Weird problem with my Car's A/C. Any Suggestions?

1993 Mazda Miata has plenty of pressure in the system and no leaks. I can jumper the clutch with +12v and it shows proper high and low pressures. After about 4 minutes it overpressured and vented a little refrigerant at the compressor, but not all of it. Still have plenty in the system.
The truly weird thing I am puzzled by and can’t find is that I am getting negative .8 volts at the wire that’s supposed to provide the +12 to the solenoid for the clutch. I can only find an underpressure switch on the lines. Any Ideas where I can look for an obviously malfunctioning switch that might throw voltage the wrong way?