My 2002 S80's AC woork fine, then just stops cooling. Air keeps coming out of the vents, it's just not cold air. Car has about 125K miles on it. Is this a common problem on these cars?

 

The AC compressor has a magnetic clutch. Over time the clutch wears and the space increases to a distance where the magnet can't engage the clutch any more. This clutch engages/disengages and will usually work in the morning when the engine is cool and will stop working when the engine compartment heats up. To confirm you can try the broom stick test (but be careful). When the AC is on but not working press the AC clutch with a broom stick to see if you can engage it and get it working. I have read that people have wedged things behind the clutch (like the plastic clip from a bread bag) to decrease the distance of the clutch. In the end you will most like just have the entire compressor assembly replaced.

 

Could also be you are low on Freon. The system has a pressure sensor. If the system pressure drops below the threshold, the compressor will not engage or it will cycle on and off. Either way, it will not cool very well. A simple pressure test with a set of manifold gauges will indicate the amount of charge in the system. You may just need a squirt of freon. Not unusual for older cars to start leaking a little, especially over winter. I've had cars that I need to "top off" the system once a year.

 

on my '01, i've done the "Ghetto Clutch fix" and it helped, I also bypassed the thermal cutoff switch on the back side of the compressor and that has helped. Still, every now and then the AC still cuts off. I have to minimize hard accelerations as the engine is programmed to momentarily cut out the AC for power management reasons. Sometimes my AC will have a difficult time re-engaging. Just my 2 cents.