Howard_Woodard

Hi. Does anyone have an electrical diagram for this car that includes the auxiliary battery circuit?

I'm getting a constant message of "Auxiliary Battery Malfunction". This is the small 12 VDC battery that mounts under the driver-side dash behind the left-side snap-in dash panel. Once it is installed it is impossible to get in there to take live voltage readings. I may end up putting a pigtail on those leads long enough to reach outside the dash but if there's a chance of avoiding R&Rng this little hand-scratching, skin-scraping dickens I will.

Both, the main and auxiliary are new.

The following measurements are without the auxiliary installed:

The main battery reads 12.75 VDC. The auxiliary battery [disconnected] reads 12.57 VDC. The [disconnected] leads that connect to the auxiliary battery read 11.21 VDC.

The 1.54 VDC drop from the main tells me there's something in there and, whatever it is, causes current to flow from the auxiliary to power whatever systems it powers when the ignition is turned off. I assumed that when the alternator is putting out its 13+ VDC while running it would be powering everything plus charging the auxiliary battery. However, when I measured the two leads coming to the auxiliary battery while the car was running, they still read 11.21 VDC which, instead of charging the auxiliary, would still be drawing from it. No surprise that putting a charger on the main battery doesn't change the reading on the auxiliary battery leads either. The fuse that is clipped to the battery mount is good.

Does anyone have the schematic/diagram for the auxiliary charging circuit or know what causes this condition?

Thanks in advance for your help.

Howard in Seattle
2011 Mercedes Benz E63 AMG W212 M156

PeterUbers

Have you run icarsoft or similar product to check for codes?

Howard_Woodard

Hi, and thanks for your reply. Yes, I ran iCarSoft and it had no codes, pending, active, or stored.

Howard_Woodard

I did find this. It's helpful if true.

"This battery is a very common failure on all the eco mode equipped cars. Normally, it fails the internal resistance check - it's not that the battery is bad yet. Most of them, when I test them after removal, have 13+ volts and show as a very good battery, but once the internal resistance is over 1.76 Ohms, it fails and has to be replaced. Any normal powersports battery that size could be used, it doesn't have to be some special MB battery, although it must be sealed."

While logical in and of itself, it doesn't help in understanding the voltage drop from the main that is preventing any current flow/charging to the auxiliary battery.

I'm hoping someone has the complete wiring diagram for the auxiliary battery circuit(s).

PeterUbers

@CaliBenzDriver will be brilliant here .... standby he will chime in soon

CaliBenzDriver

I don't have that driagram and Ill save us all the fun reading it... check the AUX fuses.

Discharged AUX had a swing at its fuse last time car was left to drain flat.

Go grab some pics around your AUX... look for a 7.5A Dark-Red

Howard_Woodard

Hi and thank you for your reply and recommendation. Unless there are more than one, the fuse you refer to is clipped to the auxiliary battery's metal mounting frame and it tested good. Since you used the plural of fuse, I pulled the fuse locator sheet but didn't find any fuses that mentioned the auxiliary battery.

I found a lot of good info from you and a couple others here: 2011 E350 Aux Battery Malfunction Error - MBWorld.org Forums

I should be able to use that info to TS my issue.

Howard

CaliBenzDriver

Howard, yes there is a second fuse for AUX but it's only involved with the discharge path to protect shorts. It's a 100A fuse-link built-in to protect the big red cable.

Here I think we are troubleshooting lack of charging voltage, right?

The spot were two big red cables land is a relay, a solid state relay, a fancy automotive SSR. They are super rugged switches controlled by rear SAM. Use scanner to read battery data and what SAM has to report.

AUX sports it's own Painted GND Post nearby: vlean it!

Howard_Woodard

Thank you. I'm heading up to visit a friend in the hospital now but will dive back into it [with your info] as soon as I return. Will update afterwards.

Thanks again,

Howard

Howard_Woodard

Hi again. Just checked all of the connections. They read 0 ohms, but I removed, cleaned and reattached anyway. No difference.

I don't have two big red cables and none of the big black ones come completely together, though they both connect to the same device and are joined electrically. That auxiliary relay would be a good place to monitor/confirm a few things, but I don't have what you described or anything that I suspect might be it. I've done quite a bit of looking at related threads for something that identifies the relays and their function. One guy -- I think you had several posts on the thread -- did a great job of identifying all of the fuses and did identify the relays to the power circuits they were feeding but that didn't help me. I don't have enough documentation to relate to power circuits -- I need something that either says that it's for the auxiliary battery or that it's "K57/2".

A friend who lives about 50 miles aways has my iCarSoft scanner. I'll see him this weekend but, in the meantime, between my VOM and 4-channel oscilloscope I think I would be able to figure this thing out if I knew where that relay is located and it's probable.


Battery Component Compartment

Negative Battery Post

Front Fuse/Relay Box

CaliBenzDriver

your AUX battery is in the rear trunk compartment or inside fashboard side ??

Howard_Woodard

Inside driver-side dashboard - LHD.

CaliBenzDriver

Then this is where you can find a 5Amp fuse that protects overcharging a flat battery.

Here are selected docs and diagrams fetched from WIS by MS.
🤞

Howard_Woodard

Thanks! I'll let you know how this turns out.

Howard

Howard_Woodard

Well, not what I was suspecting at all. Since static readings produced reasonable voltages for the new main and the new auxiliary batteries [to battery neg and body grounds] I would have bet it was either the auxiliary relay or the n10/73 driver(s) for it.

Nope. While the new auxiliary battery read the correct charged voltage, as soon as it was loaded - any load - the voltage would drop significantly. Internal resistance or just unable to preform as advertised. At any rate, it was defective and was replaced by the seller.

No more malfunction error and the voltages for both batteries show the correct readings for when the auxiliary is being charged by the alternator and for when it’s the power source. Scanner is iCarSoft MR BT.

Thanks for all the help. In retrospect, it could have been fixed without the wiring diagrams but they certainly helped with perspective.

Problem solved with new aux battery - thanks all.

Howard

PeterUbers

Nice work