Does anyone have any ideas about my A/C mystery?

Ever since I got the car about a year ago, the A/C compressor has always come on when the blower in the cabin is turned on. The "ac off" switch does nothing. Even with the heat at full power the compressor runs. If you forget to turn the blower switch off before starting the car, the compressor engages while I'm cranking the engine, causing rough startup and lots of wear on the starter - there's no timer delay function. I just got the system recharged today, and the mechanic said that this was bad for the compressor, since it's supposed to cycle on and off periodically when in use. Is this really true? I've already tried a couple of replacement control units from junkyards which I was (fortunately!) able to return when they all did the same thing. So I know it's not the control unit.

Does anyone have some good knowledge of the electrical system for this car, or any similar experiences? I'm thinking it must be some kind of electrical short which is bypassing the control unit in some way. Something must be getting through, since when you turn the blower off the compressor disengages. The mechanic (a certified Volvo specialist) couldn't think of anything apart from the control unit, which I know to be good. He also said there were lots of different sensors in the system which could be to blame, but didn't want to get involved with tearing everything out and doing extensive testing.

Sorry for the long post, but I'm a little worried about burning up my compressor!

Many thanks,

Ben