[dontlifttoshift]

I answer trinary switch questions at least once a week. Most of the confusion comes from the fact that all of the aftermarket controllers want a 12v feed to the fan controller. The theory being that most people will just tie it to the wire that turns the compressor on. But we are smarter than that, we know the fan doesn't need to run when we are doing 70 mph down the freeway.

So there are 4 wires on the trinary switch. It is important to note that it is just a pressure switch, it is not magic.

The 2 black/green wires are for the hi/low safety shutoff. If pressure is too high or low, the contacts open and the compressor shuts off. One wire goes to the compressor and the other wire goes to the ECM (genIV) or the thermostat (genII)

The 2 blue wires are for the fan control function of the trinary switch. If your fan was just wired with a simple relay, not a controller, you would ground one of these wires and the other would go to the "85" or "86" terminal of the relay. When pressure reaches 254 psi the contacts close, creating a ground path for the coil side of the relay and the fan runs.

When using a fan controller that needs a 12v signal to turn on we need to use the fan control function of the trinary switch to switch the power side instead of the ground side. Remember, it is just a switch, it doesn't care. So with a fan controller, one blue wire hooks to switched 12v and the other side to the "AC" input on your controller. Now when pressure reaches 254 psi, the contacts close and send a 12v signal to the fan controller and the fans turn on. The only thing that this 12v wire does is tell the controller that we need the fans on.

I typically run both blue wires from the fan switch to the fan controller and get the switched 12v from there.