If your voltage regulator can be changed independently of the alternator, I would check that out. You may be wearing the carbon bushings on the reg to the degree where idle speed no longer provides a charge, either way, it will soon go altogether.

Worth a check, voltage regs are the cause of a majority of MB charging issues, and generally cost $10 bucks at your local parts shop.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)