Hi all,

My 940 has a little button on the dash which is pressed in to switch to full
recirculation and and pressed again to have fresh air coming in through the
vents. Does anyone else who owns a 940 on this list notice that when you
switch to fresh air (button out) there is a noticeable hiss?
My car has been doing this for ages and I was wondering if it is normal.
The hiss is definitely coming from around the back of the control unit in
the dash.

Any thoughts?

Thanks
Brad
91 940GL

It's correct for there to be a hiss which diminishes over the course of a few seconds. It should do this even after the engine has been stopped. -It's because the flaps in the ventilation system are operated by the vacuum system.

If you have a continuous (still hissing after 10 seconds or more) sound then you might want to check for a leak, but if you're just curious at to whether a few seconds of hissing is normal, then it's perfectly fine.

Keith

While there is a hiss while the vacuum door moves the flapper, there should be no continual hissing. If so, your control unit has a vacuum leak, probably a split or loose hose at the back.