On dads 04 f350 the 4wd first wasn't shifting to 4lo. Would do 4hi and 2wd fine. Then motor on t-case went bad. Replaced motor 2wd and 4hi still good still no 4lo. Then just 2wd so we unplugged t-case motor and power probed motor got it to do 2wd, 4hi, and 4lo. So we thinking bad dash switch but power probed it and its working fine. So we r thinking VP4C3F-14B128-A/ chassis computer bad just behind/above glovebox. R there any test we can do to confirm its bad? We called local junk yards to see bout getting one, nobody had one. If this is indeed bad where is cheapest get one? What all years use this certain chassis computer??

 

Maybe first test what you get at the plug on the transfer case [from the computer]?

Does it get the proper signals when you switch modes?

 

no no signal at all. Checked relays. we Power probed power to 86 and got motor to move.

 

Check that the wiring is still OK [continuity between the connector at the computer and the plug on the transmission].

And that the relay is properly working when you work the switch.

 

the wiring is fine cause we can power probe to relays and unhook wire at chassis computer and get motor to work as it should. That's y we are thinking computer junk

 

plugged that chassis module back in and 4wd hi works. I don't know if maybe we just didn't have brake pushed trying it maybe. 4lo still wont engage never has for us. Just power probe jumping power at motor connector. We have never got it to by switch.