Air Conditioning compressor fails to cycle

[plymouthcranbrook]

I have a 2001 Astro with 151,000 miles on it. The Wife told me earlier this week that the Air Conditioner did not work when she tried to use it on an 80 Degree day. I expected that the system was low on R134 and bought a couple of cans to put in. When I went out to check it the compressor would not kick in. Jumping the connector at the low pressure switch did not work. I checked and there was no voltage at the switch on either wire. I went to the fuse box under the hood and checked the relay and also the relay that activates the air conditioning relay. First I switched the fuel pump relay with the air relay and the truck started. Relay ok. Then switched the preactuator with the headlight relay and that worked as well. One lead to the Air relay has hot all the time and was. No power except that one was available anywhere in any location of both relays. However there is continuity between all wires to the low pressure switch and also from there to the compressor. The fuse in the underhood location is good. Does anyone have any ideas to help me locate this problem? Thanks if you do, as I am stumped. Also does anyone know where I might find a wiring diagram for this area of the truck?
I would appreciate any ideas.

[chevymaher]

On the back of the compressor there is a high pressure switch. Or shall I say most have it. Maybe it bad check it.

[plymouthcranbrook]

Thank you for that information. I will see if I can check it this weekend.