I have a 86 300sdl. I bought a used alternator for it and installed it and after 5 miles of driving the belt came off. The pulley had become very loose on the alternator. I got home and took the alternator off and found the woodruff key missing. I ended up taking the new brushes out of the used alternator and putting them in my old alternator and put my old alternator back on.
I then put the used one away and forgot about it.
Moral is... there is a woodruff key that helps hold the pulley to the shaft. Make sure you have not lost yours.

John

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John
1986 300sdl
1985 380se