If all of the working fluid has leaked out or if it drops below a certain point there is a low pressure switch that will stop the compressor from engaging. This is to stop the compressor from being damaged from lack of lubrication. This is fairly easy to test for - if the switch is grounded, you don't have enough R12/R134a/whatever. Mine is located on the receiver drier.

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86 560SL
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85 380SL
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