Hello I’m a new user and have never used a forum before but I’m at the end of my rope and really need help here. My 98 f150 4x4 was working great until it wasn’t. It try’s to shift into 4 wheel on its own. The 4x4 light flashes on and it wants to rip apart. But it won’t go in when I shift it nor will the light come on. Here is what I’ve checked so far. The vacuum is good. Both solenoids at the firewall are in working order. The vacuum unit that moves in and out engaging the front wheels has been replaced and works fine if I manually ground the solenoid. I also replaced the sending unit at the transfer case that tells it to shift. When testing this unit I manually grounded one side and got the low range light. But the other wire does nothing. ( no 4x4 light ). Was told it was the gem module. So I replaced that also. NO CHANGE. I know a loss of vacuum can cause it to shift on its own. But will that illuminate the 4x4 light also? I’m pulling my hair out here. Can anyone help?

 

Watch this and see if it helps.
and this

 

Thread has been moved out of introduction section, and into 10th Generation sub forum.


You said manual shift 4x4, but you're talking about shift lights, and solenoids. That sounds like electronic shifting (ESOF) transfer case. Manual shifting 4x4 is a shift lever on the floor of the truck.....

If you had a vacuum issue, and you were actively pulling the transfer case into 4wd, then yes the shift light would illuminate. Think of it as just a position sensor letting you know what mode its in.