Get your hands on a wiring diagram or better yet a shop manual that will describe how power gets to the compressor and what interrupts are in place to disable the compressor in the event of a system anomaly.

If the relay compares compressor rpm to engine rpm, diagnosis will require more than a simple electrical tester unless the problem is one of the simpler interrupts.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD