2000 Bravada, even with the temp control all the way on coldest setting, the damn vents are blowing hot air. I noticed also a strange sound coming from under middle of dash. The sound would come approximately every 13 seconds and would make a creeking door closing sound. That sound quit but the heat on all the time is killing me. The blower control set to off still blows enough to feel air coming out of the vent which it never did, when the blower **** used to be off, NO air would be coming out of vent or perhaps it was but if so it was cooler air and I never noticed it before. I dropped the lower dash panel and the only thing I could see is a round vacuum bulb hanging freely by a metal tang slipped up over a lever that goes into ducting. It just hangs there and it has 2 vacuum ports with no hoses attached to it??All the functions of the climate control work , and one note, the A/C does NOT work, the compressor does not turn on. I jumped it out and it does run but no cooling, lines get hot hot, so another issue but is it related or not , I don't care about A/C but though since it is related in HVAC, I included that info.

There is always natural air flow through the duct, as long as you are not in the recirc mode....

Your temp, is controlled by an electrical actuator.....could be the gear is stripped in the actuator, the actuator is not getting a signal from the control head, or the linkage to the door is broken....

Best move, is to remove the actuator and manually move the door to see if you can change the temp......next thing, grab the actuator with it still connected, and see if it can move when commanded....if it does move, apply resistance to see if it still moves or if the gear is stripped.....

If it doesn't move, check the three wires to it......brown is battery, black is ground, and light blue should vary in voltage as the temp switch is varied.....if voltages ok, and temp door can be moved manually, then you need an actuator....

Is access possible to get to this door by removing glove box? I noticed under hood at heater core area shroud is a "cut for service" imprint. I'm not cutting that, it will destroy the housing, but is that where the actuator is?

Maybe one of the guys can give you a diagram of it'* whereabouts, since I am not up on Bravada'*.......it should be under the dash, not sure if passenger or driver'* side.....the actuator looks like a little black box(half he size of a pack of cigarettes)......look for a wire harness going into it with the colors I previously mentioned......