Jerome Casi

My heater control unit hisses, I replaced the dealy everyone says to, fixed the switching between vents, defrost, etc. I still have a hiss when you slide the hot/cold to anything but cold (far left).

I disconnected a vacuum line behind my engine and the hiss went away. I can still select hot or cold and it adjust accordingly, engine didnt run funky or anything, what is that vacuum line supposed to do? Did someone have that line run wrong, or does the heater need it for some reason I am not catching?

rgarcia63

Of the two vacuum lines that go through the firewall one is the supply vacuum to the A/C control head the other returns from control head to the hot water valve which is vacuum operated. That is why it hisses when you select heat, its trying to open the valve to allow hot coolant to the heater core. Plug the line, no more hiss, and no more heat.

Jerome Casi

Any idea, why the hiss goes away when I unhook the vacuum line, but I still can select everything, and the temperature changes when I use it? I figured it would stop working, like you state above, but it works the same as it did, just no hiss now? The heat has always got warm (Not really hot), could the hiss be something with the heater core not functioning correctly. Thanks again.

rgarcia63

Where exactly are you disconnecting the vacuum line.
Heat maybe from partially stuck open valve.
Hiss is usually form small leaks like when you breath in with mouth slightly open compared to mouth wide open, same as disconnecting the hose. You'll have to check behind the control head or along the hoses.

Jerome Casi

I have a vacuum line from around air cleaner on top of engine. It then splits to two separate vacuum lines, both of which run through the firewall. If I disconnect one I loose my multi switch selector, for defrost, vent, heat, etc. If I disconnect the other the hiss goes away from when I slide the temperature selector to anything but cold.
Whether its plugged in or not I still get temperature control, even though the hot doesn't get much but warm. I took out the unit and can hear the leak and see it is coming from the temp selector, tried reseating the connection but that didn't work. Is there something further down the line I should check that may be stuck or is it in the control unit? Thanks for the help, getting colder here so trying to get it figured out.

rgarcia63

If both lines from the splitter go through the firewall, what's controlling the heater coolant control valve?
Find the heater coolant control valve on one of the heater coolant lines and follow the vacuum attached to it, find where it goes.

oldskwl81

On mine there is a diaphram that did that on the sliding track. I found it by running the car with the conrol head in my lap. Then it looked like gm used a grease to seal it and lubricate it. If its the diaphram do just that

kman71

I am having the same issue, though it was the main switch, so did that fix and now the selector switch works great, strong air from where it is supposed to come from by the selector.

I am still getting the hissing though as soon as I move the temp selector from cold to hot. No hiss if it is all the way cold, but starts as soon as you slide it hot. I removed the two vacuum lines that are on the drivers side back of the control unit and the hiss goes away, not the selector switch vacuums. So if I understand you right, the two lines are for the ac and heat selector. My AC is currently down and needs to be connected and charged, I just had the engine rebuilt. So do you think the hissing is due to AC being down or is it another vacuum line leak? It is so weird though, the hissing sounds like it is coming right from the control unit and I cannot pinpoint where the leak is actually.
Thanks for the help and advice. Great thread.

91charlene

I know this is an older thread, but I am dealing with the same thing. From my research the hiss can come from either the function selector switch on the right side of the control unit that creates a hiss when the ventilation is on anything but off ( not what my 91 is doing), or the left side temp selector that creates the hiss any time the temp selector is on anything but cold. The selector shuts hot water off to the heater core to make the AC more effective on cold (Max) AC. I guess my questions are 1) does the heater control valve turn on or off with vacuum or the other way? If the control unit turns the vacuum off, then the hiss is there because the valve cant handle the negative pressure, and is sucking air. It also means that there wont be heat to the core (or not much) when heat is selected. in which case one solution is to bypass the heater control and disconnect the head unit(?), but then AC won't work quite as well. 2) Why is it that only seems to be later model Camaro's, and is the part in earlier model control units compatible? 3) Is it possible to swap the whole control form an earlier model (although I would rather not)? 4) Is there a possibility that the vacuum lines in the engine compartment, even though it seems pretty obvious where they go, could be reversed (i.e. the supply is hooked to the switch and the switch leg is attached to the source (engine))? 5) Is there any advice on fixing the AC system (I will look in a different thread if suggested), I have access to 80 lbs of R12 but, I am not sure for how long. Would I be better off converting to R134a? 6) Ive seen many different opinions on how to do it, so does anyone have advice (e.g. flush and fill vs. replace receiver dryer) on the best way to check for vacuum leaks (mine wont hold a vacuum) and convert? I know this was a long one but any help or advice would be appreciated.