I think I have read just about every post on this forum about problems with the truck going into 4H. I've studied the electrical diagrams. I've taken the motor off, taken it apart, and confirmed that it is working. I have also replaced the GEM module and checked every fuse in the truck.

Today I noticed something odd. When switching from 2H to 4H, the relay(s) under the dash click once, then a few seconds later they start clicking again and keep clicking every second or so till the motor turns to the 4H position.

I hooked a volt meter to each of the wires going to the motor on the transfer case shift motor. I found that both of the wires are getting 12V applied to them when I hear the click. The motor will only turn when voltage is applied to just one of the wires. I don't know the exact timing of the voltage being applied, but I think this may be the problem.

When looking at the motor shaft as the system is switched from 2H to 4H, the motor moves just a little bit with each click of the relay(s). When shifting from 2H to 4L or 4H to 4L, the voltage on one of the wires stays high until the motor has moved, and when shifting from 4L or 4H to 2H, the voltage on the other wire stays high until the motor has moved.

Anyone else seen this? If I hadn't replaced the GEM module (same result with old and new module), I'd think it was bad. Any known fixes?

 

I would suspect a grounding issue. The GEM controls these things by providing a path to ground to complete a circuit, in this case the relays for the ESOF. I am at home right now and my wiring diagram book is at work so I can't give you much detail for troubleshooting the grounds. I would look in the driver and/or the passenger kick panels for problems. Good luck!

 

Grounding would be a good guess. I checked the grounds in both kick panels, removed the screws and then drove them back in nice and tight. No change. I hooked an oscilloscope to the two leads at the relay going to the motor. When going from 2H to 4H, the signals behave quite differently than when going from 2H to 4L or 4L to 4H or 4L to 2H or 4H to 2H.

Motor was replaced not too long ago, and when bench testing the motor with 12V, it spins just fine.

Stumped.

 

Well, I got to the bottom of it. The switch assembly on the back of the motor on the transfer case was not working right. I took the assembly off the motor and took it apart. There are five beryllium copper fingers that contact a ceramic plate with the contact material plated on it. One of the fingers was not bent down far enough to make good contact with the plate. I bent all of them properly and put it back together and now it works perfectly. I suspect that the fingers were not bent properly at the factory, and as the copper aged, it stopped making contact.

Yahoo!

 

Glad that you figured out what the problem was. It sure sounded like a ground problem but an open switch will sure throw a monkey wrench into things too.