I have two '93s, and when I first got my first one, I was surprised that the fan wasn't running whenever the A/C was switched on. But looking at my (Volvo "green" book's) wiring diagram for the '93 model year, it's easy to see that the fan is *not* supposed to work like I (and you) initially expected.

I think your '91 is like my '93 (but I'm not sure, as I've never seen a '91 wiring diagram).

It's controlled by a relay (in the '93, on the driver's side fender's relay support bar) that's activated only when an A/C pressure sensor (actually, a switch called a "presso-stat") closes from high pressure. So the fan should properly work intermittantly if at all, and only when A/C pressure is high. In cool weather, the fan may not even turn on at all.

That pressostat is located (again, on my '93, and maybe on yours, too) on a Y-branch (like a railroad side track) from the main A/C line -- look down under the lower (output, or return-to-engine) radiator's coolant hose, where you'll see a metal A/C line terminate on an electrical connector.

If you want to test it, remove the plug (depress the wire to release it) and put a paper clip across the terminals -- the fan should start, showing that the relay and fan motor are good.

Hope this helps.