Now it is a few days later. I have confirmed that the system was converted to R134 a little over a year ago, and the A/C worked fine over the long Tucson summer.

I am beginning to think that the pressure switch down by the dryer? (bottle with the little window in it) should be always "made" if there is freon stuff in the system. This would explain how the "signal" from the a/c controls inside the car gets through the switch and on to the control unit behind the battery before the compressor starts running. So if the pressure switch is not "made" (mine is always open) that means there is not enough coolant or freon like stuff in the system. If that was the only problem though, I would have expected the following test to have caused the compressor to run: (For this test I replaced the control unit in its 12-pin connector, connected a jumper across the pressure switch's terminals, started the car, turned on the A/C with temp set to minimum, still the compressor did not run.) So I need to check the R134 level. If that is good and the pressure switch is good, then the control unit must be bad.

I have read in some posts a couple of years old which say R134 is a real problem. Is that still the case?