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We have a 2016 E220d on around 45000 miles. Recently it popped up the ‘Auxiliary Battery Malfunction’ fault which I guess would be expected for age and mileage.
I replaced with an aftermarket part from Amazon earlier today but sadly that did not make the fault go away.
I am thinking that the part was at fault and have a replacement coming. In the meantime, is there anything else I should be checking?
I’ve been doing some digging and it suggested that it might be a fault with one of the fuses under the hood. Location 121. That was a 10S fuse and although it looked OK I have just swapped for a standard 10 and the car now seems to be fine. Fault gone.
I’ll check the 10S for continuity when I have a moment but aware the S designation is, I think, for a slow fuse type. Perhaps the new voltage converter just needed some time to clear the codes.
Let’s see, I guess relay is next if needed. Hopefully the fuse has failed and lack of continuity would be a nice simple confirmation.
Last edited by breeze247; Aug 21, 2022 at 03:04 AM.
it is all confusing, specially when in the 1st place: the display message call this part the wrong former name
It used to be a tiny SLA battery in-dash to lock the steering column, then a trunk AGM to backup power during star/stop then now a bank of 1Farad capacitors that suffer from extreme currents. shrunken AUX now a module 😒
Question : what happens to unsold new parts that are superseded with a newer version ???
... YT has now provided me a long answer on spare parts mgt.
Last edited by CaliBenzDriver; Aug 22, 2022 at 01:01 PM.
EDIT: This is quite an old thread so I won't bump back up. I thought I had updated but apparently not. In summary, this finally got a new voltage converter (or whatever it is called) from the Mercedes dealer and the problem went away. Has been fine for a long time. My advice? Buy genuine parts in this case.
Using Xentry (MB diagnostics) erase all stored codes & perform quick test for definitive answer. Reluctant to do so 2809 would most likely be aftermarket or used unless supplier is liquidating old MB supply new, try current 3414 MB genuine.
Thank you for your thread. S213 2017 e250 wagon with m274
i had a low voltage on main battery because i had not driven it for 6 weeks
started getting the annoying auxiliary battery malfunction message.
i replaced the main battery (was old anyway)
This didn’t fix.
so I went and replaced the voltage converter module under the passenger floor carpet.
this didn’t fix.
checked engine bay fuse box, found the 10s fuse you mentioned and it was blown.
replaced this and the error message went away.
hope this information helps anyone in future with same problem and saves them from some expensive diagnostics with Mercedes mechanic
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