Air Conditioner problems

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.

did you already try to introduce a can into the low side port of the system?

sometimes, adding a can will give enough pressure to engage the compressor after the low pressure switch senses some R134a freon into the system...

Yes, that is what I was trying to do when I was jumping the compressor. I thought that it was low and hoped to pull enough in to get it to cycle on it's own.

The high pressure cut off switch on the rear of the compressor must show less than 5 ohms resistance between the 2 pins. If not, the switch is faulty.

The clutch cycling switch on the accumulator must show less than 5 ohms between the 2 pins. If not, the switch is faulty, or the system has less than ~42psi and needs to be evacuated and recharged.

FYI, these systems must be evacuated for a minimum of one hour, then charged with the proper amount of R134a refrigerant and oil to operate properly. They're not like the old systems that you could just "add a can". If you use that method with this system there's a real possibility of destroying the compressor, and/or blowing a line. If the system is low on refrigerant, there is a leak. A very common place for these systems to leak is where the case halves come together on the Harrison H6 compressor, referred to as a "belly leak". Rule of thumb is, if it's not leaking now, it will shortly, (they all do at some point in time). Sanden makes a replacement compressor that is designed differently and does not leak.

Thank you for the detailed assistance. I will check it out as soon as I get a minute

Turns out that a wire was broken or fried between the pressure switch and the compressor. Replace the harness and all is well