Paul:

the pressure switch is on the accumulator (big aluminum can in the low pressure side) -- probably over on the passenger side, but I'm not sure. Unplug the wires and jump the connector with a paperclip or something. If it runs, most likely is low on freon, expecially if it has been cycling excessively and not cooling well. If it cools well, you can replace the switch by simply unscrewing it and screwing another in -- there is a built in Shraeder valve.

Otherwise, check the wiring on the compressor -- if the ground for the clutch is bad you may no longer have a connection and the compressor won't run.

Squeals can be from a loose belt (you have a serpentine, right?), or from a worn-out clutch friction plate. Jingling could indicate a worn out of damaged clutch, in which case it simply failed and won't engage.

I wouldn't try to put any freon in without putting a gauge set on to see what is in there -- fully charged it approaching the limit of what I consider safe for freon cans with the compressor off.

You can directly jump the compressor by disconnecting the clutch lead somewhere and jumping to the positive battery post -- if the compressor runs, and it cools, find the bad connection (probably pressure switch). If it runs and doesn't cool, you are low on freon and need to find the leak and fix it before charging. Most likely leakage points are the o-rings in the line connections and front seal on the compressor.

You have a cylcling clutch orifice tube system -- the pressure switch on the accumulator cycles the compressor to maintain a certain pressure/temperature at the evaporator. There is no expansion valve, just a fixed orifice tube in the high pressure line going to the evaporator.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!