Volvo had the conversion from R 12 to R134 in mind when they started producing the '91's. Your car should have an electric "pusher" fan in front of the a/c condenser and the single blue switch on the console that operates the system. Volvo also started using a larger evaporator in the dash, so the car is just about ready for the conversion without too much fuss. All it should take is an evacuation and flush of the system, adding a 134 adapter to the fill port checking for leaks and then charging the system. Before that, though, I would check to make sure the wire to the compressor is in good shape and connected, as well as the wiring to the pressure switch on the tubing going to the condenser. Check the fuses. If all is well, then the system probably doesn't have enough pressure to make the compressor cycle. Bring in a pro to evacuate, flush and charge the system, they have the proper tools and expertise.
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