[ls1nova71]

The 3 wires you will need to keep from the PCM connectors are-

C2-17 dark blue - A/C request. This is what tells the PCM to turn the A/C on, it hooks to the 12v wire coming from the controls that is hot when the air is on, or in other words, the wire that originally went to the old compressor. It will go from this compressor wire (dark green on a 67-72, and probably later) to the high pressure sensor, which is the purple sensor on the back of the compressor, and then from there back to the PCM (C2-17)

C2-43 dark green/white - A/C clutch relay control. This will hook to the #85 pin on the A/C clutch relay, it supplies a ground when the PCM commands the relay (and therefore the compressor) on.

C2-55 dark green - A/C cycle switch. This will go to one side of the low pressure switch, (usually mounted on the accumulator) the other side of the low pressure switch will go to ground.

The rest of the wiring is the remainder of the A/C clutch relay wires, and the actual compressor clutch it's self. The A/C clutch has two wires, the black goes to ground, and the dark green goes to pin #87 on the relay. Relay pin #86 will be an ignition on hot through a 10a fuse, and pin #30 will be a constant hot through a 10a fuse.

You didn't say what year your truck is, but this is the way I have mine hooked up and it works flawlessly. However, I do have a later model evaporator with an accumulator and low pressure switch rather than the original POA valve. The PCM controlled A/C is designed to cycle the compressor on and off depending on pressures, if you're using a POA, it's designed for the compressor to run continuously, so I wouldn't try to use the PCM controlled A/C in that case.