It's not easy to get at the compressor but here is the procedure. Myself, I would remove the cmpressor and recharge the system.

Volvo 850 AC Compressor Clutch Removal

1. Don't discharge the A/C system.
2. Remove the bolt holding the receiver dryer bracket to the car, but don't disconnect the receiver/dryer.
3. The manual tells you to remove the 2 radiator bolts and swing the radiator forward. This looks like a bad idea to me. If you do that, nothing is holding in the radiator but the hoses. I removed the right hand bolt and loosened the left hand bolt, so that the right side could swing forward.
4. Unbolt the compressor and move it to where you can access the center bolt on the clutch plate. Remove the bolt. You probably need an air impact wrench to get it off, unless you can stop the plate from turning when you try to unbolt the bolt. (Don't over torque it going back in. The spec is 10 ft/lbs.)
5. The manual tells you to pull off the clutch plate. It will not come off. What they don't tell you is that there are 3 5MM holes in the plate. You need to get three 5MM bolts, about 1 1/2" long, and thread them into the holes. Tighten them evenly, and it will pull the plate off. If you don't do it evenly, the plate will get cocked and stop coming off.
6. Using a six inch wide, 2 jaw puller, remove the pulley. You have to rig something up to protect the end of the compressor shaft. If you damage this shaft, you will be buying a new compressor.
7. Using an impact screwdriver, remove the 3 screws holding the clutch coil to the compressor.
8. When reassembling, don't forget to put in the clutch plate spacer shim(s). The new clutch comes with 3. I used the one shim that was in the old clutch. (It was stuck inside the clutch plate, so you have to go look for it.)

Re-assembly is the reverse of dis-assembly.
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Tom 69-1800S, 72-1800ES, 96-850T