[cruiser_guy]

Stone, connect the wire from the A/C compressor clutch to power (12v or 24v whichever it takes) there is only one wire. If the clutch closes, you're OK on the clutch, if not, and I'll bet that it won't close, the clutch is toast.
It can be replaced on the truck if you know what you're looking for. You'll need to loosen the 12mm nut holding the outer part of the clutch and pull it off. There will be spacer washers behind there, be careful NOT to loose them as you need them on reassembly. There will then be a large inside spring clip holding the pulley or electromagnet (whichever is next) on and another spring clip holding the last part on. The biggest problem will be pulling the large spring clips off with the compressor mounted. I've done it but it's not easy.
If the clutch does close then the pressure switch would be suspect and for that you need to discharge the system.