Just one addition to what jerryb stated. I happen to mention to the parts manager at my local mb dealer that I was planning on adjusting the alt compensator (over 2 years ago). He warned me to be sure to saturate the adjusting screw with penatrating oil. He said that he's a few do-it-yourselfers and independent shops snap the adjusting screw. BTW, adjusting the compensator made a noticeable difference on my 240d.

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1980 Mercedes 240D
1982 Mercedes 300SD
1985 Mercedes 300D
1993 Dodge Cummins Turbo Diesel