The pushbutton controller decides on the need for the compressor based on the sensors and the settings. The signal in most cases is a ground signal. The compressor controller (Klima, MAS, BASE) sees this signal by the control leg being pulled to ground and turns on the compressor (if all its conditions are met). This MOST important of signals passes through the low/high pressure switch.
In your case if batt voltage is on one leg that tells me that the compressor controller is prepared to see the signal. You said one wire was batt voltage and one was open. The switch is closed if there is any gas in the system at all. Check it with an ohm meter or place the batt voltage lead on the switch and see if the other terminal is also battery voltage. Check this with a volt meter as a test light will look like the ground signal and probably turn the compressor on.
If the system has gas on it then place both terminals back on and see what the voltage looks like on either. Since the switch will be closed the same voltage will be on both sides.
With the car running and the system in the EC position (Compressor desired off) the voltage should be battery voltage on both legs of the switch (with both wires in place; pull up slightly and check each term at the base). When the system is engaged (compressor desired on) both sides of the switch should be at about 0.15 volts to grounds. If at any time the voltage is different across the switch then the pressures are wrong or the switch bad (they leak but never go bad electrically).
The pressure switches pur[ose originally was to protect the system from running "dry". At a real low pressuree (this is on the high side so the pressure of any gas will get greater when the compressor engages. In the early 80's the switch was changed from only a low pressure switch to also cut out at high pressures to protect the system from a blockage or aux fan failure. On 134 conversions this is an absolute necessity.
Be real carefull where you might buy this switch as most are only low pressure. The parts procurers see that one number converts to the other and assume they are the same. The early switch is much cheaper. Only the smartest parts houses know the difference. Almost everyone selling this switch has it wrong.
Bosch Master, ASE Master, L1
33 years MB technician