I have a 99 s10 ZR2 with the 4.3. I've had issues with the 4wd for over a year now. It worked for years and then would get stuck in 4wd so we changed the dash switch and encoder motor. It then worked okay up until this winter and now it won't go into 4hi or lo it just flashes then gives up. The actuator is not seized up but doesn't move when button is pushed. No vacuum leaks on or around the transfer case. Fuses are good. I have a p0440 code and am starting to wonder if I don't have a huge vacuum leak somewhere throwing the code and killing the 4wd? Would it be the little 3 way vacuum fitting on top of the transfer case?

Thanks
I used to be on here before years ago for my s-blazer and you guys saved me so many times.

 

I was having the same trouble with my 2002 blazer. Changedthe switch and it too worked for a while. What it turned out to be is the plug connectionon the control module. Mine is located behind the passenger side kick plate.They designed it to be cooled with air flow from the fender well and thiscauses the corrosion. I clean it up and then lubed it with dielectric grease. Ialso mounted a screen in the opening to act as a water filter.