A/C over pressure switch. Trips if the head pressure gets too high. Someone replaced the compressor with an older style. Millions of the older vehicles had no overpressure switch and they worked just fine. You may want to pull that jumper and make sure it's clean and making a good connection then seal it up good and make sure it can't get against ground. A better fix would be to cut the wire,slip a piece of heat srink tubing over one of the wires,strip,solder,slide the heat shrink back up over the connection and shrink it down.
Or,,,,,,,,,,,,,go buy a compressor with the head pressure switch.
.035 SS welding wire makes good jumpers for such rigs.