I don't know anything about the S/V90, but on the 7/9 series Volvos, the AC compressor is grounded by a wire that eventually breaks. This problem has fooled non Volvo specialists before. Get a pair of alligator clip wires and rig up a temporary ground from the compressor body or bolts or brackets to a chassis/frame/battery ground and eliminate this possibility.

Another possibility: On the 7/9 series with automatic climate control, the climate control computers can fail. I assume the S/V90 uses something just as fancy and complex and failure prone. They are expensive new. Try to find a used one if the diagnosis points this way.

Philip Bradley