Chances are its a defective alternator/voltage regulator. Chances of it being a bad wire are slim but double check the negative batt cable.

I had this problem, replaced alternator(with comes with a voltage regulator) for $120 and the light did not come on again. These cars on great, they can run without alternator, voltage or belts. I made it home 500KM with none of these and did not overheat... lol. I will never.. ever... ever sell my 240D.

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1976 Mercedes 240D (Sold)
1985 Mercedes 300D Turbo Diesel: 500,000KM
1986 Mercedes 300E (Sold)
1988 Mercedes 300E (Sold)
2002 Mercedes C240 (Sold)
2008 Mercedes C350 4matic

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Industrial Rubber