1991 745 T, R-12 system. A/C got a bit weak last summer, so a local "Volvo Specialist" "topped it off" for me. Worked fine until earlier this summer. By the way, am in Florida, so we use the A/C even in the winter sometimes.

Early this summer, cooling performance began to drop. Eventually, about a month or so ago, we noticed the problem is that it is not running the compressor. Sometimes, it will start up the compressor and run properly for a long time, from several hours to at least a half hour. Cooling performance is very good to excellent then. But eventually, when it cycles the compressor off, it will not restart it. Or it will restart the compressor, only to shut it off in seconds. If the A/C is left running long enough, and it can be several minutes to nearly an hour, it will sometimes begin to behave normally again. For a while.

So, in other words, sometimes it runs fine for quite a while, sometimes it cycles the compressor on for just a second or two, then off for a short time, and other times, it does not even try to cycle the compressor on at all for a long time. Then, it can, somehow, seem to fix itself and return to normal operation for a while. Usually, but not always, it does better after the car has been shut off for a while, more than 10 minutes, but not long enough to cool off, then restarted.

Also, when at idle, sometimes everything is normal, and the A/C works fine, sometimes it does not engage the compressor. But, sometimes at idle, the compressor runs, cooling is fine, but the compressor shakes the car like a wet dog. It is incredible. But this is not every time, and I can't seem to figure out a pattern to when it happens.

A month or so ago, another, far better local Euro import shop checked the system for Freon leaks. They put some dye in and found no leaks. I don't know if they checked system pressures or not. But the system worked just fine for them.

My diagnosis is leaning toward the MCC controller box, which was replaced in July of 2003. The system was also recharged, and dye was added to the system then.

I have read the FAQ, and am going to check the low pressure switch today. Aside from that, what steps should I take next to diagnose?

Thanks in advance
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)