I own a 91'Volvo 940 Se Turbo when using a/c or heat when accelerating all are goes to the floor airflow stops blowing through the vents per dealer both one-way check valves were replaced I still have the same problem when accelerating w/ a/c or heat in use dealer now giving me the run around. Any and all ideas,diagnosis,or suggestion will be appreciated.

Thank You,
Chooselife

check your (hard plastic vacum hoses)carefully,sometimes they rubb against parts under the hood and create vacuum leak.vacuum hoses inside the car rarely fail.good luck.charles.

...topic is covered in considerable detail in the FAQ's. Go to:

http://www.brickboard.com/FAQ/700-900/HeatingAirConditioning.htm#960ACVentStopsWorkingUponAcceleration


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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD; plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)

I'm not sure but what your experience is fairly normal. Often, on a long uphill climb with the accelerator floored, my vents will change just like yours. As soon as I let up on the accelerator, the vents change back.

The actuators for the vents require a constant vacuum. With the accelerator depressed, the throttle plate opens and the intake manifold vacuum drops way down. The check valve is supposed to prevent the higher pressure in the manifold from bleeding into the heater/AC vacuum reservoir. If there is even a tiny leak in all those hoses and actuators under the dash or in the check valve, the vacuum in the reservoir is "used up" and the actuators return to the rest position which is defrosters and floor. Since you've replaced the check valve, it seems that there is a small leak in those tubes. This would be expected as they age and harden.

In my case the problem is slight and I can live with it. If your system changes over too often, you may have to go under the dash and replace all the vacuum hoses and maybe an actuator or two. I haven't done this on a 900 so I don't know how hard a job this is. I have heard that some of the actuators are a PITA to get to.

This is sound and accurate advice!