My volvo 940 decrease the fan flow when I speed up the car, and it is independent if it is in auto or not , or vent selector in any speed, I think it is a vacuum problem, I do not feel that the flow change for the differents ducts, I only feel that it goes down when I press the acelerator and if I maintain the car speed I have not problem at all.
Is there some vacuum diagram available?
Looking at the engine, I found a rubber hose (4 mm interior diameter aprox)coming from the intake manifold intentionally blocked for someone, and also near to the hoses that goes to interior of the car I found a plastic rigid hose (2 mm diameter aprox) that is broken, but no idea where it goes. In spite of the different diameters I connected both hoses with no results in the blower fan, but listening a hiss in the interior of the car when I speed up, I come back everything to their position again.
I would appreciate any help. Thanks in advance

 

Do you hear the actual fan slow when you hit the gas, as in spin slower, or does just less air come out but the fan sounds like its spinning the same speed?

 

Ok hows this theory sound...?
the blower motor is being affected by the airflow from the icrease in speed (increase in speed = increased resistance to the fan slowing it down)caused by the stuck air valve not allowing recirc flow and a weak fan motor... any takers?

 

I will check tomorrow if the spinning of the fan decrease, the flow depend on the effort the engine made, for instance if I drive going up an slope I have no flow at all , the same when I start moving, if I decrease the speed I have flow again, I will try to hear if the rpm of the fan changes setting the **** in 5 and speed up.

 

Fernando, I have the same problem. I have been told that the bellows that control the vacuum on the airbox under the dash are bad. They (nasty dealers) told me there are 3 or 4 bellows, depending on the model and they are cheap plastic that break down over time. They also said it is a $1,000 USD repair, because they have to take the entire dash out to fix them. I have a hard time believing them, since everything works fine otherwise. I have been toying with the idea of throwing in a vacuum reservoir in the line to the air box.

 

I had the same problem with my '87, 740 GLE. The fan speed did not change but the air flow did. I have over 400,000 miles on the car so I let someone buy it form me and moved on. That was the only thing wrong with the car. It looked like it had 75,000 miles on it. I was also told it was the bellows. Looks like 4 tennis ***** connected to each other with a vacuum tube coming from them.

 

If the fan motor speed is constant, then most likely the issue is a faulty vacum check valve that sits on top of the intake manifold. It is the job of this check valve to keep the vacum up when accelerating, etc. This vacum is needed to open/close the flaps that run the heat in the cabin.

I had a similar problem on my 240. Another symptom was that you could hear a hissing sound in the area of the center console every time I turned the engine off.

You can easily check to see if this is the problem. The valve is black and white plastic with one end connected to the intake manifold and the other has a small tube that goes through the firewall. Remove the valve and try blowing through the larger opening. If air can pass through, then it needs replacement.

 

In the 940, the check valve is right up under the hood against the firewall on the right side looking towards the rear of the car. it looks like a black and white marsh mellow. follow the tubing coming from the top of the intake manifold if you cant spot it. You can get them for $6 at ipdusa. http://www.ipdusa.com/product.asp?st...hStartRecord=1

Also check all that little tubing that plugs into the manifold. Sometimes there will be a crack in one especially in the rubber connectors. In my case, i some pieces of gas line tubing or something similar from behind the desk at a local autozone and cut small 2" pieces and made my own replacements.