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 A binary or trinary switch will cut off the compressor clutch at 30 or 406 psi. A binary switch is used if a mechanical fan is used. This is a trinary switch, which also sends a signal to engage an electric fan at 254 psi. It is wired into the same relay as the engine temperature switch, so the fan will engage with either engine temperature or A/C pressure. It can be mounted anywhere between the condenser and evaporator, in this instance just behind the radiator support panel between the dryer and condenser.