Diagnostic Aids

WARNING: This page is about a different car, the 1995 Ford Explorer and 1995 Ford Aerostar. However, it is still accessible from the selected car via links, so may be relevant.

Perform this test only when directed by QUICK TEST. To prevent replacing good components, be aware following non-EEC areas may be at fault:

Engine base condition (compression, valve timing, etc.).
Brakes.
Transmission fluid, friction elements and cooling.
Transfer case linkage or internal condition. This test is only intended to diagnose:
Harness circuits (4x4 LOW, TCIL and TCS).
Faulty PCM.

Fig 1: Identifying 4x4 Low Switch Circuit (Touch Drive Option)

Fig 2: Identifying 4x4 Low Switch Circuit (W/O Touch Drive Option)

Fig 3: Identifying 4x4 Low Switch Circuit (Explorer)

1) Codes 633 & 691 Code 633 indicates 4x4 low selector lever is not in 4x2 or 4x4 HIGH position during KOEO SELF-TEST. Code 691 indicates 4x4 selector fault is set when KAM value for engine speed/vehicle speed disagrees with PCM logic regarding engine speed/vehicle speed verses TCS position. Possible causes for these faults are:

Faulty 4x4 Low switch or 4x4 selector lever position.
Transfer case damaged internally.
Wiring harness circuit open or grounded.
Faulty PCM.

If 4x4 low selector lever was in 4x2 or 4x4 HIGH position during KOEO SELF-TEST, go to step 4). If 4x4 low selector was not in 4x2 or 4x4 HIGH position, repeat KOEO SELF-TEST with lever in 4x2 or 4x4 HIGH position.

2) Codes 632 & 653 Codes 632 and 653 indicate TCS is not cycled between engine ID code and WOT check during KOER SELF-TEST. Possible causes for these faults are:

Faulty TCS or switch not cycled.
Wiring harness circuit open or grounded.
Faulty PCM.

Ensure TCS is cycled between engine ID code and WOT check during KOER SELF-TEST, and go to step 4).

3) Codes 631 & 623 Codes 631 and 623 indicate TCS light circuit fault during KOEO SELF-TEST. Possible causes for these faults are:

TCS bulb burned out.
Wiring harness circuit open or grounded.

Ensure TCS is cycled during KOEO SELF-TEST, and go to step 5).

4) Cycle 4x4 Or Overdrive Cancel Switch Turn ignition off. Disconnect 60-pin PCM connector. Inspect terminals and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Turn ignition on.

To check 4x4 LOW circuit, measure voltage between test pin No. 12 and test pins No. 40 and 60 at breakout box while cycling switch in and out of 4x4 low position.
To check TCS circuit, measure voltage between test pin No. 41 and test pins No. 40 and 60 at breakout box while cycling overdrive cancel switch.

On all models, if voltage cycles, replace PCM and repeat QUICK TEST. If voltage does not cycle, go to next step if diagnosing a trouble code or step 7) if diagnosing driveability symptom only.

5) Check Circuits For Short To Ground Turn ignition off. Leave PCM disconnected from breakout box. Disconnect switch connector. Inspect for damage and repair as necessary.

To check 4x4 LOW circuit, measure resistance between test pin No. 12 and test pins No. 40 and 60 at breakout box.
To check TCS circuit, measure resistance between test pin No. 41 and pins No. 40 and 60 at breakout box.

Also measure resistance between test pin No. 32 and pins No. 40 and 60 at breakout box. If all readings are more than 10,000 ohms, go to next step for Code 631 or 623, or step 8) for Code 633, 632, 633 or 691. If any reading is 10,000 ohms or less, repair short circuit. Repeat QUICK TEST. If code is still present, go to step 7).

6) Check Power Through TCIL Circuit Turn ignition off. Leave PCM disconnected from breakout box. Measure voltage between test pin No. 32 and test pins No. 40 and 60 at breakout box. If voltage is more than 10.5 volts, replace PCM and repeat QUICK TEST. If voltage is 10.5 volts or less, go to next step.

7) Check Output Driver Voltage Signal Leave PCM disconnected from breakout box. Turn ignition on.

To check 4x4 LOW circuit, disconnect switch. Connect DVOM positive lead to test pin No. 12 and negative lead to PWR GND terminal of breakout box.
To check TCIL circuit, connect DVOM positive lead to test pin No. 32 and negative lead to PWR GND terminal of breakout box.

On all models, if either voltage reading is less than 2 volts, check bulb and fuse. If bulb and fuse are okay, repair open circuit and repeat QUICK TEST. If all voltage readings are 2 volts or more, go to next step.

Fig 4: Identifying KEY POWER Terminal in Fuse Panel (4.0L)

8) Check Circuit Continuity Turn ignition off. Leave PCM disconnected from breakout box. Leave switch disconnected.

To check 4x4 LOW circuit, measure resistance between test pin No. 12 at breakout box and 4x4 LOW LED circuit at switch wiring harness connector.
To check TCS/TCIL circuit, connect DVOM positive lead on KEY POWER terminal of fuse panel and negative lead on power side of TCS wiring harness connector. Also connect DVOM positive lead on test pin No. 41 at breakout box and negative lead on power side of TCS wiring harness connector.

On all models, if any resistance reading is 5 ohms or more, repair open circuit and repeat QUICK TEST. If all resistance readings are less than 5 ohms, go to next step.

9) Check Circuits For Short To Power Turn ignition off. Leave PCM disconnected from breakout box. Leave switch disconnected.

To check 4x4 LOW circuit, measure resistance between test pin No. 12 and test pins No. 37 and 57 at breakout box.
To check TCS circuit, measure resistance between test pin No. 32 and test pins No. 37 and 57 at breakout box. Measure resistance between test pin No. 41 and test pins No. 37 and 57 at breakout box.

On all models, if each resistance is more than 10,000 ohms, replace switch and repeat QUICK TEST. If any resistance is 10,000 ohms or less, repair short circuit and repeat QUICK TEST.

10) Code 633 & 691 (Explorer) Code 633 indicates 4x4 low switch is not in 4x2 or 4x4 HIGH position during KOEO SELF-TEST. Code 691 indicates 4x4 low switch failure. Possible causes for these faults are:

Faulty 4x4 low switch
Faulty Generic Electronic Module (GEM).
Wiring harness circuit open or grounded.
Faulty PCM.

If 4x4 low switch was in 4x2 or 4x4 HIGH position during KOEO SELF-TEST, go to next step. If 4x4 low switch was not in 4x2 or 4x4 HIGH position, repeat KOEO SELF-TEST with switch in 4x2 or 4x4 HIGH position.

11) Check 4x4 Low Signal Turn ignition off. Disconnect 60-pin PCM connector. Inspect terminals and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Turn ignition on. Measure voltage between test pin No. 42 and test pin No. 40 or 60 at breakout box while cycling 4x4 low switch. If voltage cycles, replace PCM. Reconnect all components and repeat QUICK TEST. If voltage does not cycle, go to next step.

12) Check For Intermittent Fault Turn ignition off. With 4x4 low switch in 4x2 position, measure voltage between test pin No. 42 and test pin No. 40 or 60 at breakout box. Voltage should be 5 volts. Tap, shake, move, wiggle wiring harness between transfer case and Generic Electronic Module (GEM), and between GEM and Powertrain Control Module (PCM). If voltage fluctuates, isolate fault in wiring harness and repair as necessary. Reconnect all components and repeat QUICK TEST. If voltage does not fluctuate, go to next step.

13) Check 4x4 Low Wire Continuity Turn ignition off. Disconnect GEM connectors. Inspect terminals and repair if damaged. Measure resistance of Light Blue/Black wire between test pin No. 42 at breakout box and terminal No. 10 at GEM 22-pin connector. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit. Reconnect all components and repeat QUICK TEST.

14) Check For Short Circuit Ensure ignition is off, and PCM and GEM are disconnected. Measure resistance between test pin No. 42 and test pin No. 40 or 60 at breakout box. Also measure resistance between test pin No. 42 and test pin No. 37 or 57. If any reading is 10,000 ohms or less, repair short circuit. Reconnect all components and repeat QUICK TEST. If both readings are more than 10,000 ohms, diagnose transfer case.