There is not supposed to be any power at that switch. It supplies a ground input to the PCM.
You need to start at the relay and see what fuunctions are missing. The PCM controls the relay by applying ground to it. You need to make sure the relay has 2 power supplies. The high pressure switch is powered through the control head on to the PCM for the A/C request.
Here's the wiring diagram.let me know if you need some help with it.