Check the serpentine belt. If it is loose, that may be your problem. If it is the original A/C system, chances are you have a leak. If the leak is in the evaporator area, that means big $$$$$
Have a pro look at the system if you are lost. It does not cost that much to diagnosed the problem. Most mechanics use a dye to look for leaks.
Good luck either way!

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J.H.
'86 300E