Check the climate control fuse if you have climate control on the Volvo.

Also, you may want to jumper the compressor clutch to see if it turns. If it does, and you get cold air, the problem is in the switch. If it runs, but you dont' get cold air, you have a big leak and no pressure.

I'm not familiar with the 850 AC system, but on the 740, I had a bad switch assembly. BIG PITA to figure out the swap, but fairly easy when I did it. AC wouldn't go off in vent mode, one of the contacts had come adrift in the slide switch.

On the 740, Volvo used GM type AC (Cycling clutch, Orifice Tube or CLOT) -- if the pressure switch on the accumulator is bad, the compressor won't run. Unplug and jumper it -- if the AC works, get a new switch. If not, big leak.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!