I had an epiphany about mysterious under-dash hose
 

Years ago, when I was removing bundles of under-dash wires from the non-functional alarm system, I found a disconnected hose. I couldn’t figure out for what. My ‘83 had Cruise Control that I finally tried yesterday. The Cruise Control didn’t work. Then it dawned on me “THAT HOSE! IT’S A VACUUM HOSE FOR THE CRUISE CONTROL!” But sadly, I clipped off that hose and I don’t think I’ll reconnect it. But oh well, now I can stop worrying about it:)

924/944 cruise control does not use vacuum.

Ahhh..,thank you! Well, now the mystery continues about that mysterious rubber tube under the dash. My car seems to drive perfectly, so I’m not going to worry about it.

Could it be a drain?

it might be a hose that runs to the manifold , through a heat or AV vent comntrol and then to an air powered actuator. in a volvo and I htik in a 944 as well they used vacuum to move these things.

check if all you air and heat flaps work or not.

If you cut the line it may lead to the manifold and that could be a source of unmonitored intake air. it could cause rough idle or troubles starting.

if this is the case try plugging the hose off, you can mend it later.

the defroster may not seem too important in summer but it may be undrivable if the windows are not able to clear once weather is worse.

a volvo has a "vacuum storage tank" it is part of this system. it can be in various places, in my volvo 740 it is hidden inside the front bumper! Thats about the last place one would look for the thing !
in a 944 , not sure.
I think the sliding controls work a valve linked to this vacuum driven actuator system.

if the actuators do not work thats a second issue but if you have a vacuum leak it may be affecting the engine's ability to control the Air fuel mixture.

a vacuum line may be easy to pull off when working , since it is conducting vacuum and not pressure it may not have or need a hose clamp because vacuum will tend to pull the line on, not push it off.

WM, that vacuum tank, you referred to, is located by the battery and looks like a cylinder with a "rounded" bottom.