the pressure switch used by patc is a GM unit used in turbo 350 transmissions-----they charge about 20 times more for it than it costs-----the AC pressure switch turns on at 30 psi and turns off at 20psi (for instance)---this prevents the AC unit fluttering on and off at 25 psi and destroying the unit.....you can find a multitude of psi ranges available-----if you are wiring both lockup and overdrive you want to have the activation points at different spots.....i bought the patc kit for my 64 savoy with a 500 tranny---i was less that thrilled with trying to adjust the vacuum switch they provided---i run without it------i bought a 92 diesel truck with a bad o/d switch---it had been bypassed with just a toggle-----i put the purloined AC switch on it and drove it a year without issue.......