I recently purchased a 1971 Skylark it has AC and blows cold however it only blows out the defrost vents and the heater never gets hot (engine temp 190). There's also an audible hisss sound coming from the HVAC controls so I'm guessing it has a vacuum leak at the control unit and that control unit controls the airflow and the vacuum operated heat actuator installed inline on the heater hose. Does the entire HVAC control head need to be replaced or can you buy replacement pieces? I have not had the control out yet to know how serviceable it is. Thanks in advance for passing any knowledge forward... Is the duct work flexible hoses that connect the ac box to the dash vents or plastic ducts like in newer vehicles? Curious if those are remanufactured somewhere or available for purchase, or just wrecking yard items? Its an interesting system all the vacuum operated stuff
If you mean the mode select switch in the dash, they're out there. I've seen them on Ebay. I do agree with Mark though it's worth a good look down there. I once took my first GS apart to redo the stereo, and when I put it back together, I had the same issue. It was literally I'd knocked the vacuum hose off the little pod on top of the heater box while reassembling the dash and AC components. This is also where it isn't a bad idea to get ahold of the shop manuals for your car.
Sure enough the vacuum line that feeds vacuum to the control head had fallen off. There are two side by side not sure which one gets the main feed. I have more problems after poking my head under the dash though. The distribution box that bolts to the main big heater box that feeds the dash vents is missing as is the foot heater outlet vents. Basically this whole thing here...
