Battery control unit, but still--"convenience functions unavailable". What's left--alternator, voltage regulator? Why no other symptoms? Thoughts/experiences appreciated....

Voltage regulator on alternator, then alternator

My battery issue thread
https://mbworld.org/forums/w211-amg/...ry-thread.html

I've replaced the main battery, the aux battery upfront, and brand new alternator.

They all had issues even though the aux was MB factory one & only 2 years old. The main one was the original one.

If you are due for belts, get that alternator replaced too. It's more costly than the voltage reg alone, but it's not that bad if you are keeping the car & drive it often.

so you got new batteries back and front, changed the battery control module, the voltage regulator is a good bet if never done but also something to consider..... before start up in the morning check the voltage of your "new" battery. make sure you do not have a "parasitic draw" causing your battery's voltage to drop and throwing that convenient functions. if you find no low battery voltage, there is the front aux relay under the hood on the passenger side that maybe faulty also. on the parasitic draw: there could be a draw that still allows your car to start up but the bcm senses that the voltage on the battery is "low" and throw that message for convenient functions on your screen. so you have to actually check that battery after sitting overnight. these alternators are very robust, not often is it the actual alternator but........ exception happens.