Confused about how pressure is controlled in an onboard air system? The compressor has an electromagnetic clutch. Give it 12 volts and it energizes to spin the compressor’s internals. Take power away and it disengages to free-spin. The brain of the system is a pressure switch plumbed into the air system. When pressure drops below a certain point, the switch closes and engages the compressor clutch. Once pressure builds, the switch opens and cuts power to the clutch. A toggle switch on the dash should be wired in series with the pressure switch to completely defeat the system (just like the original A/C switch). Wire the whole thing into a switched ignition circuit.