I took my car over to a GM place last year to fix the heat they told me some ridiculous price like $600. Heater core is bad (which i could do myself) and i believe he said some kind of actuator was bad because the doors werent switching when you hit vents or defrost of any of those buttons. I remember him telling me there was like 3 of them and one was different from the other 2 and that one was bad. this is what he typed out on my receipt "Air valve actuator not changing vacuum for switching mode positions" what is that?

 

There's only ONE thing that can keep coolant from flowing through your heater core in a 3rd gen (besides a plugged heater core).... the heater core bypass valve under the hood. It has a vacuum line running to it (from the HVAC switches in the dash) and 3 heater hsoes going through it. One from the back of the manifold (where the hot coolant comes from), one to the heater core, and one to the heater core bypass line that goes down to the metal return line on the pass. side frame rail. That valve is SUPPOSED TO close any time you have the Hot/Cold selsctor on the dash pushed all the way to to COLD position (applies vacuum to the valve). Valve could be bad, HVAC selector in the dash could be bad, etc. Sounds like you know how to do some diagnostics so I'll leave it at that.

The actual hot/cold selector on the dash is, I beleive CABLE OPERATED on almost all 3rd gens (digi-dash GTAs being the only exception I am aware of). That being the case, unless that cable is broken, slipped off a fitting or whatever, the mix of hot/cold air coming out your vents can't really be affected by a vacuum problem.

The only thing vacuum CAN affect directly is which vents the air blows out of. There's a vacuum system from the intake manifold that supplies vacuum to the HVAC controls.... starts under the hood with a hook up to the intake, plus a one-way vacuum valve, a vacuum reservoir, and, of course, ultimately a vacuum line going through the firewall to the controls. If that's screwed up the air may not blow out of the proper vents. If there is NO vacuum present (line off, disconnected or a pretty big vacuum leak) a 3rd gen will default to blowing out of the DEFROST (windshield) vents. If you've got vacuum but it still isn't changing which vents it blows out of, it may well be the problem your dealership mentioned on the receipt- the vacuum selector valve that is part of your HVAC controls (the vacuum switch to send vacuum to the different actuators is physically part of the Defrost/heat/vent selector switch in the valve- and there are several vacuum servos that are controlled by this valve and physically open/close different flappers to change which vents the air blows out of).

Not the answer, but hopefully something to point your diagnosis in the right direction.