After an exhaustive search of the car and the forums for help, I still can't locate the auxiliary battery.

Most references point to the location as under the cabin filter housing unit, but there is a heat exchanger type of setup with water hoses under that, no battery, battery terminals or cables at all. Popped the end of the dash off and looked there, no luck... some have it located there.

Nothing I have seen in any DIY reference, youtube, or MBenz forum has the right location for my model vehicle.

My battery issues started with getting intermittent "red battery warning" in my dash panel for last couple months, would pop up and go away after a few seconds. Last few days it came on and stayed on for 10-15 minutes, then went out, then it came on during the morning commute last Wednesday and never went. On the way home (1hr commute one way) it again stayed on and when I got home and turned the vehicle off, it would not start again, seemed like a simple dead battery... checked it and appears it is the original main battery, that would make it almost 10 years old, very admirable to have lasted that long. Replaced the battery and the car started right up but the red light stayed on... so right away tells me there is a charging issue, and probable BCM, voltage reg, or alternator problem... one of those three, or a combination of the 3. Being a 2008 and presumably all parts are original, might just bite the bullet and replace all 3 so parts not replaced don't fail 1000 miles later.

I'm thinking if my main battery was the original then the auxiliary battery is most likely the original as well and good idea to change it, but can't locate it to verify the voltage it is putting out. I've seen 2 different auxiliary batteries, one is a motorcycle size/type looking battery p/n 2115410001. The other is a small battery p/n 000000004039, almost like for a R/C controlled car about 2"x4".

Can't locate either one.

If anyone has some hints to the location, would much appreciate the assistance in locating this elusive battery.

 

You have SBC brakes on your '08 E320?
No?
You don't have an auxiliary battery either...


(if I'm mistaken about the SBC brakes, then I'm also mistaken about the aux battery...)

 

'08 does not have aux battery, before replacing parts have some pull fault code(s) using MB DAS (diagnostics) see attachment.

 

Hello bradleyroo1 I have an o8 e550 and I am experiencing the same battery issue you described and the same as you I am unable to locate an auxiliary battery. Were you ever able to resolve your issue and if so what was the fix. I would really appreciate your help

 

Dear hottrod67:
Did you bother to read the two posts prior to yours? The forum helps those who help themselves.

 

Indeed, that is correct, no extra battery in the 2008 E320 Diesel (CDI). In the location where the aux battery is described in other W211 models, we have the on board computer. Changing the battery is best done with an auxilliary keeper, else the clock goes to back to noon, and the date to 01/01/2003..... why 2003 ??? I expected 2007... However the odometer mileage is kept