ncbitman

Hello. I have a 2013 E350. After getting the "Auxiliary Battery Malfunction" message, I replaced the aux battery under the dashboard. I still get the message. I removed the battery and see that it is fully charged, outputting 13v. I do see a 5a fuse on the battery enclosure, but it's hard to get out. As far as I can tell that fuse is okay. I don't think this is a relay problem, as the main battery and aux battery are charged.

Is there another component that might be causing the message to still be displayed? BTW, As far as I can tell, only the brake HOLD function isn't working without a working auxiliary battery.

thank you

juanmor40

Scanned for codes?

konigstiger

Battery correct, fuse fine auxiliary battery provides backup power for transmission lever monitored by rear SAM located in trunk, as mentioned have some with Xentry (MB diagnostics) erase stored codes & pull current codes.

ncbitman

Yes. There aren't any codes being thrown.

ncbitman

Thank you. I only have an OBD for reading codes.

CaliBenzDriver

How often is this car driven?

How often do you charge or float the batteries ?

How old is the main battery ?

How old is the AUX Mfg date code?

ncbitman

The car is driven every day.
The main battery is 3 years old
The Aux battery has a 12/22 date.
I do store the care in the Summer. I keep it on a float charger. However, I've been driving it since October.

thanks for your response.

CaliBenzDriver

NCB, you check all the boxes for perfect owner maintenance...

Recap:

• prefacelift car witout ECO-stop,
• new batteries,
• frequently charged,
• tests show good voltage,
• no fault codes,

Something electrical is wrong with your car!
> Use your scanner to go through SAMS + ECU live data to hopefully find a reason: a low voltage measurement.

> Obviously a voltage related to the AUX circuitry. German use numbered references to codes their circuit paths. Never mind the naming, just read voltages "as seen by your car". No DVM needed for now.

> Once you spot your faulty circuit, we'll try to identify actual reasons and fixes for it.

Stop throughing money at problems, it seldom helps.
🤞

rapidoxidation

I'd start by investing in an icarsoft scanner or other inexpensive equivalent.

Senecat

It should clear itself after a few drive cycles. I don't know how many for MB, but that function (to self-clear) is pretty standard in modern vehicles. Once the ECU detects proper values for (whatever) it removes the DTC.

ncbitman

I purchased an iCarsoft tool. The error that I get is "The buffer battery has a malfunction. There is an open circuit". Does this refer to the fuse on the aux battery, or could the circuit be opened elsewhere?

CaliBenzDriver

... good thinking about the meaning of things 👏

Vague message may point to Rear-SAM gone wild from crazy KeylessGo or other factors like swampy collection of radars modules after salty wet roads.

If the AUX voltage is found above 12.6V then the 7.5A charging fuse is in mint condition.

>> Then what...: REBOOT therapy
Disconnect both batteries for 15mn will refresh both SAM to "best behavior condition" for a while.