There is no provision for cutting the compressor off at full throttle on the '91 model schematic. The later trucks may have a provision for this, but I do not see it in their schematics. The switch that is mounted in the compressor is, as I said, wired in series with the low pressure switch and the clutch. It cuts power to the clutch if head pressure gets too high. All you have to do is install it, get the proper connector to hook it up, and splice it into the power wire going to the clutch. This will give you additional high pressure protection. The third switch mounted on the AC line at the compressor is more or less for diagnostics. It lets the AC control head know when the AC is working and how often it is cycling. It allows the controller to shut the system down if cycling is too rapid or there is insufficient pressure in the system. There may be more, but I can't locate a FSM to find the theory of operation. Your AC control head doesn't have this function, so the switch is unnecessary. Again, wire in the high pressure cutout and you will have all you need for safe operation.
Edit: I looked at the later model schematic again, and it uses a relay to control power to the AC clutch, instead of the control head powering it directly. The relay may be grounded by the ECM, and therefore shut off at WOT. I can't seem to dig up the grounding for the relay, so this is just speculation. If somebody has the schematics for both years, that would be helpful.
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