a/c compressor not switching on

My dad has a 93 chevy cheyenne with a 4.3 v6. A couple weeks ago i added some freon to the system and everything was fine. After running the system for two or three days it stopped working. I thought it may have been a leak so i tried to add freon and it wouldn't take. After looking over it i see the compressor is not kicking on. when i jump it, it comes right on. I can't find any relays or disconnected wires. What else should i be looking for to fix this?

Too much pressure in the system will lock out the compressor. There is a pressure switch on the dryer (up near or on the firewall) that won't allow the compressor to kick on if too high or too low. IF you jump the compressor, bypass the switch, it will turn on. If your freon pressure is good, then the switch may be bad.

There are two pressure switches, one for too low, and one for too high. A manifold gauge set is the best way to determine if the system is overcharged. The dinky quick gauges sold with refrigerant cans don't show this high-side pressure.

Was your system converted to R-134 or did it come from the factory that way?