12. A trinary switch is used to shut off the A/C compressor when it detects extremely high pressure in the A/C system. It is also is used to turn on the A/C fan when pressure reaches a predetermined amount. When the pressure is reduced by the A/C fan pulling colder air across the condenser, then the trinary switch turns off the fan. If you have two fans or a high- and low-speed fan then hook the trinary switch to the secondary fan or high-speed fan and not the primary engine-cooling fan.