Hello,
I am having a problem with the heater control on my car. I have had a slight "hissing" sound in the area of the control pushbuttons for some time and an issue with the vents switching modes on their own when the engine is under load (passing, climbing long hill, etc.) I have checked and replaced the main feed line from the intake manifold and put in a new check valve. I have also swapped out the control head itself (twice). No change. I still hear the noise and when I shut down I can hear the vacuum slowly escape. I did get the replacement controls from a salvage yard, but it's hard for me to believe that three of them would all be bad. The problem seems to be isolated to the "recirc" button. When I push that one in, the hissing seems to go away. Where should I look next? Thank you for your help.

Thanks to everyone for the replies and tips. The recirc door -does- operate normally when I activate the button: In: door opens (recirc mode) Out: door closes (fresh air mode) There is a constant hiss when the button is out. The hoses are all connected at the control head and the hiss seems to be right at the control area. This is the third control I have tried. Granted, the two replacements were from the bone yard, but it's hard for me to believe all three would have the same problem. I'll try and trace back to the vacuum motor for the recirc door. Can anybody give me a hint where it is back there and what side it will be easiest to get at it? I can't see it at all back there. Thanks again, I hoping to get this fixed this weekend with a little luck!

it may be a loose/punctured hose that is bypassed when the button is in but when out, sucks in air. Try clamping off a hose until hiss disappears.
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1980 245 Canadian B21A with SU carb and M46 trans

It's got to be a bad (missing) connection behind the push buttons. Maybe you have a bad vacuum hose in there.
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85 240 DL, 89 245 DL, 419,000 miles total

There's gotta be a leak someplace, and boy, they're hard to locate sometimes. I just found a kinked T-connection to my floor ducts and replaced it. Anyway, when you press RECIRC, vacuum is blocked to an actuator behind the heater unit, which allows the inside door above the trans tunnel to swing open. You can see it happening, right up at the firewall. With the engine running, and the button out, vacuum is applied... thus holding the inside door closed and allowing outside air in.

If the hissing stops when you depress the button, you must be blocking the leak. See if the door is actually moving; I'll guess that it isn't. One of four conditions is probably the culprit:

1. Hose needs to be connected at the actuator.
2. Hose has a leak somewhere along its length.
3. Actuator itself has a puncture, or has come apart.
4. Hose is connected to the wrong port on the control head. The rear port, farthest from the button face, is the one you want.

The hissing on engine shutdown is merely vacuum dissipating from the tank, so not to worry. I played around with this silly system for a month, and it was actually entertaining to figure it all out. A diagram from Volvo, glued onto the heater case, would certainly have been welcome!! Good luck.