allows a voltage signal to pass through when it senses detonation. The MCU uses this information to signal ignition module. The MCU otherwise does not control ignition timing.

Barometric Switch

This switch informs the MCU of altitude. Based upon this information, the MCU modifies the air/fuel ratio to provide accurate mixture for all altitudes.

Self-Test Connector

The MCU can self-diagnose most common operating problems. In order to initiate and read the diagnostic program, connections are made to the Self-Test connector. It provides voltage pulses which can be read by Self-Test Automatic Readout (STAR) tester (Rotunda 007-00004) or a dial-type voltmeter.

ENGINE CONTROLS

The following MCU controlled devices are used to accomplish the task of improving driveability and reducing emissions. These devices are all electrically controlled.

Thermactor Controls

These controls direct the flow of air from air pump to either the exhaust manifold, catalytic converter, or atmosphere. A pair of solenoid valves control vacuum flow which operates a Thermactor Air Valve assembly. These valves are called the Thermactor Air By-Pass (TAB) and Thermactor Air Diverter (TAD) valves.

In normal operation, the air is injected into the catalytic converter to improve reduction of emissions. When the engine is idling or decelerating for long periods of time, air is diverted to atmosphere. When the engine is first warming, air is injected into the exhaust manifold to help heat exhaust gases before they reach the converter.

Air/Fuel Controls

The MCU provides current to a feedback motor in the carburetor, also known as a Feedback Carburetor Actuator (FBCA). This motor extends and retracts a shaft which alters a bleed-air orifice in the carburetor.

Canister Purge Solenoid

The canister purge solenoid is controlled by the MCU. When engine conditions are correct, the solenoid is opened and the fuel vapor canister is purged.

Throttle Kicker

The throttle kicker is used to improve idle. The MCU applies voltage to a solenoid, providing vacuum to operate the kicker.

Ignition Control

The knock sensor signals the MCU for required ignition advance modification. The MCU sends a signal to a 3-connector ignition module, which then retards timing to reduce detonation.

PREPARATION FOR TESTING

The MCU system is capable of diagnosing some problems which may occur. To determine which components should be checked, perform the FUNCTIONAL TEST .

If problems do exist, a service code will be displayed (as pulses on a voltmeter). Locate the appropriate test chart and follow the repair procedure as instructed. DO NOT use the test charts unless referred to them by the FUNCTIONAL TEST or you may replace some components unnecessarily. Testing procedures require the following equipment

1. Needle-Type Voltmeter (0-20 volt scale)
2. Digital Voltmeter (DVOM - Min. impedance 10 megohms)
3. Vacuum Gauge (0-30 in. Hg)
4. Vacuum Pump
5. Tachometer
6. Jumper Wire
7. Timing Light
8. Test Lamp (12-volt)
9. Torque Wrench with deep 1 1/8" socket
10. Steel Rod (socket extension) and 4 oz. Hammer
11. Watch with second hand

1. Check vacuum hoses for leaks, cracks, or improper routing. Repair or replace as necessary. Check coolant level.
2. Check electrical connections. Repair any frayed or broken wires. Ensure that all connections are clean and tight.
3. Turn all accessories off. Place transmission in Neutral and set parking brake. Warm engine to normal operating temperature. If air cleaner must be moved, leave all vacuum hoses attached. Check to see that battery voltage is present at electric choke terminal while engine is running. NOTE: If vehicle will not start, see NO START TEST.
4. Turn ignition off. Locate Self-Test connector (on left fender panel, near MCU) and insert a jumper wire between ground and Trigger sockets. (Scheme 43) Connect the positive lead of a needle-type voltmeter to vehicle battery positive terminal, and the negative lead to Self-Test Output socket.
5. Set voltmeter on 0-20 volt scale. Battery voltage may be shown. Remove PCV valve from valve cover. Using a vacuum tee, connect a vacuum gauge to canister purge solenoid valve hose, on canister side of hose.
6. Models equipped with a delay valve have a tee and restrictor in the thermactor by-pass vacuum control line. Uncap restrictor while performing test. Recap after test.

Scheme 43

FUNCTIONAL TEST

Note. Service codes are shown by voltage pulses. The first digit is indicated by a series of pulses, then the needle drops to zero for 2 seconds, then the second digit of the code is displayed. After all service codes are displayed, the throttle kicker will retract.

Key On, Engine Off Test

Turn key on, but do not start engine. Watch voltmeter for code pulses which should appear within 5-30 seconds. Ignore any initial surge of voltage when ignition is turned on. Record code(s) and repair any problems indicated by codes.

Note. If voltmeter and vacuum gauge do not pulse, or show steady high or low readings, see NO SELF-TEST OUTPUT. Engine Running Test

1. Verify that engine is at normal operating temperature. Run engine at 2000 RPM for 2 minutes. Turn key off and immediately restart engine. Observe voltmeter and vacuum gauge for initialization pulses after restarting engine. Throttle kicker should come on (engine speed increases) at this time and remain on throughout test.
2. To test knock sensor, have steel bar (socket extension) and 4 oz. hammer ready. As soon as 4 initialization pulses appear, hold steel bar against manifold near knock sensor. Tap bar lightly with hammer for 15 seconds. NOTE: If more than 4 initialization pulses appear, check for open circuit between coil and MCU connector pin 8.
3. Observe and record service codes. When codes are complete (about 90 seconds), throttle kicker will retract. Stop engine, remove test equipment and reconnect PCV valve. Perform tests for codes in the order that codes were displayed.

Note. When code 11 (code 62 for vehicles above 4000 ft.) is displayed for vehicles with a drive complaint of detonation (at WOT only) and/or poor performance, and vehicle is equipped with a universal ignition module, perform SPARK KNOCK TEST. On all other vehicles, testing is complete and MCU is functioning.

No Start Test

This test identifies faults in the MCU only, not other causes of No-Start condition.

1. Turn ignition off. Disconnect coil "horseshoe" shaped connector from coil, and vehicle harness from ignition module (4-wire connector). Check resistance between coil connector "Tach Test" terminal and ground. If resistance is less than 1000 ohms, go to next step. If greater than 1000 ohms, MCU is okay.
2. Disconnect vehicle harness connector at MCU. Measure resistance between pin 8 of harness connector and ground. (Scheme 44) If greater than 1000 ohms, replace MCU and retest. If less than 1000 ohms, repair circuit from MCU to ignition coil connector, then retest.

Scheme 44

No Self-Test Output

Disconnect jumper wire from Self-Test Trigger in Self-Test connector.

1. Disconnect harness connector at MCU. Connect a voltmeter between pins 14 and 20 in harness connector. With ignition on, 10.5 volts or more should be present. If so, go to step 3). If not, go to next step.
2. Connect voltmeter between pin 20 and ground. If voltage is more than 10.5 volts, repair circuit from pin 14 to ground, then retest. If not 10.5 volts, repair circuit from pin 20 to battery, then retest.
3. With ignition off, check continuity of circuit from Self-Test connector (Blk/Lt. Grn) to ground. If resistance is less than 1000 ohms, go to next step. If greater, repair circuit.
4. Check for continuity between MCU harness connector pin 19 and Self-Test Trigger socket (Tan/Red). If resistance is less than 5 ohms, go to next step. If greater than 1000 ohms, repair circuit, then retest.
5. Check continuity between harness connector pin 8 and "Tach Test" lead at coil "horseshoe" connector (remove connector from coil). If resistance is 5 ohms or less, go to next step. If greater than 5 ohms, repair circuit, then retest.
6. Disconnect harness connector at canister purge solenoid. Check continuity between MCU connector pin 9 and Gry/Yel Hash wire at canister purge connector. Also check between pin 20 and Gry/Yel Stripe wire. If resistance of each circuit is less than 5 ohms, go to next step. If greater than 5 ohms, repair circuit, then retest.
7. Connect ohmmeter across canister purge solenoid. Resistance should be 50-100 ohms. If not, replace solenoid. Retest. If no service code is present, replace MCU. If resistance meets specifications, go to next step.
8. Connect ohmmeter between engine ground and Gry/Yel Hash wire of harness at canister purge solenoid. If resistance is less than 1000 ohms, go to next step. If greater than 1000 ohms, repair short circuit to ground, then retest.
9. Check to ensure canister purge solenoid passes vacuum when energized with 12 volts, and blocks vacuum when de-energized. Check housing for leaks. If solenoid is okay, replace MCU, then retest. If not, replace solenoid and retest.

Scheme 45

Spark Knock Test

Note. Test should be performed if spark knock is detected only at wide open throttle, and/or exhibits poor performance or mileage.

Scheme 46

1. Disconnect knock sensor, check ignition timing and reset as necessary. With engine at normal operating temperature, disconnect 2-wire ignition module connector (Yel and Blk/Wht wires). Jumper wires together at module. (Scheme 46)
2. Check ignition timing. If timing retards 16-20°, reconnect module and go to next step. If timing does not retard, replace module and repeat test.
3. With vehicle at normal operating temperature, set fast idle screw on cam step that will maintain engine speed above 2400 RPM. Read and record ignition timing.
4. Locate zone vacuum switch assembly, then remove and plug vacuum supply hose. (Scheme 47) Recheck timing. If timing retards more than 5°, go to step 6). If timing does not retard 5°, go to next step.
5. While engine is still at 2400 RPM, disconnect 2-wire connector at ignition module. If timing retards, check Yel wire for short to ground. If okay, replace MCU and retest. If timing does not retard, check Yel wire for open circuit. If okay, replace MCU and retest.
6. Reconnect knock sensor. Repeat ENGINE RUNNING TEST, but DO NOT simulate knock by tapping rod on intake manifold. If service code 25 is obtained, MCU system is okay. If any other code is observed, replace knock sensor and retest.

Scheme 47

INSTRUCTIONS FOR USING THE SUB-ROUTINE TESTS

Sub-routines are performed to correct a service code. Be sure to perform check as instructed. After replacing components or repairing circuits, repeat FUNCTIONAL TEST and check engine operation.

Observe the following instructions when performing sub-routines

1. DO NOT measure voltage or resistance at MCU, or connect test lamps to it (unless specific instructions say to do so).
2. Disconnect both ends of a circuit when looking for continuity or shorts. Be sure ignition is turned off.
3. Disconnect solenoids and switches from harness before measuring resistance or continuity.
4. When more than one service code is indicated, start service with the first code received.
5. Use diagrams to locate pin locations and connectors.

Note. Partial wiring diagrams are provided for most SUB-ROUTINE TESTS to aid in servicing. For complete system wiring diagram and connector terminal locations (Scheme 59)

DIAGNOSTIC CODE 12 - IDLE SPEED INCORRECT

Adjust carburetor idle speed and retest. Check all connections to carburetor, throttle actuator, and throttle kicker solenoid. Repair if necessary, then repeat.

1. Disconnect Self-Test input. Tee vacuum gauge to throttle kicker. Start engine and run at idle. Turn A/C on; vacuum should be present. Turn A/C off; no vacuum should be present.
2. If vacuum switches, turn ignition off and continue test. If no vacuum is present or vacuum does not switch, check vacuum line for leaks, blockage and correct routing. If system is okay, go to step 4).
3. Turn ignition off and apply 10 in. Hg vacuum to throttle actuator. Arm should extend and hold position while vacuum is applied. If not, repair or replace actuator, then retest. If actuator operates properly, go to next step.
4. Disconnect wiring from throttle kicker solenoid and measure resistance of solenoid. If resistance is between 45-90 ohms, go to next step. If not, replace solenoid and retest.
5. Check circuit resistance between throttle kicker relay connector and solenoid, then between solenoid and ground. If resistance is less than 5 ohms, go to next step. If greater than 5 ohms, repair circuit and then retest.
6. Disconnect harness connector from MCU and throttle kicker relay. Check circuits for continuity between MCU connector pin 12 (Red/Lt. Grn wire) and throttle kicker relay connector, and pin 20 and relay connector (Gry/Yel). If resistance is more than 5 ohms, repair circuit then retest. If less than 5 ohms, go to next step.
7. Connect ohmmeter between MCU connector pin 12 and ground. If resistance is less than 1000 ohms, repair circuit then retest. If more than 1000 ohms, go to next step.
8. Reconnect throttle kicker relay and solenoid to harness. Leave MCU connector disconnected. Make sure A/C is off. Turn ignition on, leaving engine off. Connect voltmeter across solenoid terminals (with connector on solenoid). Ground pin 12 of MCU connector and observe voltmeter. If greater than 10 volts, verify that circuit 69 is not shorted to circuit 687. (Scheme 48) Repair harness if necessary, then replace MCU module. If less than 10 volts, replace relay then retest.

Scheme 48

DIAGNOSTIC CODE 25 - KNOCK DETECTION SYSTEM

Ensure that intake manifold was tapped with steel object within 2" from knock sensor. If not, retest, then remove jumper wire from Self-Test Trigger.

1. With ignition off, disconnect harness from knock sensor. Using 1 1/8" deep socket, loosen sensor and tighten to 12-18 ft. lbs. (16-24 N.m). Go to next step.
2. Disconnect MCU connector. Connect ohmmeter between engine ground and pin 13 of MCU connector. If resistance is less than 1000 ohms, repair short circuit to ground, then retest. If more than 1000 ohms, go to next step.
3. Check continuity between pin 13 and knock sensor connector (Yel/Red wire). If less than 5 ohms, go to next step. If more than 5 ohms, repair circuit, then retest.
4. Check continuity between pin 14 and knock sensor connector (Blk/Lt. Grn wire). If less than 5 ohms, connect MCU and go to next step. If more than 5 ohms, repair circuit, then retest.
5. Reconnect Self-Test Trigger jumper on Self-Test connector. Connect a test lamp to battery positive terminal. Disconnect knock sensor. Perform steps 1) and 2) of ENGINE RUNNING TEST. Instead of tapping manifold with rod, tap test lamp to Yel/Red contact in knock sensor connector for 5 seconds.
6. Observe voltmeter. If code 25 appears, replace MCU, then retest. If any other code appears, replace knock sensor, then retest.

Scheme 49

DIAGNOSTIC CODE 41 - FUEL ALWAYS LEAN

Run engine at 2000 RPM for 1 minute, then retest. If code still appears, disconnect jumper wire from Self-Test connector and proceed.

1. Turn ignition off. Disconnect oxygen sensor. Connect jumper wire from oxygen sensor to harness. Connect DVOM between engine and jumper. Connect vacuum pump to zone vacuum switches.
2. Run engine at 2000-2300 RPM while applying 10-12 in. Hg to vacuum switch assembly. Wait 2 minutes, then check voltmeter. If voltage remains below 0.5 volts, go to next step. If voltage remains above 0.5 volts, go to step 8). If voltage alternates above and below 0.5 volts, go to step 17).
3. Turn engine off. Reconnect oxygen sensor. Disconnect thermactor air hose from air pump. Disconnect harness connector at MCU. Connect DVOM between MCU connector pin 3 and engine ground (pin 14). Run engine at idle.
4. Place carburetor on high idle cam. Depress CVR rod on top of carburetor to force system rich and observe voltmeter. If voltage is less than 0.5 volts, go to next step. If greater than 0.5 volts, go to step 8).
5. Turn engine off. Disconnect oxygen sensor (harness at MCU is still disconnected). Connect ohmmeter between MCU connector pin 14 and engine ground. If resistance is more than 5 ohms, repair circuit, then retest.
6. Check continuity, of Dk. Grn/Pnk wire. If resistance is 5 ohms or less, go to next step. If resistance is over 5 ohms, repair circuit.
7. Connect ohmmeter between engine ground and Dk. Grn/Pnk wire. If resistance is less than 1000 ohms, repair circuit, then retest. If resistance is more than 1000 ohms, replace oxygen sensor, then retest.
8. Thermactor hose should still be disconnected from air pump. With ignition off, reconnect harness to MCU and oxygen sensor. Disconnect harness from feedback motor on carburetor. Using a known good feedback motor, connect harness to motor and fully depress shaft. DO NOT remove motor from carburetor. Apply 10-12 Hg vacuum to zone vacuum switches.
9. Start engine and set fast idle cam on low speed step. Observe feedback motor shaft and remove PCV hose from carburetor. Vehicle may run rough due to vacuum leak. If feedback motor shaft fully extends, reconnect PCV hose and thermactor hose, then go to step 13). If shaft does not extend, reconnect PCV hose and go to next step. Disconnect substitute motor.
10. Turn ignition off. Disconnect harness connector at MCU and feedback motor. Check continuity of circuits from feedback connector to MCU connector. If resistance is more than 5 ohms in any circuit, repair circuit(s), then retest. If less than 5 ohms in each circuit, go to next step.
11. With ignition off, connect 1 ohmmeter lead to FBCA terminal 687 (Gry/Yel wire) and other ohmmeter lead to each of 4 outside FBCA terminals in turn. Measure resistance of each FBCA winding. If all resistances are 50-175 ohms at room temperature or above, go to next step. If any reading is not within 50-175 ohms, replace FBCA motor, then retest.
12. Connect 1 ohmmeter lead to engine ground. Check all circuits in feedback harness connector (except Gry/Yel wire) for continuity with ground. If resistance is more than 1000 ohms in all circuits, replace MCU, then retest. If less than 1000 ohms, repair circuit, then retest.
13. With ignition off, connect ohmmeter lead to feedback motor case and other lead to terminal 687 (Gry/Yel wire) of motor. If resistance is less than 190,000 ohms replace motor, then retest. If greater than 190,000 ohms, go to next step.
14. Remove feedback motor from carburetor. Check pintle shaft for binding or damage. Connect harness to motor and turn ignition on. Shaft should extend, then retract when ignition is turned off. If pintle binds, replace pintle and retest. If motor is damaged and/or if pintle does not move, replace motor, then retest. If motor is okay, reinstall motor in carburetor and reconnect harness.
15. Turn ignition off and disconnect vehicle harness from TAD solenoid. Connect an ohmmeter across terminals at TAD solenoid. If resistance is 50-100 ohms, go to next step. If resistance is less than 50 ohms or greater than 100 ohms, replace TAD solenoid, then retest.
16. Disconnect harness connector at MCU. Measure resistance between ground and pin 10 (Wht/Blk wire) in CU connector. If resistance is less than 1000 ohms, repair circuit, then retest. If more than 1000 ohms, reconnect harness at CU and go to next step.
17. Check connections and operation of TAD solenoid and valve. If wiring and hoses are properly connect, check for carburetor leaks, hose routing errors, and operation of bowl vent solenoid.

Fuel Always Lead Test Connections. Scheme 50

DIAGNOSTIC CODE 42 - FUEL ALWAYS RICH

Vehicle must be at normal operating temperature. Disconnect jumper wire between ground and Self-Test Trigger. Ensure power is present at choke cap, cold enrichment circuit works, and choke is off.

1. With ignition off, disconnect thermactor air hose at pump, and vehicle harness at oxygen sensor. Connect a voltmeter between oxygen sensor and ground. Disconnect PCV hose from carburetor. Run engine at 2400 RPM for 60 seconds, then observe voltmeter.
2. If voltage is greater than 0.5 volts, replace oxygen sensor, then retest. If voltage is less than 0.5 volts, reconnect PCV and oxygen sensor. Go to next step.
3. Stop engine and turn ignition off. Disconnect vehicle harness from feedback motor on carburetor. Connect a known good motor (shaft fully depressed) to harness; leave original motor on carburetor. Turn ignition switch to "RUN" position. If shaft extends, then retracts slightly, go to next step. If motor does not extend, go to step 5).
4. Using a vacuum pump, apply 10-12 in. Hg vacuum to zone vacuum switches. Start engine and place on high step of fast idle cam. Let engine run for 2 minutes, then force system rich by depressing CVR rod on top of carburetor. Observe test feedback motor. If shaft does not retract, disconnect motor and go to next step. If shaft retracts until flush, turn ignition off and reconnect thermactor hose. Go to step 7).
5. With ignition off, disconnect harness connector at MCU and feedback motor. Check continuity of all wires between feedback motor connector and MCU connector. If resistance in any circuit is more than 5 ohms, repair circuit, then retest. If all are less than 5 ohms resistance, go to next step.
6. Connect 1 lead of ohmmeter to engine ground, and test pins from feedback motor connector (except Gry/Yel wire). If resistance of each circuit is more than 1000 ohms, go to next step. If resistance of any circuit is less than 1000 ohms, repair circuit, then retest.
7. Connect 1 ohmmeter lead to feedback connector terminal 687 (Gry/Yel wire) and other ohmmeter lead to each of 4 outside terminals in turn. Measure resistance of each FBCA winding. If all resistances are 50-175 ohms at room temperature or above, replace feedback motor and retest. If any winding is not 50-175 ohms, go to next step.
8. With ignition off, connect ohmmeter between feedback motor case and terminal 687 (Gry/Yel wire) of motor. If resistance is less than 190,000 ohms, replace motor, then retest. If greater than 190,000 ohms, go to next step.
9. Remove feedback motor from carburetor. Check pintle for binding or damage. Connect harness to motor and turn ignition on. Shaft should extend, then retract when ignition is turned off. If pintle binds, replace pintle and retest. If motor is damaged and/or pintle does not move, replace motor, then retest.
10. If motor is okay, check vacuum line routes, evaporative emission system, bowl vent solenoid, and crankcase for fuel contamination. If these items are okay, replace MCU and retest.

Scheme 51

Scheme 52

Scheme 53

1. Warm engine to normal operating temperature. Upper radiator hose should be hot and pressurized. Turn engine off. Restart engine at curb idle and immediately initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) within 10 seconds and record codes. If any other codes are displayed, go to appropriate tests. If code 11 is displayed, continue with «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test). If code 44 is displayed, go to next step.
2. Turn key off. Disconnect MCU from vehicle harness. Measure resistance from pin 11 (circuit 190) to dump relay pin 3, and from dump relay pin 4 to TAB solenoid. (Scheme 53) If both readings are less than 10,000 ohms, go to next step. If not, repair open in appropriate circuit.
3. With key off, measure resistance between dump relay pins 3 and 4. If reading is less than 5 ohms, replace dump relay and retest. If reading is greater than 5 ohms, go to next step.
4. With MCU disconnected, measure resistance from pin 11 (circuit 190) to TAB solenoid. Turn key on to close dump relay contacts. If reading is less than 10,000 ohms, go to next step. If not, check MCU to TAB solenoid circuit and repair as necessary.
5. Reconnect all electrical and vacuum connections. Perform «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) and record codes. If code 44 is not displayed, service other codes as necessary. If code 44 is still displayed, go to next step.
6. Remove vacuum hose from TAB valve and connect vacuum gauge to hose. Start and run engine above 2000 RPM for 2 minutes. Stop engine, restart it and initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test). Observe vacuum gauge. If vacuum is always above 5 in. Hg, go to step 8). If vacuum is always below 5 in. Hg, go to step 11). If vacuum pulses above and below 5 in. Hg, go to next step.
7. Reconnect hose to TAB valve. Remove vacuum hose from TAD valve and connect vacuum gauge to hose. Start and run engine above 2000 RPM for 2 minutes. Stop engine, restart it and initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test). Observe vacuum gauge. If vacuum pulses above and below 5 in. Hg, check and/or service thermactor air system. If vacuum is always above 5 in. Hg, go to step 14). If vacuum is always below 5 in. Hg, go to step 16).
8. With key off, measure resistance between MCU connector pin 11 (Wht/Red wire) and ground. (Scheme 54) If reading is less than 1000 ohms, repair short to ground, then retest. If reading is greater than 1000 ohms, go to next step.
9. Measure TAB solenoid resistance. If reading is not within 50-110 ohms, replace TAB solenoid and retest. If code 44 is still displayed, replace MCU. If resistance is correct, go to next step.
10. Check TAB solenoid output to make sure vacuum is not present when solenoid is de-energized. Disconnect TAB solenoid from vehicle harness. Start and run engine at idle. If vacuum is present, replace TAB solenoid, then retest. If vacuum is not present, replace MCU, then retest.
11. Turn key off. Check vacuum hose between TAB valve and TAB solenoid for blockage or leaks. If hose is damaged, repair as necessary, then retest. If hose is okay, go to step 9).
12. Check TAB solenoid output to make sure vacuum is present when solenoid is energized. Install a jumper wire from TAB solenoid circuit 190 to ground. Start and run engine at idle. If vacuum is not present, replace TAB solenoid and retest. If code 44 is still present, go to next step. If vacuum is present, go to next step.
13. Turn key off. Measure resistance between TAB solenoid and MCU (circuits 687 and 190). If reading is more than 5 ohms, repair circuit and retest. If reading is 5 ohms or less, replace MCU, then retest.
14. With key off, measure resistance between circuit 200 and ground. If reading is less than 1000 ohms, repair short to ground. If reading is more than 1000 ohms, go to next step.
15. Check TAD solenoid output to make sure vacuum is not present when solenoid is de-energized. Disconnect TAD solenoid from vehicle harness. Start and run engine at idle. If vacuum is present, replace TAD solenoid, then retest. If vacuum is not present, replace MCU, then retest.
16. Turn key off. Check vacuum hose between TAD valve and TAD solenoid for blockage or leaks. If hose is damaged, repair as necessary, then retest. If hose is okay, go to next step.
17. With key off, measure TAD solenoid resistance. If reading is not within 50-100 ohms, replace solenoid, and retest. If code 44 is still displayed, go to step 19). If resistance is correct, go to next step.
18. Check TAD solenoid output to make sure vacuum is present when solenoid is energized. Install a jumper wire from TAD solenoid circuit 200 to ground. Start and run engine at idle. If vacuum is not present, replace solenoid and retest. If vacuum is present, go to next step.
19. Turn key off. Measure resistance between TAD solenoid and MCU (circuits 687 and 200). If reading is more than 5 ohms, repair circuit and retest. If reading is 5 ohms or less, replace MCU, then retest.

DIAGNOSTIC CODE 45 - THERMACTOR AIR DIVERTER

1. Ensure that TAD vacuum line restrictor was uncapped and free of obstruction. If plugged, clean out and retest. If okay, go to next step.
2. Cap vacuum restrictor and remove vacuum delay valve from TAD vacuum control line. Reconnect system without delay valve and retest. If code 45 reappears, go to next step. If not, replace vacuum delay valve and retest.
3. Remove vacuum hose from TAD valve and connect vacuum gauge to hose. Start and run engine above 2000 RPM for 2 minutes. Stop engine, restart it and initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) . Observe vacuum gauge. If vacuum pulses above and below 5 in. Hg, check and/or service thermactor air system. If vacuum is always above 5 in. Hg, go to step 8). If vacuum is always below 5 in. Hg, go to next step.
4. Check vacuum hoses between TAD valve and TAD solenoid, then between TAD solenoid and vacuum source for blockage, leaks, and proper routing. If hoses are damaged or improperly routed, repair as necessary, then retest. If hoses are okay, go to next step.
5. With ignition off, remove TAD connector and measure TAD solenoid resistance. If between 50-110 ohms, go to next step. If not, replace solenoid, then retest.
6. Check TAD solenoid output to ensure vacuum is present when solenoid is energized. Using a jumper wire, jump circuit 200 (Wht/Blk wire) to ground. Start and run engine at idle. If no vacuum is present, replace solenoid, then retest. If vacuum is present, go to next step.
7. With ignition off, disconnect harness connector from TAD solenoid and MCU. Measure resistance between MCU connector pin 20 and TAD solenoid Gry/Yel wire, and between pin 10 and Wht/Blk wire. If resistance is less than 5 ohms, replace MCU, then retest. If greater than 5 ohms, repair circuits, then retest.
8. Connect MCU. Leave TAD solenoid disconnected. Check that TAD solenoid output to make sure vacuum is not present when solenoid is de-energized. Start and run engine at idle. If vacuum is present, replace solenoid, then retest. If vacuum is not present, go to next step.
9. With ignition off, disconnect harness connector at MCU and TAD solenoid. Measure resistance between circuit 200 and ground. If reading is less than 1000 ohms, repair short to ground, then retest. If reading is more than 1000 ohms, replace MCU, then retest.

Scheme 54

DIAGNOSTIC CODE 46 - THERMACTOR AIR BY-PASS

1. Ensure that TAB vacuum line restrictor was uncapped and free of obstruction. If plugged, clean out and retest. If okay, go to next step.
2. Cap vacuum restrictor and remove vacuum delay valve from TAB vacuum control line. Reconnect system without delay valve and retest. If code 46 reappears, go to next step. If not, replace vacuum delay valve and retest.
3. Start and warm engine to normal operating temperature. Ensure upper radiator hose is hot and pressurized. Stop engine and restart at curb idle. Immediately initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) within 10 seconds. Observe DVOM for codes. If any other code is displayed, go to appropriate test. If code 11 is displayed, continue with «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) . If code 46 is displayed, go to next step.
4. Remove vacuum hose from TAB valve and connect vacuum gauge to hose. Start and run engine above 2000 RPM for 2 minutes. Stop engine, restart it, and initiate «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) . Observe vacuum gauge. If vacuum pulses above and below 5 in. Hg, check and/or service thermactor air system. If vacuum is always above 5 in. Hg, go to step 8). If always below 5 in. Hg, go to next step.
5. Check vacuum hoses between TAB valve and TAB solenoid, and between TAB solenoid and vacuum source for blockage or leaks. If hoses are damaged, repair as necessary, then retest. If hoses are okay, go to next step.
6. Check TAB solenoid output to make sure vacuum is present when solenoid is energized. Jumper TAD solenoid circuit 190 (Wht/Red wire) to ground. Start and run engine at idle. If vacuum is not present, replace solenoid, then retest. If vacuum is present, go to next step.
7. With ignition off, disconnect harness connectors at TAB solenoid and MCU. Measure resistance between MCU pin 11 (circuit 190) and TAB solenoid Wht/Red wire. If reading is more than 10 ohms, repair circuit, then retest. If reading is less than 10 ohms, replace MCU, then retest.
8. Connect harness connector at MCU. Leave TAB solenoid disconnected. Check TAB solenoid output to make sure vacuum is not present when solenoid is de-energized. Start and run engine at idle. If vacuum is present, replace solenoid, then retest. If vacuum is not present, go to next step.
9. Measure resistance between circuit 190 and ground. If reading is less than 1000 ohms, repair short to ground, then retest. If reading is greater than 1000 ohms, replace MCU, then retest.

Scheme 55

DIAGNOSTIC CODES 51 & 55 - ZONE VACUUM SWITCH OPEN

1. Ensure that vacuum is present at switch during idle. Code 51 is for Hi/Low Vacuum switch, code 55 is for Mid-Vacuum switch. If vacuum is 10 in. Hg, go to next step. If vacuum is less than 10 in. Hg, check vacuum hoses for kinks, cracks, blockage, mechanical problems, etc. Repair as necessary.
2. Turn key off. Apply vacuum to zone vacuum switch assembly. If assembly does not hold vacuum, check vacuum hoses for leaks. If no leaks are found, replace zone vacuum switch assembly, then retest. If assembly holds vacuum, go to step 3) for code 51, and/or go to step 5) for code 55.
3. Turn key off. Disconnect Hi/Low Vacuum switch from harness. Measure resistance across Hi/Low Vacuum switch by inserting ohmmeter probes into switch connector. If resistance is less than 5 ohms, go to next step. If greater than 5 ohms, replace switch assembly, then retest.
4. Disconnect MCU 24-pin connector. Measure resistance from MCU connector pin 5 to switch connector (Dk. Grn/Lt. Grn wire), then measure resistance from pin 14 to switch connector (Blk/Lt. Grn wire). If resistance is 5 ohms or less, replace MCU and retest. If greater than 5 ohms, repair circuit, then retest.
5. Turn key off. Disconnect Mid-Vacuum switch from harness. Apply a minimum of 15 in. Hg vacuum to switch. Check resistance of switch (Yel/Blk-to-Blk/Lt. Grn wires). If resistance is greater than 5 ohms, replace switch assembly, then retest. If resistance is less than 5 ohms, reconnect hose and go to next step.
6. Disconnect MCU 24-pin connector. Measure resistance from MCU connector pin 18 to switch connector (Yel/Blk wire), then measure resistance from pin 14 to switch connector (Blk/Lt. Grn wire). If resistance is less than 5 ohms, replace MCU and retest. If greater than 5 ohms, repair circuit, then retest.

Scheme 56

DIAGNOSTIC CODES 53 & 54 - TEMPERATURE SWITCH OPEN

Code 53 is for Dual Temp. switch, code 54 is for Mid-Temp. switch. Before testing, ensure coolant temperature is 140-200°F (60-93°C).

1. Disconnect vehicle harness from switch. Connect ohmmeter across switch terminals and measure resistance. If resistance is greater than 1000 ohms, replace switch. If less than 5 ohms, go to next step.
2. Connect one lead of ohmmeter to Lt. Blu/Org Hash wire, then Org/Red Dot wire, and the other lead to ground. If resistance is 1000 ohms or less in either location, repair circuit. If resistance is 1000 ohms or more, go to next step.
3. Disconnect MCU harness connector. Check continuity of circuits from MCU side of connector to switch connector. (Pins 7 and 14 for Dual Temp. switch; pins 14 and 17 for Mid-Temp. switch). If resistance is greater than 1000 ohms, repair circuit, then retest. If less than 5 ohms, replace MCU module.

Scheme 57

DIAGNOSTIC CODE 62 - BAROMETRIC PRESSURE SWITCH

1. Make sure that the barometric pressure switch connector is mated correctly with wiring harness connector. If not, reconnect and repeat «FUNCTIONAL TEST»(/ford/ltd-crown-victoria/i-1983-1991/remont/testing-diagnostics/#58l-mcu-system__functional-test) .
2. Determine local altitude. Below 3500 ft., switch is normally open. Above 4000 ft., switch is normally closed. DO NOT test switch at altitudes between 3500 and 4000 ft. as switch may be in either condition. Disconnect harness connector from switch. Connect ohmmeter across switch terminals.
3. If resistance is greater than 10 ohms at altitudes above 4000 ft. or less than 1000 ohms below 3500 ft., replace switch and repeat test. If resistance is less than 10 ohms at altitudes above 4000 ft. or more than 1000 ohms below 3500 ft., go to next step.
4. Disconnect MCU harness connector. Check wires leading from MCU pins 14 and 6 to switch for continuity. If resistance of either wire is less than 10 ohms, replace MCU and retest. If resistance is over 10 ohms in either wire, repair open circuit, then retest.

Scheme 58

DIAGNOSTIC CODE 65 - ZONE VACUUM SWITCH CLOSED

Ensure that engine is at normal operating temperature and that vacuum hoses are connected to zone vacuum switch assembly.

1. Turn key off. Disconnect zone vacuum switch from vehicle harness. Connect ohmmeter across Mid-Vacuum switch connector (Blk/Lt. Grn and Yel/Blk wires) and measure switch resistance. If reading is less than 1000 ohms, replace switch assembly, then retest. If reading is greater than 1000 ohms, reconnect vacuum hose, and go to next step.
2. Disconnect vehicle harness from MCU. Connect ohmmeter between MCU connector pin 18 and engine ground. If resistance is greater than 1000 ohms, replace MCU. If less than 1000 ohms, repair circuits.

5.8L Police MCU System Wiring Diagram. Scheme 59

See also:
• FUNCTIONAL TEST