My cabin fan works great when I step on the brakes. Sometimes. When it is working it seems to function normally; there is no strange noise and the heater or A/C works well. When I'm driving the fan may or may not work, and sometimes varies its speed regardless of anything I do. Mostly if I step on the brakes to slow or stop, the fan comes on to its nominal speed. But not always.

It doesn't matter whether or not the fan speed control is on Auto or manual. When I first start the car, the fan may or may not work, but if it doesn't, it usually kicks in when I stop at the end of my street.

I am constantly adjusting the windows to correct for the problem, which is very unsafe.

If you know anything about where to start looking for this problem or have a similar experience, please help.

Thanks!
--
98 V70 GLT black 54k

Sorry to say this, but you have to go to the dealer to have the codes read. You will never determine the problem otherwise. Just went through similar problem my self. Turned out to be a bad motor control of a dampener.

They got you with these newer cars. The computer will tell all, but your chance is remote.

I tend to agree. This my 2000 V70R is my first Volvo. I like the car, but the at ~90K miles, The electronics can be a nightmare. I had a similar problem, that my motor would run intermittently and unpredictable. Codes were read and they came back as " blower motor overcurrent drawn" or something to that effect. Sometimes with the fan on high speed I could hear a noise, like the blade rubbing also. Turns out that a louvre door arm was broken and would cause the noise, probably the noise an overcurrent as well. I replace both. Too bad the bury all this stuff so far in the dash.

In the process they found that the MOTOR that switches the ducts from recirc to fresh air doesn't work (I miss the good ole days of a mechanical lever/cable). I didn't have it replaced because of the sticker shock from having all the other stuff done, but perhaps I should have. Fans and A/C works fine now, but I am noticing something similar in that the flow of air is MUCH more with the car at rest vs. at highway speed (with no adjusting of the fan speed). I figure it has something to do with the recirc door and some effect of airflow at high speed?