I’ve started receiving the Aux Battery Malfunction message upon startup on my 12 CLS550 after sitting for a day or 2 in the cold weather. After driving for a while I can stop and start it up again and all is fine so I assume the battery is starting to go but charges sufficiently for now.

Owners manual says it is an aux battery for the automatic transmission?? But the only symptom I have when this message comes up is the disabling of Distronic Plus.

I’ve reviewed all threads on this forum and several other MBWorld forums and cannot determine exactly where the aux battery is. I’ve seen writeups saying it is in the trunk, under the cabin air filter and under the dash. Can anyone state conclusively where it is on a 2012 CLS?

 

Instrument panel, OEM current part # 000000004039.

 

 

 

 

 

Had a malfunction display on my 2012 CLS 550 when I started my car yesterday so I quickly looked up help and came up on this thread.
yes it is located right underneath the steering wheel to the left and a quick trip to the Mercedes dealership for a new battery cost me $50 and a Benz mechanic I know only charged me $30 to fix it .😄 A quick 30 minute job and Distronic features is restored and all.
thanks for the posting on this
Really helped !

 

Konig is a huge help.

 

I have the same auxiliary battery malfunction message on my 2017 CLS550, how do I fix this?

 

Usually you should be good with a replacement battery, unless AUX was discharged, BCM is not passing through.

 

Thank you! Rookie question, what is BCM? I don't understand what you mean by "unless AUX was discharged, BCM is not passing through"

 

Battery Control Module is a glorified RELAY (SPST/SSR) operated by the rear SAM.
BCM manages the connection of AUX Batt to the circuit.

BCM keeps AUX batt. normally disconnected unless it needs to charge or supply power. SAM tests AUX under load at every drive cycle. It then calculates its "internal resistance" for having a higher than normal internal resistance. Poor connections affect that result directly.

> PRACTICALLY:

1. Check that AUX gets alternator voltage through BCM,
2. Float-charge AUX battery full one time
3. Check connections are not loose
4. Inspect chassis connection of brown cable!
5. Reset related DTC fault to grant it last chance of survival