what exactly is wrong with the alternator that you need to change it?

In some cases you only need to change the regulator which screws into the back of the alternator (not sure if this applies to your car.)

Also if needing to replace it, may as well try to find a higher output alternator from another model benz that will work on your car.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U