2004 Mercury Monterey

Air Conditioning problem
2004 Mercury Monterey 6 cyl Front Wheel Drive Automatic 150000 miles

My AC was making a lot of grinding noise when I truned it on everytime the clutch kicked in. I had cold air all this time. Now the clutch will not kick in. I am sure I need to replace the clutch, but the system does not turn on. How do I verify the system still works, (Pressure switch, What should presure of ambient in off position be for this car?) How do I test the pressure switch?
I replaced the relay already and nothing. I intend to replace the Compressor/clutch assy and fill the system. I want to make sure that is all that is wrong.
March 17, 2010.

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