My HVAC box does some weird things. First of all, I always seem to hear some mild hissing coming from it while driving around. It REALLY hisses loud if I slide the control switch to different functions. Is this hissing normal? I need to know which settings are supposed to pump air from where. The box is totally confusing to read (Maybe i'm just an idiot, but I cant understands whats what).

 

it uses engine vacuum to move the servos that control doors, if it's hissing continuously, that means you have a vacuum leak either in the line that goes to the HVAC control or in the HVAC control itself, normally all you hear is a quick hiss then a thud when the vacuum servos move and the doors open and close, and if the foam rubber sealer is still good (good luck...these cars are too old for that) you won't even hear the thud, if it only hisses in one setting that means that settings' servo or vacuum line is leaking

 

I had the same hissing problem.
I thought it was a break or cut in one of the Vac lines, but turns out that the selector and switch assembly has a rubber "disc" that "switches" to what servo the vacuum goes, there is a piece of springs steel that puts pressure on it and a plastic (of course) clip that holds in together and one of the ears of that clip was broke off. I took it apart and carefully superglued the clip in.
That may be what your problem is and you'll know if you look at the back of the control panel while moving the selectors.
Good luck and let us know how it goes.


-Birdofprey83

 

My main question though is each selector. Where is air supposed to come out of for each selection?