The MS3 has a dedicated A/C control option, allowing the ECU to take several actions to control the A/C. These include a full throttle cutout, increasing idle speed while the A/C is active, compressor overspeed protection, and more. The wiring needed is very straightforward. You’d need an on/off switch on the dash, a cycling pressure switch in the A/C system which closes when pressure allows the A/C to operate and opens when the pressure is out of acceptable range. The pressure switch is also called a trinary switch because it responds to three possible conditions: It will turn the A/C off when pressure is either too low (because your refrigerant has all leaked out) or pressure is too high (the refrigerant has backed up between the compressor and evaporator, and so the compressor needs to cycle off), and let the A/C operate when the pressure is in the correct range. The pressure switch and activation switch are simply wired inline to an available switched input.

The MS3 can use any switched input for the A/C input side, and any ground switched output to drive the compressor relay. Here’s an MS3Pro example that used Digital Switched In 1 as the A/C input, and the High Current Out 2 to control the compressor.

MS3Pro A/C control wiring diagram