At the back of the alternator there is a little unit that is secured in with a few screws. It has brushes at the end...
If yours are completely worn, replace them and you probably wont have to replace the whole alternator.
The brushes wear over time. Unless the bearings in the alternator have failed, the brushes might cure your problem.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.