Is the fan supposed to run at low speed when the ac is engaged? The fan only turns on when the high side reaches 350psi when the ac is engaged. Low and high pressures are a lot higher than they should be and cooling is less than optimal.

I can ground high, low, and engine temp sensors and the fan actuates. Therefore, not a relay, wiring or fan issue, and I expect the low pressure pressostat is faulty but I do not know how to diagnose a pressostat and i also do not know the psi level where the low pressure pressostat should ground the fan relay. When I ground the low speed fan circuit, ac levels are where they should be and cooling is much better.

Yes the radiator fan is supposed to run at low speed when ac is engaged. Current modern cars instantly turns on their radiator fan when AC is switched ON but not on our 940 Volvo. Our low speed fan circuit is controlled by the grey pressostat (located on condenser) which only turns on the fan when it senses the refrigerant pressure more than 270psi. The brown pressostat (located next to the grey) turns on the fan at high speed only when pressure is more than 334psi. This could mean that your grey pressostat is faulty.

How to test? Unless you have a pressure tester with a gauge and proper fittings, you can't. Get a new one to compare with the old.

The grey/brown pressostat fan circuit is electrically independant of the ac compressor electrical circuit. If you park your car in very hot weather and underhood temp is high enough to raise the refrigerant pressure in the ac pipes then the fan could automatically start spinning until it cools down. This with engine off, ac off.

Amarin.