I have the Volvo manual for comfort control and would like to share how to know if your compressor is faulty.
If compressor will not run or runs intermittently
Disconnect connector from pressostat(what we've been calling low pressure switch). Connect ohmmeter across pressostat pins and check for 0 ohm reading

A) If ohmmeter reads infinite resistance across pressostat pins:
connect filler unit low pressure side to service valve and read pressure on low pressure side. If pressure is less than 46 psi, then add 500 g of R134 and see if compressor goes on and off less often. check for leaks. (there are other threads that go into leaks!)
If pressure is more than 46 psi, then pressostat is faulty and must be replaced! then go to PERFORMANCE TEST, which consists of:temp switch to full cooling, A/C switch ON, mode control to ventilation, Recirculation switch ON, fan switch on full speed, hood close, all windows and doors shut, engine RPMs to 1500, put thermometer in center panel vent and allow system to stablize for 8 minutes. Temp should be in mid-40's!

B) If ohmmeter reads approximagely 0 ohm across pressostat pins:
Allow engine to idle and turn A/C switch and fan ON. Connect voltmeter between green wire on pressostat connector and ground. Voltmeter should read battery voltage.

a) If no voltage, check circuit from A/C switch via the A/C relay to the high pressure sensor. (i'm assuming that the relay is the one near glove box). With engine idling, A/C switch and fan ON, connect voltmeter between blue/gray wire on sensor connector (pos. 4) and ground. Voltmeter should read battery voltage.
If no voltage, refer to ignition system manual!
If voltage, turn off engine and check terminals of sensor between 3 and 4 with ohmmeter and see if it is 0 ohm. if ohmmeter reads infinite resistance, replace high pressure sensor
If ohmmeter reads about 0 ohm, check green wire between pressostat and high pressure sensor for breaks/short, repair and run performance test

b) If voltage is present between green wire and and ground:
turn off engine, connect ohmmeter between GRAY wire on connector and ground

If resistance is less than 5 ohm, disconnect compressor connector and measure resistance between engine block and cable to compressor clutch.
if approx 0 ohm, then clutch is faulty and need to replace compressor.
if resistance is 2-5 ohms, then check gray wire between pressotat connector and compressor for breaks and repair and go for PERFORMANCE TEST.
If resistance between gray wire and ground is greater than 5 ohms:
disconnect compressor connector and measure resistance between engine block and cable to compressor clutch. If 2-5 ohms, check wire for breaks and do Performance test. If greater than 5 ohms, then thermo connector or compressor clutch is faulty. repair as necessary and do Performance Test.

That's it, folks.