Sounds pretty normal.

I remember buying my '91, it was $74,000 plus change and a luxury tax. Reading the manual it mentions that with all electrical accessories on, the battery can discharge at idle, ... my '85 Jetta didn't do that! So, I realized that Mercedes didn't put a lot of emphasis on charging systems, the alternator was the same output as our old '85 Jetta, and electrical loads were much more. The rear defroster will turn off temporarily sometimes when the voltage drops at idle (mentioned in the operator's manual also) as do the seat warmers. This short-cycling / discharging actually shortens the life of a lead-acid battery a great deal, another discussion that.

Anyway, the original alternator is weak at idle, normal. If your battery is in excellent shape, you will not notice this as much, your battery might be getting old and losing capacity. Many of us over on the Diesel forum upgrade our alternators to the later 110amp or better to give us adequate output at idle. My 124 is running a later E 430 alternator with better low-RPM output.

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Gone to the dark side

- Jeff