Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the following:

• Electronic shift control module and mechanical 4WD LOW wiring harness circuit (4WD LOW). See Fig 1 and Fig 2 .
• Powertrain Control Module (PCM).

Fig 1: Identifying Electronic Shift Control Module Circuit (Electronic Shift)

Courtesy of FORD MOTOR CO.

Fig 2: Identifying 4x4L Switch Circuit & Connector Terminals (Mechanical Shift)

Courtesy of FORD MOTOR CO.

NOTE: Transmission Range (TR) sensor must indicate Neutral before shifting into 4x4L.
1. 1) Continuous Memory DTC P1729 & KOEO DTC P1781: Verify 4x4L Selector Position
   DTC P1729 will be set when PCM receives too high of a voltage on 4WD LOW circuit when 4x4L switch is ON (indicating open or short in 4WD switch circuit). DTC P1781 indicates 4x4L switch position closed during QUICK TEST. Possible causes are:
   • 4x4L Circuit Open Or Shorted
   • Faulty Electronic Shift Control Module Or Generic Electronic Module (GEM)
   • Faulty Powertrain Control Module (PCM)

   Ensure vehicle is equipped with 4WD. If switch was not in 2WD or 4WD HIGH position during KOEO SELF-TEST, select 2WD or 4WD HIGH and repeat QUICK TEST . If switch position was as specified, go to next step.

2. 2) Check 4x4L Circuit For Intermittent Fault
   
   Connect scan tool to Data Link Connector (DLC). Turn ignition switch to ON position. Using scan tool, select 4x4L PID from PID/DATA MONITOR & RECORD menu. Cycle 4x4L switch to 2WD. Check for indication of fault while performing the following:
   • Shake, wiggle and bend small sections of wiring harness starting at transfer case working toward PCM.
   • Lightly tap on transfer case harness connector.
   • Shake, wiggle and bend small sections of wiring harness starting at instrument cluster working toward PCM.

   Fault will be indicated by 4x4L PID value changing from ON to OFF. If any faults are found, isolate fault and repair as necessary. If no faults are found, go to next step.

3. 3) Check 4WD LOW Signal Voltage To PCM
   
   Turn ignition switch to OFF position. Disconnect PCM connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Turn ignition switch to ON position. While cycling 4x4L switch, measure voltage between negative battery terminal and PCM harness connector terminal No. 14. See Figure, Fig 1 or Fig 2 . If voltage cycles with switch position, replace PCM. If voltage does not cycle with switch position, go to next step.
4. 4) Check 4WD LOW Circuit For Open In Harness
   
   Turn ignition switch to OFF position. Disconnect electronic shift control module. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. On electronic shift models, measure resistance of 4WD LOW circuit between PCM harness connector terminal No. 14 and electronic shift control module harness connector 4WD LOW terminal. See Fig 1. On mechanical shift models, measure resistance of 4WD LOW circuit between PCM harness connector terminal No. 14, instrument cluster and 4x4L switch. See Fig 2. If resistance is 5 ohms or less, go to next step. If resistance is more than 5 ohms, repair open in 4WD LOW circuit.
5. 5) Check 4WD LOW Circuit For Short To VPWR Or Ground In Harness
   
   Reconnect electronic shift control module. Using a DVOM, measure resistance between PCM harness connector terminal No. 14 (4WD LOW) and negative battery terminal. Also, measure resistance between PCM harness connector terminals No. 14 (4WD LOW) and No. 71 (VPWR). If both resistance measurements are more than 10 k/ohms, 4WD LOW circuit is okay. Check transfer case for mechanical fault. See appropriate OVERHAUL article in TRANSFER CASES. If any resistance measurement is 10 k/ohms or less, repair short in 4WD LOW circuit.