2002 F250 with 7.3. 4WD spontaneously stopped working. Used it approx 5 thousand highway miles ago and worked perfectly. Has been converted to Mile Marker manual hubs many miles ago (which have performed flawlessly somewhat suprisingly). No 4 wd light (neither 4H or 4L) will light up in cluster. No light ON the 4 wd switch itself (do not recall if it was lit or if it even is backlit before). No power whatsoever to switch itself. Checked all pertinent fuses (visually and ohmed) and then some and they are all good. Checked the two relays and they both switch perfectly and have good continuity when switched. The relays however do NOT click while installed (only do it when I remove them and test with jumper harness) There is 12 volts TO the sockets of both relays. The sockets themselves are not corroded and look good. I have read about checking the "two wire pin" on transfer case but it appears to have 4 wires to me! Also how is that plug removed? I slid the red piece back and still cannot get it off of the plug on transfer case, do I need to just get more vigorous? I have not checked power at that plug as I have not had it off yet obviously. Visually inspected the wiring and it all appears unharmed and not pinched, etc. Have been unable to find a good wiring diagram despite searching. Suggestions as to what I might do?

 

Sorry, I haven't had to deal with this yet but maybe this will help you out.

 

Many thanks for that wiring diagram, it helped immensely and is exactly what I needed! After hours of troubleshooting with the help of it I am 99 percent sure that I have my problem located. My particular truck has the 4 wheel drive module mounted above the glove box and despite having new windshield installed and sealed about 2 years ago it appears that water still entered and shorted out circuit board. Will attempt to show pictures. Ordered a salvaged one and will be trying it with update as soon as I get it. I also realized that the wiring to transfer case is more involved than is commonly believed. There are actually 2 different plugs that go into transfer case, one on the driver’s side that has 6 wires and then the actual transfer case motor plug on the bottom passenger side that has two wires. Thanks again to Mike1!!!!!

 

 

Installed salvaged 4 wheel drive module and everything works like a dream. Didn’t even have to get it reflashed!

 

Superb troubleshooting skills, ElCampo!

— Dave

 

Glad it all worked out. With as many windshield leaks that seem to be happening over time, I think I'm going to make some sort of a deflector shield to go over some of the components that are getting water damage.

 

Thanks! I siliconed all of the slots and openings in the top side of the plastic module cover. I’m not sure if it’s the best idea but I did it anyways. A deflector would probably be ideal. Looking closely at the module it appears that most if not all of the water ingress occurs through the slots where the copper colored bracket inserts. If only one place was sealed up this seems like the best place to do it .