mm9351

Just wanted to pass this along....for those of you who don't cancel the ECO function. Should your Aux Battery run down....WARNING - when the engine attempts to restart say after a traffic stop, you will experience what feels like a rear end collision! In my case, the engine restarted and the car leaped forward with a BANG and almost felt like the rear end fell out! So after some research I discovered the AUX battery is primarily used as a starting battery after a normal ECO shutdown. At this point the ECO function should also disable (cannot turn on or off). Later on i finally got the message "aux battery malfunction" Luckily I had my MB dealer replace the battery (under warranty) and all is good. If you experience any unusual roughness when the engine restarts on ECO, these are definite warning signs to replace the aux battery soon!

TechSuperstar

I know this is a year old thread by I just got this same message although I didn't have any rough starts or anything. Everything has been working fine until today I started getting this message upon starting the car, and I did notice the car wasn't taking advantage of ECO mode... So I just hooked up my charger to it to see if i could give it a little charge and my charger read 13.7v saying its at 96% charged. What gives? I suppose the error could be from a loose wire.. anything else?

ygmn

Did you disconnect from car cables before measuring voltage?

TechSuperstar

No. Didn't think it needed to if the car is off. I would hope there's no affect from the main battery.

ygmn

try it disconnected.

I have been thru 2 of these batteries this .. ERRRR last year (Dayum New Years ALready)

TechSuperstar

Ok, disconnected and tested. Still reading up around 14v. I put it on my charger which only charged for 5min before the charger said it was fully charged. Could there be other causes for the malfunction message? Just don't want to spend the money for a battery and find out its not the issue.

ygmn

I would re-install and drive around to see if message comes back....
Maybe battery had a bad day from sitting too long?
I would remove battery and have load tested at auto parts store?
They have replacemnts for about $100.

there is thread about some relay or something that prevets aux battery from properly charging but it is rare.

Wehn mine went bad (warranty) both dealers checked the charging system to the aux battery and said ok.
My guess is original battery sat so long before car was sold it died.
Then first dealer replaces with bad battery which happens way more then people think.

TechSuperstar

Ended up going to my local "Batteries Plus" store. Sure enough it read 14v on their tester but failed the load test. Ended up paying $112 for a Duracell Ultra, part # CYL10001 which fit perfect. No more error. Note: The dealer wanted over $300!

ygmn

yup.... dealers have to eat tooo but guess they are too good for hot dogs.