This is a very common problem in the 960 and is almost always vacuum related. 1st, check that the check valve in the vacuum line on the passenger's side firewall (engine side) is holding vacuum. If it is, then it's typically the dual acting vacuum servo on the driver's side under the knee panel. A temp fix would be to put a golf "T" in the yellow vacuum line and see if the problem is solved.

The fan isn't stopping, the low vacuum in the system is allowing the flapper to move to the floor position during acceleration, then when you let up and vacuum builds up again, the flapper moves back to dash vent position.

A 3rd, less likely cause could be the vacuum solenoid block near the vacuum reservoir under the passenger's side dash, but it's typically the first two items that cause +90% of the problems.

You can also check the FAQ's for permanent repairs.

DEWFPO
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1998 S90 077,400 and 1995 964 154,100