Sounds like an alternator problem. Perhaps the belt is too tight causing drag.

On my car the alternator is located on the lower passenger side.

Why did you take it to the shop in the first place? If I were you I would call them back and ask what they did and tell them that you still have a problem. Asking them to fix the problem for free/cheap would not be out of line since they didn't fix it in the first place!

I know your car is not a diesel, but there are many knowledgable people over in the diesel discussion. Perhaps you want to pose this question over there.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD