Low voltage issue
Thread Starter
Junior Member
Joined: Sep 2003
Posts: 40
Likes: 11
From: W Hollywood, CA
'06 SLK55, '14 E63S Estate
Low voltage issue
Recently had my '14 E63 S Estate's main battery disconnected for a few days while attending to a coolant leak at the back of the engine. After all work was completed, I put everything back together and drove the car around for a few hours. However, just as I pulled in I got a message "Comand shutting down" and it did just that. I'd seen this before, last year when my main battery failed. I shut the car off and could not restart. I checked the battery voltage which was 10.9V so obviously not enough to start the car.
I charged the main battery fully (aux battery is 8 months old and sits at 12.9V currently) then connected it back up. Started the car, and the voltage rose to about 14V which made me think maybe the alternator's OK but the battery's bad? However, after driving around for about 10 minutes with my hand held tester connected I see where the voltage diminished, down to 12.0V and occasionally dipping a little below.
The main battery was under warranty so I replaced it with a new one just now. Started the car, took it for a brief ride and it seems to only ever show a voltage in the 11.9-12.7v range which seems low to me. Even with everything turned off that's where the voltage sits.
The only code I can find anywhere in the system is U041208 which states "Unreasonable data were received from control module "Battery"". I cannot seem to clear this code.
Please share your thoughts, I'm not convinced that I need an alternator or voltage regulator but I just don't know what else there is?
I charged the main battery fully (aux battery is 8 months old and sits at 12.9V currently) then connected it back up. Started the car, and the voltage rose to about 14V which made me think maybe the alternator's OK but the battery's bad? However, after driving around for about 10 minutes with my hand held tester connected I see where the voltage diminished, down to 12.0V and occasionally dipping a little below.
The main battery was under warranty so I replaced it with a new one just now. Started the car, took it for a brief ride and it seems to only ever show a voltage in the 11.9-12.7v range which seems low to me. Even with everything turned off that's where the voltage sits.
The only code I can find anywhere in the system is U041208 which states "Unreasonable data were received from control module "Battery"". I cannot seem to clear this code.
Please share your thoughts, I'm not convinced that I need an alternator or voltage regulator but I just don't know what else there is?
Out Of Control!!
Joined: Jun 2004
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2014 E63S; AMS 100 octane ECU dyno tune; EDOK TCU tune; BB intakes; sprintbooster
I would first trickle charge your new battery for 24 hours, then start her up and do your subsequent testing to see how the car drives, what codes you get, and what the resting voltage is.
you can make any determination on what parts you need after that.
you can make any determination on what parts you need after that.
Thread Starter
Junior Member
Joined: Sep 2003
Posts: 40
Likes: 11
From: W Hollywood, CA
'06 SLK55, '14 E63S Estate
New battery's charged
I did indeed charge the battery prior to install, although it was brief (used an intelligent charger that said it was fully charged).
Car is running perfectly, and the only code anywhere in the system remains U041208
Car is running perfectly, and the only code anywhere in the system remains U041208
Last edited by Red Bull; Aug 21, 2025 at 08:53 PM.
Out Of Control!!
Joined: Jun 2004
Posts: 13,799
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2014 E63S; AMS 100 octane ECU dyno tune; EDOK TCU tune; BB intakes; sprintbooster
Change the aux battery too?
the code may just clear on its own after driving a few days
and did you connect all the battery wires correctly when you put the new battery in? I'm not doubting your technical skills. I'm just asking the obvious questions.
the code may just clear on its own after driving a few days
and did you connect all the battery wires correctly when you put the new battery in? I'm not doubting your technical skills. I'm just asking the obvious questions.
Last edited by PeterUbers; Aug 21, 2025 at 09:01 PM.
Thread Starter
Junior Member
Joined: Sep 2003
Posts: 40
Likes: 11
From: W Hollywood, CA
'06 SLK55, '14 E63S Estate
Aux battery
The aux battery is relatively new (replaced 12/24) and with the car off it measures 12.9v which I think is actually on the high side? When I connected it to my intelligent charger it sees that battery as fully charged.
Thread Starter
Junior Member
Joined: Sep 2003
Posts: 40
Likes: 11
From: W Hollywood, CA
'06 SLK55, '14 E63S Estate
Voltage resolved itself
All,
Apparently having left the main battery out of the car for several days required the car to sort a few things out for itself!?! It's all back together now and running very well, just wanted to let everyone know
Apparently having left the main battery out of the car for several days required the car to sort a few things out for itself!?! It's all back together now and running very well, just wanted to let everyone know
Out Of Control!!
Joined: Apr 2019
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
DISABLING LOW VOLTAGE
Do disconnect both BATTS because AUX's job is to provide power backup in conditions when Main voltage has gone dangerously low: "PRND" ISM shifts into Park.
> FREEBIE...
Having done a reboot you may realize your car steering, engine, brakes...now perform noticeably better for a few days - Someone has noted "this is not a placebo!"
Should you find benefits, you can get used to scheduled "Reboots" & easy "disconnects" while car is unused.
> EYES ON DASH...
Set your IC-Display to show battery voltage + current so you can witness 11Volts drain on longer drives. Seeing 14.9, 14.4, 13.7, 12.6v are not sign this is working well.
> LASTING RELIEF...
NEW ALT + new Batteries do nothing to prevent this "drain by driving".
Voltage may be experimentally self-regulated under IC supervision in steps above.
> "ALL INCLUDED"...
Low chassis voltage is known to deliver :
- Limp-modes tranny
- XMas trees dashboard
- "Pre-Safe" faulty warnings
- Slow cranking
- Short lived AGM
- ...
Simply bypass the buggy control.
Last edited by CaliBenzDriver; Aug 27, 2025 at 04:37 PM.
