Hi guys, first I want to say how cool this site is, I've learned alot from the posts on this site. I am also lucky enough to live only a few miles from one of your awesome members plgebbia (HI Ian). I've owned 4x4 trucks my whole life but I'm new to powerstrokes, and this is my first diesel. Its a late 99 f250 lariat with esof. Here's the problem I'm having, and it has me and Ian both stumped, One night got in my truck to go home after work, truck would not run over 25mph, didn't know what to do. got in it next day to go to town same thing, Got to checking things out, and find ...........Its in 4low........**** on dash is in 2hi, 4x4 light on dash is lit. Called Ian he comes over and confirms it is in 4lo. I crawled under truck and got it out of 4x4 by unpluging the shift motor and shifting it with 12v jumper wire. Me and Ian both assume its a bad gem. Bought new gem plug everything back up............same issue. Pulled relays, bench tested fine, not sure if the switch could be causing this or not, pulled switch but couldn't test because I was unable to find what the resistance specs should be. Don't have any idea what to try next. Hoping for some answers here

 

I would check the switch. I don't know how to test it either, but you said you could jump the motor manually to switch between 2wd and 4x4, so that is good and the relays are good, so I would buy a new switch. Throwing parts at something sucks, but it might just fix the problem and those switches aren't too much. I had to buy one because the bulb in mine went out and the bulb is not sold separately.

 

i think you can open them up and put a led light in them
i would open it up and see if it looks bad or has corrosion in it.