Air vents won't shut off
 

The air vents won't shut.
I don't know if this happens with the car stationary at idle, but while driving it is really bad, especially at night when it is cold. The vents keep pumping cold air into the cabin (the heater matrix is diverted) even when the blower is off. What may be the cause of this and how can I fix it?

I'm pretty sure it is usual to get airflow through the cabin at moderate and higher speeds.

It's caused by high pressure around the base of the windscreen.

I'm pretty sure the Volvo ACC (Automatic Climate Control) even reduces the fan speed accordingly at higher road speeds.

it can't be that. I am not talking about a slight gust, a gentle draft. This looks like blower is on. It's freezing our feet. The mid vents are pointed upwards because even though they are shut, they keep blowing, and so does the windshield vent!

This has to be some kind of gasket that is faulty or I don't know what. But it can't be by design. It's like you basically have the blower turned on one (at least) with all vents opened at the same time.

I don't kow how this system is designed. What is it that make sthe air circulate one way or the other when you turn the vents knob? Is it a flap inside the ducts?

Fan is turned off and heat temperature control turned up shoud let warm air into cabin at all vents

Vacuum controlled
 

The air vent shutters are vacuum controlled. I am not sure if without vacuum they are spring loaded open or closed.
Do you have a manual system or ECC?

You probably found the culprit. A while ago I was checking those hoses for leaks but I did not know their purpose. I disconnected the 2 nipples shown in the pic to only later realize they might have a direction of installation. Not knowing what they were, their name, I couldn't find them online or on the catalogue, so I installed them from memory. Can this be the culprit? Right now they are as in the pic, both with the white side down.

If this is not the issue, I guess I might have a leak in this system!?

https://i.postimg.cc/fbm7NMD7/20231213-120358.jpg

Oh, ok. I searched a bit more and think I got it. It's a check valve, probably part number 9134341 and it is indeed a one way valve ins't it?

Can you know the proper installation direction just by the color white/black, or is there more to it?

I guess if it is installed the wrong way it might causing a vacuum leak!

Upon further reading, my issue seems to be consistent with either one of these valces being faulty, or a faulty Floor/defrost Vacuum Bellows (9463042).

The speed knob has Auto before 0 so I suppose it is ECC right?

I found these pictures on the internet where you can zoom in on the check valves. Yours seem to be orientated the same way. These valves will keep the overpressure in the intake manifold (from the turbo) away from the vacuum system and only let air pass if there is a vacuum in the manifold (eg idling or when engine braking).
https://berle.nl/wp-content/uploads/...285-scaled.jpg
https://berle.nl/wp-content/uploads/...291-scaled.jpg

You have indeed the ECC system, in which case the shutters are still vacuum operated, but operated by an electric valve unit, controlling the vacuum to the vacuum servo's (did you check fuse 16?).

See enclosed diagrams: as you have been fiddling with check valves item 10 and these are a common point for all valves, it would be my starting point for fault tracing.

I’ve attached a picture of these valves from my 1997 car with ECC. Mine have never been disturbed and appear to be the same orientation as yours.

Problems with the vents are often down to the vacuum servos. Your symptoms seem a little different from the normal face vents stop blowing air when the car is under acceleration There are details on replacing these units in the FAQ.

https://www.volvoclub.org.uk/faq/OEM...tVacuumMtr.pdf

It is possible to bypass the one that most often leaks by disabling the recirculation feature which might be worth exploring if only as a non-destructive test.