truck engages 4Hi on its own

[Mark31]

I was just easing along the highway between 50-55 with the Dynalocks unlocked. Suddenly, "WHAM" (since the transfer case wasn't turning. ) I pulled off the road and see my 4 HI light lit up on the electric control. I turned the knob to 4 Hi and back to 2Hi a couple of times and it lit up right. I drove on home about 30 mph. This morning, I manually turned the front driveshaft and nothing out of the ordinary--smooth, no rough spots. I switched the truck "on" and 4 Hi is lit up again. I worked the knob back and forth a few times and finally it stayed in 2Hi. I locked the hubs and drove a couple of miles down a dirt road, shifting back and forth between 2Hi and 4Hi at about 15 mph in D2. Everything worked right. I then unlocked the hubs and pulled the 4X4 fuse, number 53, I think. Then down the highway in 2 Hi. Now I have the Service 4X4 lit up. What could cause it to select 4 Hi just "out of the blue"? I plan on changing the transfer case oil tomorrow and see if there is metal in the oil. Where do I go from here? Mark

 

[JJPage]

I can't tell you where to go... but I hate that damn electric transfer case controller, I will not purchase another vehicle without a manual lever. I used 4 Lo to back up onto a portable dyno, do you think I could get the sob out of 4 Lo? Had to drive back off the dyno and fight to get back into 2 Hi, with about a 150 person audience looking on.

Hope you find no mechanical damage, I would check all wiring associated with the controller, maybe something has chafed through and is grounding, or you lost a ground.

 

[sag2]

I'm pretty sure I have covered this in detail before so you might do a search. In short, the switch uses a 5v MUX circuit to indicate what position it wants the transfer case in. You can have a problem anywhere in the 5v circuit, a short to another circuit, bad switch, or a bad shift motor. If you are not currently using 4wd just unplug the shift motor while driving it until you can get it diagnosed.

One sore spot is the A/C pressure transducer. If it goes bad it can alter the 5v and the transfer case thinks you asked for a shift. Unplug it and see if the symptoms go away.

 

[Mark31]

I'm pretty sure I have covered this in detail before so you might do a search. In short, the switch uses a 5v MUX circuit to indicate what position it wants the transfer case in. You can have a problem anywhere in the 5v circuit, a short to another circuit, bad switch, or a bad shift motor. If you are not currently using 4wd just unplug the shift motor while driving it until you can get it diagnosed.
One sore spot is the A/C pressure transducer. If it goes bad it can alter the 5v and the transfer case thinks you asked for a shift. Unplug it and see if the symptoms go away.

Sag, I went back 11 pages in your posts (as far as it would let me) and found a good thread about this, but it was for a TIPM truck. Is unplugging better than pulling the #53 fuse? ("Service 4X4" lights up. ) If the AC isn't running, does that rule out the AC pressure transducer? Any help you can give would be appreciated, as I am "electrically challenged"! Mark