Ok, I just bought a 92 S10 Blazer 4.3 Vortec AT 4 dr 4x4 last week. I already have a 99 Jimmy and an 83 K5 so this one is a little different.
It runs and drives great, problem is I am trying to fix the small stuff like wipers and recharging the Ac and whatnot. The AC works good and it also puts out heat throught the vents and the floor. I tried the defrost and it does not switch to blow out the defrost vents. You can turn the Blazer off and turn the key on and it will activate it to blow out the defrost vents. When I start it, it goes right from blowing out the defrost vents to blowing out the dash vents again. I am sure it is a vacuum problem but any info would be greatly appreciated.

 

Nevermind, it was a faulty vacuum selector switch. All is well now, it's amazing that little pieces can cause such problems.