Not knowing which sensors you have checked, or how you checked them its pretty generic of a question. First is the system fully charged? If you KNOW it is, and you are SURE that the sensors?? are working, then it might be a control head issue. This is not really something that can be checked as a seat of the pants. Which sensors were tested, and how so? and what were the results etc.
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If you just want ot test the ac clutch. Get some leads and run a line from the + terninal of the battery to the lead coming out of the ac compressor. There will be a join close to the compressor. disconnect the join and attach the lead to it. The clutch will click in and out as you apply the power .REMEMBER this line will be full power from the battery so be very careful not to allow it to touch any metal part or it will short.
I would check the low pressure switch next. The switch cuts power to the compressor if the pressure is too low. The switch can fail. Same goes for the high pressure safety switch. As for leaving the relay bypassed, you would need to use a test light or meter to see if you are using battery power with the key off. I would not recommend it.
check your fuse box for a blown ac fuse, fuse box usually under dash, sometime under the hood of the car usually on drivers side. bad switch? check if you hot wire lead might be pinched and grounded out or broken wire between battery and switch
First check the wire harness connecting to the spark plugs ignition coils f/b connections to the fuel injectors and then to the ignition circuit unit and lastly to the gearbox. Check yr all the groundings in the engine bay. If necessary install new grounding wires. As for the air conditioner, check the 2 smal valves in the hood at the firewall, if ok, then it may be one or two of the 4 bellows under the dash has given way. Also check the temperature sensor at the dash (top) front passenger side and the sensor pressure valve (connected to the evaporator piping) underneath the carriage at the front.
I AMA MASTER VOLVO ASE TECH AND RUN A JUMPER WIRE FROM A/C COMPRESSOR WITH A/C ON MAX TO POS BATTERY AND SEE IF COMPRESSOR CLICKS ON AND IF A/C RELAY GETS HOT IT IS POSSIBLE A BAD COMPRESSOR OR DOES THE FAN COME ON WITH A/C ENGAGED THE FAN IS A TWO STAGE UNIT ON 850,S ONE IS THE ENGINE TEMP RED AND THE GREEN IS FORTH A/C SYSTEM