2000 Pete 379 ac compressor not comin on

We have a 2000 379 with a cat 600. The compressor is not get its ground signal to come on. I have vaccummed the system down and recharged due to overfilling by someone else. If I ground one side of the low or high pressure switches it will come on and start cooling. When you unplug either switch I have power on both sides. I know this is not right. Please help as it is starting to get warm here.

 

Thanks, I tried jumping both switches on the firewall. I don't understand why i have power on both sides of the harness where it plugs in to the switch. One side will lock the clutch on the compressor and the other side does nothing. This was tested with a test light. I did check all fuses in the truck with the test light as well. The driver of the truck swapped the relay with several "spare" relays he had in his tool box. I checked the low pressure switch on a working truck and there was only power one one side of the pins where it plugs in to the switch.