Boring reading ahead, just ignore if you don't plan on ever putting a cummins in an Ex or Super duty
So the a/c works through the computer. And apparently the computer is fussy about what inputs it gets before it will send a signal to the a/c clutch relay. Solution is to bypass it with another relay.
This info is for a 2000 Ex with a v10. Other years/engine types may vary.
The original relay is in the power distribution box #302. Pin 86 is grounded by the computer to activate the a/c clutch.
To make it work, you pick up the wire coming from the high pressure switch to the computer. It is Tan/Lt. Green and goes to pin 41 at the pcm. You can either grab it at the switch or at the pcm, I chose the pcm since you're working in that area anyway. This wire has +12 volts on it when you select a/c and when the high/low pressure switches are in their correct positions. You will connect this wire to pin 85 on the new relay.
The wire that goes to pin 86 on the original relay is Pink/Yellow (may be red/yellow). I don't know what pin it is at the pcm, I just picked it up in the fuse box. You will connect this wire to pin 87 on the new relay.
Pins 30 and 86 are grounded on the new relay.
The idea is that when you turn on the a/c, you end up with +12 volts going to the pcm (now new relay). You need ground to trigger the a/c clutch relay, so the new relay is just flipping the 12 volt signal to a ground signal.
Recap:
Tan/Lt. Green = pin 85
Pink/Yellow = pin 87
Pins 30 and 86 are grounded.
I can draw a picture if someone needs it.