On top of the comp. there are 2 connectors.. One two wire connector and one one wire connector. The one wire connector (The front one) should have 12 plus volts going to it when air is on.. Disconnect it and check with VOM and see if you have juice there. If not , what i would do is jumper a wire from battery plus side to the wire going to the clutch, If your comp. starts then, I would bet you are low on freon. If you deside to add it, keep the jumper in place and run the engine up a little.. after connecting the freon hose to the low side of the system.. The high pressure swicth turns ON at from 196 + or - 27 PSI. It turns off at 384 + or -28 PSI.. This tells me even if you have 165 PSI on the high side it still may not be high unough to turn on the switch.. add a litle R 12 and i think you will solve your problem. Hope this helps. Jim