Volvo Repair: Volvo 850 air con, ventilation fan, volvo 850

Question
I have an intermitent problem with the air con on an 850 waggon. Sometimes you switch it on and everything is fine, another time you switch it on and nothing happens although it may come on during the journey. Once it is working it works fine providing you don't switch it off at the air con switch ( you can turn off the ignition and it will still be ok when you turn on again) but once you turn off the air con switch your problems start again. Gas charge seems ok, pressure switch is closed and the fuses are ok. If it's any help there is a groaning noise from the ventilation fan when the engine is turned off. Help! Please!

Answer
Peter, I have seen this before.  It may be 2 things.  One is a control head issue and the other is a sticky clutch.  What is the freon pressure when you are holding RPM at 2500?  If it cycles, then its still low.  The best test is when it stops(becareful doing this) use a pry bar to tap the clutch and see if it engages.  If so, there's your problem.  Its cheaper in the long run to buy a new compressor than to just replace the clutch.  Roger