My battery and alternator were both brand new and I still had the warning lights triggering. Took the battery out, had it tested, and it was good. Took the alternator out, had it tested, it was good. I narrowed it down to poor grounds (battery was under charging). I'm talking about the braided ground strap to the engine and the ground cable from battery to chassis under the rear seat. All appeared good from the outside, but when I actually unbolted these then massive corrosion was revealed. One year later, I am still running on the same battery and alternator, but the problem hasn't returned due to cleaning up the grounds.

You might wanna look into it if you hit a dead end.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280