Ford Repair: Air Conditioning, climate control unit, jumper wire

Question
1994 F150 5.0L Automatic.  Had A/C stop functioning. Charged system with R134a to normal pressure.  Compressor tries to come on, then immediately cuts off, then continues this cycle.  Took a jumper wire to Low Pressure Switch near A/C fill point.  AC System worked great.  Went to parts shop for new pressure switch.  Installed new switch and still the same problem. The compressor kicks in and then immediately kicks out, kicks in, out, in, out about once every two seconds.  Jumped switch again, all works fine.  Got ANOTHER new lp switch, same problem.  Compressor kicks in, the out.  Rechecked pressure and it is ok, checked pressure with compressor on and off, readings normal.  Pulled plug off switch to check that the switch is making contact with the plug, compressor would not come on at all, reattached plug, same problem.  Any thoughts????

Answer
It is one of two things. The freon is low meaning that the gauges are inaccurate. Adding 1/4 can of freon or more will rule this out. That little will not hurt the system if your gauge shows it to be at normal, in the middle of the green. If it needs more freon than normal operating pressure could mean that the orifice tube is dirty or clogged or the A/C compressor is going bad.. The other possibility is the climate control unit is defective.