There really is not much wrong with the way the A/C is designed other than maybe the location of the evap temp sensor, but even that is not really a problem if the correct fill of refrigerant is used. If you need to put your own control system in place I think you might be missing something.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)