GM-GMC: 1996 GMC Sierra Truck AC Not Working, compressor clutch, gmc sierra

Question
Bought this truck 2 months ago for 17 year old son and AC was working and then just stopped.  The AC light on the dashboard does not light up when pressed in.  The blower fan works.  Checked fuses I could find and none blown.  Checked the freon on low side and did not add freon as the gauge showed in the red at 100.  Checked voltage to the low side and no voltage.  Checked voltage to the compressor as compressor does not kick in and no voltage to the compressor. Checked voltage to the high side and little voltage.  Discharged battery and reconnected and no change, AC doesn't work.  Can't find an AC wiring diagram to check all the wires. Any suggestions you have would be greatly appreciated.

Answer
There are a number of possible causes for what you are describing.  The first one being the system is low.  I realize you have a pressure on the low side but I am just going through the possibilities.  Without the adequate amount of pressure the system will not engage the compressor clutch.  Another possibility is that the a/c relay is not functioning properly.  If all the circuits etc on the control side of the relay are working properly etc but the output side is not or does not, you will not get current to the compressor clutch again, no a/c.  Simply swap the relay with another one with the same p/n and try it.  A third possibility is that the control head for the a/c is not putting out the proper signal to the pcm or relay to turn the a/c on.  The easiest way to test for that is to display the outputs on a scanner and turn it off and on and see if the pcm is recieving the signal.  Unfortunately I don't have access to online diagrams for that year of vehicle, I will check the manuals when I return to work to see if there is anything else that sticks out as a possible cause for what you are describing.  I would work through those possibilities I have listed and see if you get anywhere.
Hope this helps.