I have a 2002 F250 7.3L 4x4 with auto/man hubs and it won't shift into 4wd hi or low. I replaced the transfer case shift motor and the 4wd control module under the glove box (with a used one) and swapped relays on the shift motor relay to prove out the relay. It worked fine when I first got the truck about a year and a half ago, then it just quit last fall. I've been messing with it here and there as I get time and I just can't figure it out. I checked the 2wd/4hi/4low switch and it has the proper ohm drop through the 3 different resistors, but when it is unplugged the power wire to the switch has 12vdc on it, but when i plugg it in it drops to about 4-5vdc. Can't figure that one out. Maybe its just a bad switch? Anyway, is there anyway to test the shift motor safely? Can I jump 12vdc to the motor to get it to move the transfer case, and if so, what tells it NOT to go all the way to the 4low position? I'm guessing it would be the feedback position switch to the control module? In which case, if the control module is bypassed, it would go straight to the 4low position before stopping? I can't believe it's that tough to make a 4wd system work correctly. Anyway, any help would be greatly appreciated.

 

May be reading the reference voltage..

KOEO, Do the 4x4 relays 'click' when High/Low ranges are selected? After the range switch is cycled, the system will attempt to engage the relays/gearmotor 5 times within 20 seconds.. Try the selections in neutral/brake applied?

 

I'm guessing by KOEO, you mean Key-On, Engine-Off? I've actually only tried Key-On, Engine-Running. Can't hear any attempts to cycle the gear motor from the relays. It's a brand new gear motor. No movement in any switch position. Also, doesn't seem like it's even trying as far as I can tell. Maybe I can't hear the attempts to shift because of the rattling diesel? I'll try KOEO tonight. Should I uncouple the gear motor, leave it hanging/plugged in, and attempt to cycle from switch, or from jumping relay pins across the NO contacts, simulating a pulled in relay? That might prove out the gear motor functionality and point me to either a bad relay(or two?--odd no doubt), or it might prove the control module is either not getting correct input voltage/current range from switch position, or isn't outputting correctly to the relays? Anyway, I'm assuming I should prove out the gear motor function first, then move backward toward the relays, then the control module output, then the control module inputs, then the selector switch? I have a break-out box at home, so I can use it to simulate the 4wd selector switch with the properly known resistances through each setting, but just a bit reluctant to do so until I get more knowledge on the functionality of the control module and the selector switch. I really don't want to burn something up because of my stupidity.

 

Well, after a new shift motor, and a new 4wd control module (used take out from ebay), that still didn't get it. So I pulled the 4wd selector switch again, and checked the ohms through the resistors, and they were all right on the money. So, I looked at the contacts and they looked a little weak. They are pre-bent contacts made of copper, so I put a little bit more bend in them, increasing the applied pressure on the electrical contacts when the switch is put back together. That worked! It's been working since late Nov and I've had it in and out of 4wd 20x's since then and it's never missed a beat. Thanks for the help!