Have you tried to bypass the low pressure switch to see if it is the problem?

on the passenger side of the car near the firewall, on the same low pressure line that is used to charge the system, there is a low pressure switch. it is on top of the engine compartment and very easy to check.

Unplug the wire harness from the sensor and jump the circuit in the warness plug that connects to hte switch (ordinary wire works fine). Does the compresser turn on now? if so you have 1 of two problems either you actually have low pressure in the system, or a faulty low pressure sensor.

Have you checked the pressure on the low side with a guage? is the pressure OK? if it is and jumping the switch turns on the compressor the you probably have a bad sensor.

The high pressure sensor i believe is on the back of the compressor and there is also a hihh pressure relief valve on the back of the compresser i think.

If the ssytem builds up to much pressure on the high side this will shut off the compressor also. I do not think there is a High Pressure port on 850's AC so i do not know for sure how to check the pressure on the high side, but normally the problems are on the low pressure side of the system.

Dan