Remove the connector at the low pressure switch on the accumulator (the aluminum cylinder at the firewall). Jumper across the two terminals inside the connector (a paperclip works fine). With the AC set to on, the compressor should engage. If it does, don't run it that way for long since the reason why your system isn't working is because your refrigerant is gone and it carries the lubricating oil for the compressor. Have a close look at the aluminum hoses running along the frame rail on the passenger side.

Good luck!
--
Herb Goltz, London, Ontario, Canada '92 245 w/109K mi

Unplug the connector in the black wire going into the clutch assy. If you have an ohm meter there should be a low resistance 3-5 ohms measured at the compressor side of the connector to the compressor body or engine block. You should be able to jumper this directly to the pos battery terminal and see the clutch activate. The engine does not have to be running. Because of the low resistance of the clutch coil there's going to be a little bit of a spark.

You can jumper 12 volts to the compressor from the B+ on the back of the alternator. There is a relay on the side of the heater case behind the glove box. It sure would help to know what year model it is, believe it or not all 740's are not wired the same.

Mark