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Air doesn't blow through vents

I have a 97 Volvo 960 and air doesn't seem to want to blow through the vents. It will work sometimes if I'm accelerating or braking hard and it will come out steadily (but very little) if I'm doing at least 60-70 mph. I've brought it back to where I bought it twice and they said it was some connection behind the glove box and they fixed it both times, but it stopped working soon after. Anyone else have this problem and be willing to help me out? It'll be much appreciated. Thank you

Quote:

Originally Posted by Retlaw (Post 458498)

I found this post on this board, although its about A/C issue it may be related to your problem, here's the link.

https://volvoforums.com/forum/volvo-260-760-960-28/97-960-c-problems-55306/

Read Down where the member Henry10 posted a vacuum schematic.

Quote:

Originally Posted by madman77 (Post 458511)

Yea, I had seen this and a few others and none of them seemed to be what was wrong with mine. Thank you though

Quote:

Originally Posted by Retlaw (Post 458498)

I have a 94 960 which didn't want to blow air thru the face vents under acceleration or uphill loading the engine. Mine would switch over to mostly blowing out the floor vents under acceleration. When I was cruising on level ground it would blow out of the face vents but under load or acceleration it would switch over.

There's a control valve which I guess develops a vacuum leak internally and it's a fairly common problem. And if it's the same kind of problem mine had, it's apparently a BIG deal to take most of the stuff our from behind the dash, if you replace the control unit. At least for my model, you can plug a vacuum hose or something on this control unit. My mechanic did it and it's much better, not perfect but much better. You do lose a bit of functionality like recirc and mine doesn't blow perfectly out of the floor vents when you select floor. But for me the most important function was face vents.

Yours sounds like it has either some kind of vacuum leak or something wrong with the HVAC vent control unit or whatever its official name is.

If it only switches under load, then more than likely it is a faulty vacuum check valve. It is really easy to check and replace. The check valve is a plastic black & white device with a hose connected to each end. One hose connects to the top of the intake manifold and the other (smaller) hose runs through the firewall and connects to a reservoir in center console. Remove the check valve and try blowing through it. It should only allow air to go through in one direction. If air can pass both ways then it needs replacement.