The AC switch is the driver's interface to turn on the air conditioner. Pressing the dashboard button sends a command to the body control module. The computer checks that engine RPM is steady and A/C pressures are safe before closing the compressor relay.
In the engine bay, pressure switches act as safety switches. The low-pressure cutout switch opens the circuit if the refrigerant level is too low. This keeps the compressor from running without PAG oil. The high-pressure cutout switch opens if a radiator fan fails, protecting the lines from exploding.
If a switch goes bad, the A/C clutch will not engage. Diagnosing a bad switch involves checking for voltage at the connector terminals. Replacing an inline switch is easy because they sit on sealing Schrader valves.