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Auxiliary Battery Malfunction

90K views 51 replies 28 participants last post by  olakennyfemi 
#1 ·
So, 8 weeks after the 3-year warranty expired, Red Ken decided to invoke the Law of Sod and throw up a "Auxiliary Battery Malfunction" warning. Ever the Drama Queen, he decided to flash this warning up on the dash in red letters and accompany it with a repeated warning sound to fully focus my attention towards this bowel-loosening Def Con 1 scenario.:eek: :eek: :eek: I can only put it down to his sick sense of humour as the mobile rave party eventually abated to a more sedate occasional flashing warning in silent white text.

A quick Google told me I was looking at around £400 to get this fault rectified by the Dealer and even if I consumed my own bodyweight in carrot cake and filtered coffee and filled every pocket with Molton Brown handwash during the visit, I would still be on the losing end of that deal. Digging a bit deeper, I discovered the so-called Auxiliary Battery fitted to the GLC is, in fact, just a simple voltage converter consisting of some components on a circuit board and 4 storage capacitors all housed in a plastic casing. It is primarily required for the Stop/Start facility although may also be integral to some of the other features like radar and cameras.

I decided I would just try swopping out the unit to see if the malfunction warning would clear. After all, in the words of Jeremy Clarkson, "How hard can it be?" Mercedes-Benz of Newcastle run an eBay shop where they offer genuine "re-manufactured" voltage converters for £102.99 delivered (with a hefty £1.03 rebate if you go via the Top Cashback website). But first I had to find the auxiliary battery. Google told me that it was located in the front passenger footwell in the normal C-Class so that seemed like a not too unreasonable starting point. Since modern cars are put together mainly with an Ikea-style click/lock system, copious squeezing, levering and pulling removed most of the panels which allowed me to lift the carpeting and soundproofing and eventually expose the voltage converter mounted at the base of the front bulkhead just below the subwoofer. A single 10mm nut and a latched electrical connector was all that held the unit in place. I plugged the connector into the new unit and tentatively fired Ken up. The malfunction message was gone!!! The air was filled with birdsong, a shaft of light appeared from the heavens and I swear I could hear a chorus of angels. ;)

Further reading on the internet confirms this is not an uncommon fault across the A, C and E-classes as evidenced by a succession of superceded part numbers for the voltage converter. Obviously if the fault occurs within the first 3 years it is just a simple trip over to your Dealer but, for older cars, you can get out of jail for around £100, less than an hour of your time and some sore muscles. Oh, and the experience has also taught me the colour of adrenalin appears to be brown and Artico man-made leather is remarkably stain-resistant. :D

Duff part (top) vs. Re-manufactured part (bottom)


Location of Voltage Converter (a.k.a Auxiliary Battery)
 
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#37 ·
Hello Everyone! Just wanted to share my same experience with Auxiliary Battery Malfunction fault... GLC 2016 250D 4matic, left hand drive, 115 000 Km, 6 years old (just over a month of the extended warranty!) "Start/stop" feature stopped working even if the function turned on... not that it's the end of the world (normally I switch off... like a muscle reflex) I haven't found any other unusual behavior... I drove only 20km after the fault and replaced the unit on the second day. The original manufacturer is Hella, it is the same part number as MB's part number: A 205 905 28 09 Talking to the main Mercedes dealership spare part department and an independent spare part dealership (one of the biggest chain in the country) there is no replacement part (don't get confused to noname Chinese un-labeled parts), only original manufacturer part... the new part number is: A 205 905 34 14 and here is the fault: The electrolyte leaked from the capacitors on the PCB which caused shorting... nice burn smell after opened the casing.... No needed to replace any fuse or clear any fault code (for me) Good luck everyone!
 
#38 ·
My error message has now disappeared without any intervention other than driving the vehicle more.. I think it is safe to assume that in many cases the error message is a result of lack of vehicle use resulting in low main and aux battery charge states. Except that for Auxiliary, that gets charged during over-run. That is where your foot is taken off accelerator and the vehicle coasts. I guess that's one way of squeezing more economy out of the car.

You can see how it charges by scrolling down to the fuel consumption section ( charge is displayed on the left)
i had a guy circuit check my fuse and he said it's fine. i am still getting the error message. i haven't changed out the part since i hurt my hand and it's in a cast and i'm not paying 2 hours of labor plus a part to fix something that appears to be irrelevant...
 
#41 ·
I got a 2018 GLC 220d. I can’t remember the start/stop ever working when I use the car. I don’t use the car much, but my dad the main user of the car says it’s not worked for a while. I’m getting no error messages, only get the start/stop symbol with a line through it pop up on the dash. I still got an MB extended warranty. So I’m guessing I should take it in.


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#42 ·
C
I got a 2018 GLC 220d. I can’t remember the start/stop ever working when I use the car. I don’t use the car much, but my dad the main user of the car says it’s not worked for a while. I’m getting no error messages, only get the start/stop symbol with a line through it pop up on the dash. I still got an MB extended warranty. So I’m guessing I should take it in.


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Chances are there is nothing wrong with the system. There are so many variables that will prevent the auto start stop to work I really wonder why they have such a system at all. Things like battery charge state, whether aircon has reached target temp, engine temp, and a few others such as rain/wipers etc all get factored in. Some new cars like Toyota now don't show if the start/stop is not working, presumably to avoid so many customer queries.

Mercedes has the added issue that the battery doesn't get fully charged by the alternator in the first instance. It goes to about 80% and slowly charges from there so that unless the car is used a lot, the charge state might be just below the threshold for auto start /stop to work.
 
#45 ·
Hello, great post!
I replaced my Auxiliary battery the other day , thanks for the tips.
The warning on the dash hasn’t gone yet, does it take a while to charge the Capacitors or could the fuse have blown?
The error came up after charging the Battery……??
Just seen the post about the fuse!
It was blown… changed now and Fault cleared?
you guys are the best.
 
#50 · (Edited)
Often, these errors are due to a failing auxiliary battery. I came across several YouTube videos suggesting a battery replacement. For me, the issue was resolved when I opted for a genuine Mercedes-Benz auxiliary battery. It's a good idea to monitor your battery's voltage using a voltmeter or multimeter. A healthy battery should register around 12 volts; if it's below this, it might be time for a replacement. Maintaining a good battery is crucial for your vehicle's performance and reliability, as a weak one can lead to sporadic error messages. If you're curious about the specific model I used, here's the link: Genuine Mercedes-Benz A2059053414 Voltage Converter Auxiliary Battery.
 
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