2002 blazer, front differential is engaging. The actuator under the battery tray is getting a constant vacuum. I had to disconnect vacuum to disengage front differential. I have 3 switch, on dash with no lights, except for night time illumination. No noises from transfer cases when buttons pushed, no noise from passenger kick panel, were I believe the tccm is, and As said earlier constant vacuum to the front actuator. Fuses are all good. I'm baffled on were to start trouble shooting next. Any help is much appreciated.

 

I had the same problem on my 1998 blazer Last year. When you say that the front diff is engaging and there is a constant vacuum at the actuator under the battery. Is this happening when the pushbutton on the dash is in 2hi? Then the vacuum switch on the transfer case is stuck and needs to be replaced. It about a 16 dollar part at autozone and a real pain to get to but that fixed mine.