I'm not too familiar with that model, but I'm assuming the clutch must be similar as the electromagnetic clutch on my a/c compressor. Have the car off and apply 12v to it to hear if it engages. If it's not that type of clutch and it's temp controlled, try applying heat to it to see if it will engage. If not, replace the clutch. Anyone else have anything to add?

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1990 190E 3.0L