The pulley on my 84 617 SD came off when the vibration damper separated a bit. I guess the movement of the damper spun the bolt or something. Evidently the oil and fuel which saturated the rubber in the damper and caused it to fail. Fortunately I was sitting in a parking lot at idle when the alt light on the dash came on.

Had it happened going down the road I'm sure I would have been much more radical.

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84 300SD
85 380SE
83 528e
95 318ic