Ford F-150 - 1997-2003 :: 2000 XL V6 4WD Went Out While Driving At 65 MPH

I have a 2000 F-150 XL with a 4.2 V-6, a 5 speed manual transmission and manual shift on the fly transfer case. The 4WD went out over a year ago. When it went out, I was driving at 65 MPH and heard what sounded like rapid relay chatter for several seconds (10 to 15 seconds) and it stopped. The sound seemed like it was coming from the dashboard area where the GEM is located, but may have been from the solenoids mounted on the engine compartment firewall. Actually could have been from both. When the 4WD tried to engage while driving at 65 MPH, the dash lights were flashing on and off and it sounded like the front differential was trying to engage. Of course I slowed down while this occurred to avoid damaging the truck should it succeeded to engage the front differential.

I have a Haynes manual but the wiring diagrams are some what ambiguous as my truck is not as common as those having ESOF. Even researching online I am not certain if the info pertains to my truck. The only diagram I found to be accurate is the vacuum control diagram. I believe I have either a wiring problem or the GEM is bad. When troubleshooting I like to isolate at the middle points to quickly eliminate one side or another when looking at a diagram/schematic.

Ford had a well-noted problem with the two solenoids that control the 4WD actuator and are mounted on the engine compartment firewall. So I started here, as this is a good halfway point and high failure area.Well, the solenoid power connectors had 12 volts on both connector pins (if I remember correctly as it was a year ago when I did some troubleshooting)regardless of transfer case shift position (2wd, 4wd high or low). I was able to manipulate the solenoids and verified they were operating, as I could feel and here the solenoids click when grounding the control side of the pin on each solenoid. I did check fuse 23 prior to checking the solenoids. Since 12 volts was present at the solenoids, it indicated power was getting to them.

So another common problem is the transfer case 4WDsensor. I removed, cleaned, and verified operation of the sensor. Reinstalled and check sensor state with transfer case in all 3 positions and it was working correctly. Also I have no 4WD indication on the dashboard when 4WD high or low is selected. Fuse 6 is good.

My main issue is not having confidence with the wiring diagrams I have found online. I am not sure if my truck has a MSOF relayor the transfer case 4WD sensor signal goes directly to the GEM.

I need to check:

1. Wiring from transfer case 4WD sensor to either GEM or MSOF relay, if there is a relay, will check it along with wiring to GEM,but don't believe it exists for my truck. (I need the wiring diagram to do this)

2. If the transfer case 4WD sensor output is getting to the GEM module, I would think the GEM module is bad. If possible, I would like to check the GEM module outputs to the solenoids, but since the GEM is buried behind the fuse panel, it probably is not feasible.

I plan to call the Ford service department in my area to determine what GEM I have and will buy a GEM online. Replacing the GEM does not look fun, so I have avoided it but wanted to be sure it was the problem.

So I want to know if I am tracking correctly and if there is anything else I should check, and would like the correct wiring diagram as indicated in item 1.