V8 Sub-Routines

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 and ground. If greater than 1000 ohms, replace MCU module. If less than 1000 ohms, repair circuit from MCU module to coil connector to ignition module connector.

Fig 1: No Start Test

Courtesy of FORD MOTOR CO.

Code 53 is for Dual Temperature Switch, code 54 is for Mid-Temperature Switch. Before testing, ensure temperature of coolant is between 140-200° F.

1. Disconnect vehicle harness from switch. Connect ohmmeter across switch terminals and measure resistance. If resistance is greater than 10 ohms, replace switch. If less than 10 ohms, go to next step.
2. Disconnect MCU harness connector. Check continuity of circuits from MCU connector to switch connector. (Pins 7 & 14 for Dual-Temp. Switch; pins 14 & 17 for Mid-Temp Switch). If resistance is greater than 10 ohms, repair circuit. If less than 10 ohms, replace MCU module.

Fig 2: Electrical Temperature Switch Open (Code 53 & 54)

Courtesy of FORD MOTOR CO.

Code 51 is for Hi/Low Vacuum Switch, code 55 for Mid Vacuum Switch. Use steps 1)  and 2)  for code 51, and steps 3)  through 5)  for code 55. Be sure vacuum is present at switch during idle conditions.

1. Turn ignition off. Disconnect switch from vehicle harness. Measure resistance across Hi/Low Vacuum Switch by inserting ohmmeter probes into switch connector. If resistance is less than 10 ohms, go to next step. If greater than 10 ohms, replace switch.
   NOTE: Not all calibrations use a Hi Vacuum Switch.
2. Disconnect MCU harness connector. Measure continuity from MCU connector pin 5 to switch connector (Dk Grn/Lt Grn), then measure from pin 14 to switch connector (Blk/Lt Grn). If resistance is 10 ohms or less, replace MCU module. If greater than 10 ohms, repair circuits.
3. Start engine and run at idle. Disconnect Mid Vacuum Switch hose. Check for manifold vacuum at hose. If vacuum exists, go to next step. If not, check hose for plugs, leaks, or wrong connections.
4. Turn ignition off. Disconnect vehicle harness from switch connector. Apply a minimum of 15 in. Hg vacuum to switch. Check resistance of switch. If resistance is less than 10 ohms, reconnect hose and go to next step. If resistance is greater than 10 ohms, replace switch assembly.
5. Disconnect MCU harness connector. Measure continuity from MCU connector pin 18 to switch connector (Yel/Blk), then measure from pin 14 to switch connector (Blk/Lt Grn). If resistance is less than 10 ohms, replace MCU module. If greater than 10 ohms, repair circuit.

Fig 3: Vacuum Switch Open (Codes 51 & 55)

Courtesy of FORD MOTOR CO.

Ensure that vacuum hoses are connected to switch and engine coolant is between 140-200° F.

1. Turn engine and ignition off and disconnect vacuum switch from vehicle harness. Connect ohmmeter across switch connector to measure switch resistance. If resistance is greater than 1000 ohms, reconnect switch. If less than 1000 ohms replace switch assembly.
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.

Fig 4: Vacuum Switch Closed (Code 65)

Courtesy of FORD MOTOR CO.

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

1. Turn ignition off. Disconnect oxygen sensor, then connect a jumper wire from oxygen sensor to vehicle harness. Connect DVOM between engine block and jumper wire. Start engine and run at 2000-2300 RPM while applying 9-18 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  4. If voltage alternates above and below 0.5 volts, go to step  12.
2. Reconnect oxygen sensor to vehicle harness, then disconnect Thermactor air hose at pump. Disconnect MCU connector and connect DVOM between MCU connector pin 3 and engine ground. Place carburetor on high idle cam, then depress CVR rod on top of carburetor to force system rich. If voltage is less than 0.5 volts, go to next step. If greater than 0.5 volts, go to step  5.
3. Turn ignition off. Disconnect oxygen sensor from vehicle harness (leaving MCU still disconnected). Connect ohmmeter between MCU connector pin 3 and ground. Do not touch metal portion of leads while making measurement. If ohmmeter shows an open circuit, replace oxygen sensor. If resistance is less than 190, 000 ohms, repair circuit.
4. With ignition off, disconnect MCU harness connector and oxygen sensor. Connect ohmmeter to oxygen sensor connector (vehicle harness side) and MCU connector pin 3. If resistance is less than 10 ohms, replace MCU module. If greater than 10 ohms, repair circuit.
5. Reconnect all wiring, then start engine and run on low step of fast idle cam. Disconnect harness from feedback (FBCA) motor on carburetor. Let engine run 2 minutes, then apply at least 15 in. Hg vacuum to zone switch assembly. Connect a known good feedback motor (with shaft fully depressed) to vehicle harness. While observing motor shaft, disconnect PCV hose from carburetor. If shaft does not extend, go to next step. If shaft extends in 10-15 seconds, reconnect PCV and Thermactor hoses and go to step  8.
6. Turn ignition off. Disconnect MCU harness connector and feedback motor connector. Use an ohmmeter to check continuity of circuits from feedback connector to MCU connector. If resistance is more than 10 ohms in any circuit, repair circuit. If less than 10 ohms in all circuits, go to next step.
7. Connect 1 ohmmeter lead to engine ground. Check all circuits in feedback connector (except Gray/Yellow) for continuity with ground. If resistance is more than 1000 ohms in all circuits, go to next step. If not, repair circuit.
8. With ignition off, connect ohmmeter lead to feedback (FBCA) motor case and other lead to center pin (directly below blank space) of motor. If resistance is less than 190, 000 ohms, replace motor. If greater than 190, 000 ohms, go to next step.
9. Remove feedback motor from carburetor. Check for sticking shaft or dirt. Connect harness to motor and turn ignition on. Shaft should extend, then retract when ignition is turned off. If motion is uneven or shaft rotates, replace motor. Reinstall motor in carburetor.
10. Turn ignition off and disconnect vehicle harness from TAD solenoid. Connect an ohmmeter across TAD solenoid leads. If resistance is 50-100 ohms, go to next step. If not, replace TAD solenoid.
11. Disconnect MCU connector and TAD connector. Measure resistance between ground and pin 10 in MCU connector. If less than 1000 ohms, repair circuit. If more than 1000 ohms, reconnect MCU and go to next step.
12. Check connections and operation of TAD solenoid and valve. If wiring and hoses are properly connected, check for carburetor leaks, hose routing errors, and operation of bowl vent solenoid.

Fig 5: Fuel Always Lean (Code 41)

Courtesy of FORD MOTOR CO.

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. Disconnect Thermactor air hose at pump, and vehicle harness at oxygen sensor. Connect a voltmeter between oxygen sensor and ground, then disconnect PCV hose from carburetor. Run engine at 1800 RPM for 60 seconds, then observe voltmeter. If voltage is greater than 0.5 volts, replace oxygen sensor. If voltage varies at less than 0.5 volts, reconnect PCV and oxygen sensor. Go to next step.
2. Disconnect vehicle harness from feedback motor on carburetor. Connect a known good motor (with shaft fully depressed) to harness, but leave original motor in carburetor. Turn ignition on. If shaft extends, then retracts slightly, go to next step. If motor does not extend, go to step  4.
3. Apply 10-12 in. Hg vacuum to 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, reconnect Thermactor hose to pump, and go to step  6.
4. Disconnect MCU connector from MCU, and disconnect feedback motor connector. Check continuity of all wires between feedback motor connector and MCU connector. If resistance in any circuit is more than 10 ohms, repair circuit. If all are less than 10 ohms resistance, go to next step.
5. Connect one lead of ohmmeter to engine ground, and test pins from feedback motor connector (except Gray/Yellow). If resistance of each circuit is more than 1000 ohms, replace MCU. If resistance of any circuit is less than 1000 ohms, repair circuit.
6. With ignition off, connect ohmmeter between feedback motor case and center pin (directly below blank space) in motor. If resistance is less than 190, 000 ohms replace motor. If greater than 190, 000 ohms (or infinity), go to next step.
7. Remove feedback motor from carburetor. Check for sticking shaft or dirt. Connect harness to motor and turn ignition on. Shaft should extend, then retract when ignition is turned off. If motion is uneven, or shaft rotates, replace motor. Reinstall good motor in carburetor and check vacuum lines, evaporative emissions system, and crankcase for fuel dilution.

Fig 6: Fuel Always Rich (Code 42)

Courtesy of FORD MOTOR CO.

Adjust carburetor idle speed and retest. Check all connections to carburetor and throttle actuator or kicker. If not equipped with vacuum throttle kicker, start test at step  3.

1. If equipped with vacuum actuated throttle kicker, check for manifold vacuum at kicker at idle. If vacuum is present, stop engine and go to next step. If no vacuum is present, repair vacuum lines and retest.
2. 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 does operate, go to next step.
3. 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.
4. Check circuit resistance between throttle kicker relay connector and solenoid, then between solenoid and ground. If resistance is less than 10 ohms, go to next step. If greater than 10 ohms, repair circuit.
5. Disconnect MCU connector. Check circuits for continuity between MCU pin 12 and throttle kicker relay connector (Red/Lt Grn), and pin 20 and relay connector (Gray/Yellow). If resistance is more than 10 ohms, repair circuit. If less than 10 ohms, go to next step.
6. Connect ohmmeter between MCU connector pin 12 and ground. If resistance is less than 1000 ohms, repair circuit. If more than 1000 ohms, go to next step.
7. Leave MCU connector disconnected. Make sure A/C switch is off. Turn ignition on and connect voltmeter across solenoid terminals (with connector on solenoid). Ground pin 12 of MCU connector and observe voltmeter. If greater than 10 volts, replace MCU module. If less than 10 volts, replace relay.

Fig 7: Idle Speed Incorrect (Code 12)

Courtesy of FORD MOTOR CO.

1. Check for manifold vacuum at input to TAB-TAD solenoids. If vacuum is present, go to next step. If not, check hose condition and routing.
2. Remove vacuum hose from TAB valve and connect vacuum gauge to hose. Start engine and run at 2000 RPM for 2 minutes. Stop engine, then immediately restart and allow to idle. Begin Self-Test and watch vacuum gauge. If it pulses above and below 5 in. Hg, go to next step. If gauge is always above 5 in. Hg, go to step  4. If always below 5 in. Hg, go to step  6.
3. Reconnect hose to TAB valve. Remove hose from TAD valve and connect vacuum gauge to hose. Start engine, run for 2 minutes above 2000 RPM, then stop engine. Restart immediately and observe vacuum gauge as Self-Test starts. If vacuum pulses above and below 5 in. Hg, check Thermactor pump operation. If vacuum is always above 5 in. Hg, go to step  10. If below 5 in. Hg, go to step  12.
4. Disconnect MCU connector and TAB connector. Measure resistance between MCU connector pin 11 and ground. If less than 1000 ohms, repair short to ground. If greater than 1000 ohms, go to next step.
5. Check at TAB solenoid output to make sure vacuum is not present when solenoid is deactivated (engine idling, harness disconnected). If vacuum is present, replace TAB solenoid. If no vacuum, replace MCU module.
6. Check vacuum hose between TAB valve and TAB solenoid for damage. If damaged, repair or replace. If hose is okay, go to next step.
7. Measure TAB solenoid resistance. If not within 50-110 ohms, replace solenoid. If resistance is correct, go to step 8)  .
8. Check at TAB solenoid output to be sure vacuum is present when solenoid is activated (engine idling, 12 volts applied). If no vacuum, replace TAB solenoid. If vacuum is present, go to next step.
9. Disconnect MCU connector and TAB solenoid connector. Continuity should exist between MCU pin 11 and White/Red wire at connector, and between pin 20 and Gray/Yellow wire at connector. If resistance is more than 5 ohms, repair circuit. If less than 5 ohms, replace MCU module.
10. Disconnect MCU connector and TAD solenoid connector. Measure resistance between MCU pin 10 and ground. If resistance is less than 1000 ohms, repair short. If more than 1000 ohms, go to next step.
11. Check at TAD solenoid output to ensure vacuum is not present with engine idling and TAD solenoid connector disconnected. If vacuum is present, replace TAD solenoid. If no vacuum, replace MCU module.
12. Check vacuum hose between TAD valve and TAD solenoid for blocks or leaks. If problems are found, repair. If hose is okay, go to next step.
13. Remove TAD solenoid connect and measure resistance across TAD solenoid. If not between 50-110 ohms, replace solenoid. If resistance is between 50-110 ohms, go to next step.
14. Check at TAD solenoid output to ensure vacuum is present with engine idling and solenoid energized with 12 volts. If vacuum is present, go to next step. If not, replace solenoid.
15. Measure resistance of White/Black wire between MCU pin 10 and TAD connector, then resistance of Gray/Yellow wire between pin 20 and TAD connector. If resistance in either circuit is more than 5 ohms, repair circuit. If less than 5 ohms, replace MCU module.

Fig 8: Thermactor System (Code 44)

Courtesy of FORD MOTOR CO.

1. Ensure that TAD vacuum line restrictor was uncapped and is 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 and retest. If code 45 reappears, go to next step. If not, replace vacuum delay valve with new part and retest.
3. Remove vacuum hose from TAD valve and connect a vacuum gauge to hose. Start engine and run above 2000 RPM for 2 minutes, then stop engine. Restart engine and initiate Self-Test, then watch vacuum gauge. If vacuum pulses above and below 5 in. Hg, check Thermactor pump. 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 solenoid, then between solenoid and source. Repair if necessary. If hoses are okay, go to next step.
5. Remove TAD connector and measure TAD solenoid resistance. If between 50-110 ohms, go to next step. If not, replace solenoid.
6. Check at TAD solenoid output to ensure vacuum is present when solenoid is energized by 12 volts. If no vacuum, replace solenoid. If vacuum is present, go to next step.
7. Disconnect both TAD and MCU connectors. Measure resistance between MCU pin 20 and TAD Gray/Yellow wire, and between pin 10 and White/Black wire. If resistance is less than 5 ohms, replace MCU module. If greater than 5 ohms, repair circuits.
8. Check at TAD solenoid output that vacuum is not present when solenoid is deactivated (engine idling, solenoid disconnected). If vacuum is present, replace solenoid. If not present, go to next step.
9. Disconnect MCU and TAD connectors. Connect ohmmeter between MCU connector pin 10 and ground. If resistance is less than 1000 ohms, repair short to ground. If more than 1000 ohms, replace MCU module.

Fig 9: Thermactor Air Diverter (Code 45)

Courtesy of FORD MOTOR CO.

1. Remove vacuum hose from TAB valve and connect vacuum gauge to hose. Start engine and run above 2000 RPM for 2 minutes, then stop engine. Restart engine and initiate Self-Test. Watch vacuum gauge. If vacuum pulses above and below 5 in. Hg, check Thermactor pump. If vacuum is always above 5 in. Hg, go to step  5. If always below 5 in. Hg, go to next step.
2. Check vacuum hoses between TAB valve, solenoid, and vacuum source. If hoses are leaking or blocked, repair. If hoses are okay, go to next step.
3. Check at TAB solenoid output to ensure vacuum is present with engine idling and 12 volts applied to solenoid. If no vacuum, replace solenoid. If vacuum is present, go to next step.
4. Disconnect TAB connector and MCU connector. Measure resistance between TAB connector wire (White/Red) and MCU connector pin 11. If less than 10 ohms, replace MCU. If more than 10 ohms, repair circuit.
5. Check TAB solenoid output to ensure vacuum is not present when solenoid is deactivated. If vacuum is present, replace solenoid. If vacuum is not present, go to next step.
6. Measure resistance between MCU connector pin 11 and ground. If resistance is less than 1000 ohms, repair short. If greater than 1000 ohms, replace MCU module.

Fig 10: Thermactor Air By-Pass (Code 46)

Courtesy of FORD MOTOR CO.

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 torque to 12-18 ft. lbs. Do not overtighten. Go to next step.
2. Disconnect MCU connector. Connect ohmmeter between engine ground and pin 13 in MCU connector. If resistance is less than 1000 ohms, repair circuit. If more than 1000 ohms, go to next step.
3. Check continuity between pin 13 and knock sensor connector (Yellow/Red). If less than 10 ohms, go to next step. If not, repair circuit.
4. Check continuity between pin 14 and knock sensor connector (Black/Lt Green). If less than 10 ohms, go to next step. If not, repair circuit.
5. Reconnect Self-Test Trigger jumper on Self-Test connector. Connect a 12v test lamp to battery positive terminal, then disconnect knock sensor. Perform first step of MCU Running Test, then instead of tapping manifold with rod, tap test lamp to Yellow/Red contact in knock sensor connector for 5 seconds. Observe voltmeter. If code 25 appears, replace MCU module. If any other code appears, replace knock sensor.

Fig 11: Knock Detection System (Code 25)

Courtesy of FORD MOTOR CO.

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 MCU 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 now more than 10.5 volts, repair circuit from pin 14 to ground. If not, repair circuit from pin 20 to battery.
3. With ignition off, check continuity of circuit from MCU connector pin 14 to Self-Test Connector to ground. If resistance is less than 10 ohms, go to next step. If not, repair circuits.
4. Check for continuity between MCU connector pin 19 and Self-Test Trigger socket. If resistance is less than 10 ohms, go to next step. If not, repair circuit.
5. Check continuity between MCU connector pin 8 and "Tach Test" lead at coil "horseshoe" connector (remove connector from coil). If resistance is 10 ohms or less, go to next step. If not, repair.
6. Disconnect canister purge connector. Check continuity between MCU connector pin 9 and Gray/Yellow Stripe wire at canister purge connector, then between pin 20 and Gray/Yellow Hash wire. If resistance of each circuit is less than 10 ohms, go to next step. If not, repair circuits.
7. Connect ohmmeter across canister purge solenoid. Resistance should be 50-100 ohms. If not, replace solenoid. If so, go to next step.
8. Connect ohmmeter between engine ground and Gray/Yellow Hash wire at canister purge solenoid. If resistance is greater than 1000 ohms, go to next step. If less, repair short circuit to ground.
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 module. If not, replace solenoid and retest.

Fig 12: No Self-Test Output

Courtesy of FORD MOTOR CO.