Most likely cause is a failing voltage regulator.

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x2

And what usually fails is the brushes (they wear out). They are integral to the regulator (usually).

Usually this occurs at around 150K+ but that's just a rough estimate. It can sometimes be changed without removing the alternator from the car, but it depends on the specifics.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

On the W211 E320 it's easier to just pull the alternator. There's enough room between the engine and radiator to slide out the alternator and do the work on a bench. The V8s and diesels are another story.

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