Heater ac problems

[fascat]

Hello, I have a 1996 s blazer that will not switch the air output to the different locations on the dash. The mode seems to be stuck in the blend position. (heat, defrost) I have changed the heater control unit, and changed the vacuum lines. It is getting vacuum to the back of the heater control. Can anyone help solve this mystery? I cannot seem to find any literature on this. PLEASE HELP!!!!. Thanks all.

[gchemist]

Hello, welcome to the site. I had a simliar problem. Check the main vacuum line under the engine bay. The vacuum line is located behind the plastic air intake cover. You may need to remove it to see it. Look from the driveside fender between the intake housing and distributor. Vacuum hose connects to the upper intake manifold.

[SHOguy]

I remember GMs having issues with the control heads themselves leaking all through the 80s...I wonder if the 2nd gens have the same issue? Turn off all accessories, and listen for a slight hissing noise coming from the controller while the engine is running...

I'm new here, but have a fair amount of general knowledge on these things. I hope this helps.