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On my old truck, I experimented with an adjustable pressure switch for the AC unit.
The AC on the truck would cut on when the pressure in the evaporator had risen to a certain PSI.
The PSI was set too low for long trips, the air would get too cold, so that told me the compressor was running too much, once the AC had cooled off the cab. Moving the temp select switch didn't make the compressor run less, it only mixed hot air with the cold air.
The pressure switches in this model ( 1979 Chevy) are adjustable, so i was able to adjust the switch so that it would not run the compressor as much.
Problem was it took to long to cool off with the switch adjusted for best economy.
Solution, I ran two pressure switches, one adjust high, the other low. Inside i had a double pole toggle switch that I used to pick which pressure I wanted to use to control the compressor.
On those long highway rides when the air got to cold, I simply switched to the "economy" pressure switch and enjoyed the gas savings.
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