So, just picked up a 99 GMC Jimmy for a great price and it's having some 4wd issues. It has the four button selectable 4wd (tcase NV236). I am throwing code P1875 4 wheel drive low switch malfunction. I got someone with an extended scanner to read it and they said there is also a small evap/vacuum leak, o2 sensor and abs code. The ABS light does flash intermittently, but the ABS seems to work, tested it out with all the snow and ice recently and I can feel it working, dunno if it is a symptom of the 4wd issue but figured I would include it. So when I first got it, seemed like the front axle was locked in, like hubs were locked. I press the 4L button, the TCCM clicks, I hear the selector motor working. It would go into 4L, but the 4L light would remain flashing and would not kick back out to 2H after 30 seconds. 4H and 4AWD would not cause the selector motor to engage, the TCCM would click but the light would flash for 30 seconds before going back into 2H. Now, I can't get the selector motor to do anything and the TCCM does't click everytime I try to use the 4wd. Also shifts very early in 2H, around 1500 rpm which seems like it is causing hesitation.

So what I have done so far. I pressurized my 4wd actuator under the battery, I don't think its leaking. The actuator on the firewall looks brand new. I have done fluid changes on everything, synthetic dex trans fluid and filter change, synthetic 75-90 in both diffs and Delco trac 2 fluid in the tcase, still not working. I did pull the speed sensor from the tcase and oddly looked exactly like a brand new transmission speed sensor. I am wondering if someone put in the wrong part... I have not checked the TCCM connections for corrosion yet, so I may try that next. I also know there is a vacuum switch on the tcase that I am suspecting may be the issue as well.

If you have any thoughts, recommendations or questions please let me know.

*EDIT*
So after reading the 4x4 no worky, it sounds like my front diff is locked into 4wd. The front shaft will not spin. So sounds like its going to be that vacuum leak causing this issue.. It looks like the solenoid and vacuum lines have been replaced recently and I tested the actuator and it seems to be working properly. Also just pulled the TCCM, connections are clean and dry, and the module itself looks great, no corrosion or anything. So I am thinking it is going to be the switch on the tcase, definitely looks original. I am thinking someone replaced the speed sensor (which has electrical connections) meaning to replace the vacuum switch. I am going to attempt it and see how it goes. Also noticed that if I put it down into D1 and rev it to around 4 grand before shifting into D2, it seems like it will get the 4wd to kick out every once in awhile. It shifts normally and drives normally sometimes for a few minutes, sometimes just until I stop and sometimes it will stay that way until I shut the motor off.

 

I don't believe there is a vacuum switch on your T-case , that is only the 3 button system. Will that extended scanner do the command functions like the Tech 2 ?, I can copy the diagnostic pages from the GM manual to help you sort things out.