My Other Time Sink
 

In the midst of the fork swap I have spent 5 weeks on a interior trim out of a remodeled 1920's building/now house. Then the S.O. tasks me to fix the daughters beat to $H*t '94 Volvo 940. The daughter, to her credit, did not do the beating.
Mostly simple mechanical stuff, T-belt, water pump, hoses, belts, replace A/C components, etc. The real problem is the heater does not work. Without going into all the details, I have narrowed it down to the HVAC "box" in the dash. Yes I have searched the web for answers and have come up mostly short. One thread that described the exact problem that this car is experiencing, but dead ended without resolution. Believe it was a forum specific to, more or less, this car.
Water is flowing through the heater core. The hoses in and out get hot, but not heat at the vents. My thought is one of the vacuum motors is not opening/closing a door the channel the air through the core. There seems to be a somewhat common problem with the loss of vacuum when the engine is under heavy throttle (is that high or low manifold pressure? Can never keep that straight) causing the the "door" that controls the flow through the core to, well, not control the flow.
I have the recommended fix for the lazy door situation. What I'm wondering is, will this also fix the no heat. I have put my Mighty Vac on the vacuum motor in question with no conclusive results. I would rather not do surgery on the HVAC box if I don't need to.

That repair sucks balls!! Can't help but hope you don't have to pull the core.

You are so right. I don't think I have it in me to pull the dash. Hopefully the work around hack will fix it.

Is it a vacuum hose used to control the HVAC? In my van I had a similar issue but it was all perished vac lines which of course didn't hold vac. I got a diagram and replaced a bunch of rubber tubes and it worked fine.

Did you join the Volvo model forum? Someone has solved this issue and will help.

Vacuum Sucks
 

Look for a check valve in the vacuum line to the damper controls. They can get gummed up and not hold. Especially if there is a leak downstream and lots of air is flowing when it should not be. It is there so whatever the vacuum is controlling stays in the desired position when you give the engine lots of throttle and the manifold vacuum level drops. They are usually made of plastic and about the size of a quarter. Carb cleaner spray will usually clean them out.

Oh yeah check valve. Replace that for like$1.50 and look at the vacuum accumulator or ball. Holds the vacuum during those uphill climbs.

Good to hear the universe opposes all fork swaps and not just mine. Took me 2 years of other repairs to get to mine.