I have a weird problem....(this used to work fine BTW). When I move the AC selector, air will come out of the vents on AC mode (its not cold I need a recharge), BUT when its in Vent or Heat the air always blows out of the defroster, instead of the vents (upper and/or lower). Anyone have anyidea what would cause this? The selecter moves freely and you can hear some vacuum hissing when it changes....

 

With mine when I got it, air came out ONLY at the defroster, no matter what.

I took out the HVAC control and looked it over. There's a roundish shaped disc assembly with vacuum hoses attached to it. It's actually two discs held together by a plastic clip through the middle. The whole assembly is held onto the HVAC control by two screws. When you move the control lever, it rotates the top disc, causing different tiny holes between the discs to line up and open and close the airflap doors inside your heating system.

The plastic clip holding the two discs together had broken on mine, allowing all the vacuum to escape, and apparently the defroster is the default condition (probably for safety reasons), so I got a new/old clip out of a junker heater assembly---worked like a charm.

By the way, if your disc assembly is broken, and you decide to replace the whole thing, learn from my mistake. Practically every Chevy made for the last 30 years uses the same design HVAC control, and the disc assemblies look identical except that the older ones have a metal rivet through the discs. They are not EXACTLY the same---for some wacky reason the holes inside are not in the same positions, so get your parts out of a 3rdgen if at all possible.

Hope this is your problem, it's a cheap, easy fix.

 

Yeah I think it is...now that I have played with it more...when the selector is off the AC parts..I hear a constant hissing, so I am pretty sure that part is shot....