My dashboard was showing the Aux Battery Malfunction error message on my 2011 (E-350). I replaced it with the new manufacturer's battery (Genuine 000000004039 Mercedes 000000 004039 / Sealed Lead Acid Battery) but the error message is still showing. Do you have any idea how to remove the message from the dashboard? In some of the other threads I found some of the other people also having the same problem and it was due to the fuse 448, but not sure where that fuse exists. My Aux Battery is on the driver's side of the front dashboard. I also checked my old battery with a multimeter and it's showing 12.3 volt rating so not sure if the initial problem was due to the battery or something else.

 

is your battery positively getting charged with on-board voltage, 14.Volts right?

 

How to verify. I just put the new battery (assuming it's fully charged) and drove the car for a couple of miles but the message is still coming up.

 

... yes that's why I asked!

A new battery is just like a container full of electrons... if you keep taking power out, after a while nothing is left! That's a flat battery voltage.

To check ALTERNATOR is producing running voltage, use battery display on instrument cluster or a $10 voltmeter connected on battery leads.

What is your current mileage like... 110kMi??


Beware what you're measuring:
Test the resting voltage on both batteries MAIN, AUX while disconnected.
12.3V is low without load but ok under 25A circuit load.

Old batteries typically loose the ability to hold a charge around to 3 to 5 years - Some oversized batteries go up to 10 years but with derated capacity.

 

Thanks for your reply and will check and let you know. But as I said my old battery is still showing 12.3 volts (with voltmeter) which confirms the issue may not be related to the alternator otherwise old battery voltage should be way lower than 12 volts. My current mileage is 76K and last time I changed the same Aux battery in 2017.

 

I hear you, battery stuff is expectedly basic yet you'd be surprised how these smart alternators can throw unexpected curve ***** -

I am not saying your alternator is defective either - I am trying to help you pin down your troubles.

Your car eats batteries and your alternator is supposed to be ok... then you are likely dealing with a parking drain condition.

Go ahead and scan your car for marginal modules that can impact voltage control.

 

First of all, I want to thank you for your time and detailed answers. I used the instructions given in the YouTube video ( ) and my cluster outputs were UB: 11.6V & IB: -12.0A (without the car running) but the strange part was after this test my car didn't start and it seemed the main battery (almost 5 years old) was out of juice. I replaced the main battery and the car started immediately. When I ran the same test again the outputs were UB: 12.1 V and IB: -12.3A (without the car running) and UB: A14.2 V & IB: 18.5A to 60.0A (range with the car engine running). After driving for about a mile the Aux Battery Malfunction message was back. So now I know my both batteries are new and still the Aux Battery Message popped up, which means something else is wrong with the Aux Battery, and maybe it's time to take it to the mechanic to diagnose.

Before doing that I just want to confirm if it has something to do with the driving conditions. My wife drives this car to work (about 2 miles each way) daily and not sure if that's sufficient for the battery to charge or otherwise should I drive the car for some long routes and maybe that will reset the Aux Battery Malfunction messages as in some of the posts I read people said that after Aux Battery Replacement the message was gone after couple of hundred miles of driving.

 

Great progress! Now you have reached some solid ground:
> The main new battery is good, the alternator is proven charging yet AUX still popping message.


> AUX live charging voltage:
At this stage I think you are nearly ok.
Let's probe your AUX Batt in the dash to make sure you see above 13V with engine running.

AUX does not do anything when MAIN voltage is sufficient near 12.5V. When MAIN Voltage gets below AUX battery then AUX automatically supplies a limited circuit tasked with authenticating the ignition Key (EIS + ISM).


> Good Ownership:
What that means is a low MAIN batt or poor ALT. output cause AUX to drain by working as intended during very specific times.

Practically you want to float Main with a CTEK when needed to prevent any draining of Aux.


I would advice against draining BATTS with ignition ON and engine OFF. High charge currents are not beneficial.

 

Finally, I bought the iCarSoft Scanner Tool and ran the full Diagnostic on the car. Here is the link Diagnostic-Detailed-Report-10252023 to all the Fault Codes (found) as my "Aux Battery Malfunction" message is still showing up but everything is working fine in the car, no starting problem or any electrical component not working. At this time I am totally out of options and not sure what's causing the message. Do you have any input from the attached Diagnostic Report because it seems most of the fault codes are still due to "Aux Battery" related, but not sure of the root cause at yet.

 

I read your complete fault report, lots of duplicate entries.

The large majority of faults are pointing to low voltage, not only AUX Related... we know you've had issues with that.
Now that you hold this base scan report, go ahead and reset all faulted modules then rescan again to see what comes back as current.


The big question is testing AUX: Pass/Fail.

-- Go into EZS/EIS (Ignition Switch) to read data related to voltage or use a DVM to probe AUX.

-- Read what the date code is on AUX physical casing

-- Float charge AUX once. That should be enough. These units have a really easy life, so long MAIN voltage is in range above 12.5VDC.

+++ EASY EITHER WAYS
Test before or after replacing ?
The correct answer is to test before!
Sometimes you feel lucky and some times you win!

Benz sophisticated failures can be.misleading up front.
🙂

 

Agree with Cali. The main battery is a suspect now.
How old is the main battery ?

 

He stated on post 7 that he just replaced it.

 

High chances AUX is as bad as MAIN was but we want to test it before any wet-finger decision

During low power sleep mode, MAIN batt provides power through SAM's where needed until it gets below AUX voltage. At that point AUX supplies back-up power to EIS so it can answer authentication requests. So the interesting part is draining MAIN automatically causes low AUX through diodes. Both batteries are isolated but work together side by side. (Low Main = Low Aux).
Aux charge current is not limited and not measured, AUX piggy backs Main to equally receive ALT Output.


Why the car drained and how long it took to drain is interesting to prevent repeat batteries swaps twice a year.

 

dang, I missed that. sorry. Thanks S.M

Well, grounds clean up time then............... F32 check all boths BOLTS for tightness

 

Just some additional information about the new main battery test using iCarSoft tools Voltage option:

Battery (DLC) - Car Idle Ignition On (Car not Running)
Battery (DLC) Voltage: 12.35
Min from this test: 12.31
Max from this test: 12.49

Battery (DLC) - Car Is Running
Battery (DLC) Voltage: 14.06 (Numbers were continuously changing in the same range)
Min from this test: 9.53
Max from this test: 14.53

Last week I checked the brand new Aux battery and it was showing the 12.6V and will check it again over the weekend and make sure it's still in the same range.

Thanks for all your time and help as I am new to all this and calling the MB Stealer to just diagnose the root cause was quoted to me for around $250 so just trying my best if able to find out the issue by myself, specially if it's just related to bad battery.

 

You are doing more than perfectly fine.

$250 to diagnose that sweet issue would only get you a "free car wash" and ultimately not a lasting solution.... besides another $700 battery swap regardless yours only being 1Mo old.


Glad you're starting to realize the empowerment of using scanner seriously.

From your above tests it shows AUX is not being charged.... that sounds like a bad 5¢ fuse is in your future!

AUX voltage should mirror that of Main. Its not directly connected but when main gets 14.x then new AUX can not stay at 12.6V

Let's scrutinize AUX charge circuit


> MAX/MIN Values:
Besides remind yourself to assess ALT charging voltage once we finish nailing AUX issue.
You'd expect 12.6 to 14.9v with mild Amps.

+++++ ​​​​​​

> W212 - BATTERY TOPIC :
These cars are so sofisticated that the simple battery topic has become been growned into an overwhelming chaos. MB issues are plenty but individualy simple.

Imagine a tree with many branches, conditions and priorities... To eliminate chaos from these cars power supply is essential. Thus the root KING IS ALT.


> No1 : THE ALT!
-- ALT supplies all 100% of the car electric load - Not the system batteries!

-- Quick Sanity Shortcut: unplug LIN CTL to restore good voltage regardless of Bosch control gone wild! 👍

These cars feature an unreliable smart voltage control that is allowed disfunction with zero code under the radar. 🙂

The system is distributed from front to back through many types of networks (LIN, CAN-B, CAN-C) and lacks basic failsafe so that ECU/ALT can clamp down a 90Amp short on MAIN WITHOUT CODE 🙏


> Drained by driving:
ALT output voltage varies with roles: from battery charging (14.9 to 13.7v) to basic power supply only 12.6V. - All these are perfectly good voltages so long you don't come near or below 12.3V while running... 👏

The real measure for charge level is Amp current.
​​​​​​- A healthy charge circuit quickly goes from 15A to near 1Amp in couple Miles drive.
- A circuit showing more than 40Amp means something wrong odd is up!!

You can not expect the ALT to charge a discharged low battery in 10mn drive... The high current will overheat charging circuit into disrepair.


> No2 : THE STRAP!
If ALT is King, its Queen is the undercarriage main strap.
Single chassis GND Strap is one sweet trouble maker. CLEAN IT, PROTECT IT! It's located to provide easy frequent service. All voltage dropped is current hike necessary to deliver equal power to car 50Amp average load. Poor GND deliver glitches to power circuits and reference ECU/SAM paths. Glitches stress solderless modules into soft-crash crazy modes (Reboot!).


> Drained by parking:
float tender!
Root causes: long slow polling of soft-crashed modules
(Advanced fixes: GND, Solderless, CAN-B *.*, Reboots!)


> Juvenile Batt Killers:
​​​​highly discharged!
High currents
heat stress
poor charge control


> Grumblings insurance:
At home: CTEK float charger Main only
On the road: "Jumper pack"!
Periodical ​​​​​​​Car-Reboots... monthly.


++++ Roll up:
Car was allowed to drain its dual batteries, AUX charge current exceeded its fuse link.
Use a CTEK or simply learn to disconnect the (" -") from both batteries in 5mn.

 

The fuse blown? when you removed the battery from the dash, the bracket holding the battery had a clip that held the fuse to the bracket, did you verify this inline fuse was OK?

 

My friend, I believe in the pre-facelifts, the one and only thing the auxiliary battery does is allow you to shift the car to neutral so you can tow it if the main battery goes dead?

 

No, you had it right the first time Surya, I bet its the inline fuse on the positive wire from the auxiliary battery, it's clipped to the battery bracket behind the left dash panel.

 

Thank you everyone so just to give you an update. As I also changed the Main Battery at the same time and one of my friend recommended resetting the ECU as the main battery was swapped, which I did and majority of the other fault codes are gone but still getting the "Aux Battery Message". I was thinking the same about the fuse so will check and let you know the outcome. I am traveling this week so once I get back home will check and update the post accordingly. Here is the final list of Diagnostic Codes Diagnostice Report after the ECU reset. If it's the Fuse can I just buy it from any Auto Shop or has to be specifically bought from Mercedes Dealer?

 

Really nice short fault list regarding AUX batt. - This shows an AUX relationship with ESP brake controller. DON'T LET BRAKE Module glitch on you: AUX serves its purpose in MB cars... such as when buggy ALT/ECU have arguments, AUX provide VIP stable voltage automatically 🙏

No need to special order fuse from MB. A trusted equivalent brand will work perfect.

Rebooting these cars from scratch every 2, 3 or 4 weeks is a solid practice to increase network stability.
The two sleepless SAM's power managers can get a little goofy. Flooting batteries is good but rebooting even better. There's a class of bug in there that likes reboot - Absence of faults does not mean lack of issues.
🤞

+++ shortcut:
I don't know how to easily reset modules that are not faulted - Rebooting restores some smooth performance that I attribute to F-SAM that makes its buddy ECU stumble.

 

Just a normal automotive blade style fuse, I remember it being 2-5 amps, but I'm not sure. It was in the positive line to the battery and clipped to the battery retention bracket that you unbolted through the side dash

 

Oke, here u go................ That small 1.2ah dashboard battery is called G1/7
It uses F1/1 fuse box of 5 amps



Blue-cyan arrow is when Front SAM N10/1 does charging and sending inspection signal to this G1/7 battery.
Red arrow is when this G1/7 battery is powering N73 Electronic Ignition Switch









As per Pierre information, find that 5 amps fuse , the F1/1.

==============


N73 get two power sources. From Front SAM and that G1/7 1.2ah battery





When you measure voltage for this 1.2ah battery, you must also do this :
"the negative lead of DMM goes to clean metal of car as ground and not at the battery negative."

This way you can then verify if splice Z6z1 ( I label it as 2A ) is indeed properly connected to your 1/2ah battery negative and the ground stud W15/5 or W15/7 ( I label it as 3 ) is properly connected to
car metal. Loosing connection at splice Z6z1 is the same as losing power of this 1.2ah.


U897 is for cars with Direct Select transmission lever at steering wheel and surely it will have the ISM module at the tranny to do the P-N-D by wire.
https://moba.i.mercedes-benz.com/bai...69-en-GB.html#

U940 is when the car is not using Direct Select transmission lever, example some AMG still uses middle of console regular tranny lever/knob.


===

The charging and sensing of the G1/7 battery is done by Front SAM N10/1, see image below





Attached 3 documents as PDF

Have fun troubleshooting..................

 

First of I want to thank you, everyone, for your time and detailed explanations of the suggested solutions. Finally, the issue was resolved once I checked the replaced the faulty 5 amp fuse next to the 12v AUX battery. The fuse was burned therefore throwing the error. I replaced the 12v AUX battery also but even seems the old battery was fine because it was already 5 years old and in the past was throwing random messages (once every couple of weeks). This tool was a big help to diagnose the root cause of the issue and sharing on this forum to get help.

I follow this video replacing the AUX battery which also mentioned disconnecting the main battery before replacing the AUX battery to make sure you will not short the wiring or fuse when replacing the AUX battery as the space under the dashboard is very limited. Most probably in the past when I was replacing the battery may have blown up the fuse as noticed the sparking during that time when mounting it on the dashboard.

I hope the above will help if you also experience the same kind of behavior, but for sure this forum was very helpful for me to get it resolved, and once again thanks to all of the contributors. It's saved me a lot of money and also allowed me to learn something new.

 

Here is a better video - in my opinion:

I also removed the 3 screws holding the 'kick panel' in place below the steering wheel to let it hang down and it gave easier access for my larger hands, and a light to get to the battery.