Do you have the 6500 series Basement model? If so, then the odds are the Main Control circuit board has a fault. That board controls ALL functions and logic of the Outside compressors and Blower motor and indoor blower. For some reason when the 2nd compressor comes on line, the board relay is opening for the Outdoor Fan, causing it to stop running. Since the Blower fan runs on Compressor 1, that rules out a fan capacitor problem. If your unit is the 2001 you have listed in your profile, then a Board replacement would be the first thing to try. Doug
