John Ha

There has been discussion about the "new" location (under the left rear seat bottom) of the small aux battery in the 2016-up W166 vehicles. Although several of us have found the WIS procedure for removing that battery, the procedure has always shown the battery in the "old" location - in the rear under the spare tire carrier.

Yesterday, while bumming around in WIS, I found the "Install Park Pawl Battery" instruction that shows the correct location for the small aux battery. The procedure number is AR54.10-P-0121GQB and you should be able to search for it directly in WIS by entering the number in the search field.

I've attached it to this post FYI.

An additional piece of information: there is no voltage on the terminals that connect to the battery when the vehicle is off and a battery tender is connected to the main battery (my tender connects to the terminals under the hood that are intended to be used to jump start the vehicle). So apparently the only time the small aux battery is charged is when the vehicle is running. I wonder if it might be appropriate to add a battery tender pigtail to the harness that connects to the parking pawl battery to allow that battery to be charged on occasion when the vehicle isn't being used. Just food for thought.

kajtek1

I could never find good MB note what exactly those batteries do.
For cars with ECO, suppose that keeps the lights on without dimming on restart.
But the charging module should keep it disconnected when not in use, so when I keep my vehicles sitting for few weeks, I never worry about maintaining aux battery - disconnecting only main one,

chassis

Thanks for the thread. Also included in the DIY in my signature.

John Ha

I don't know either. My bumming around actually started out looking for that very thing. I couldn't find any descriptions but did find a schematic that showed two lines from the battery going to the N10 SAM (it's the front SAM) and one going to the Electronic Ignition Switch Control Unit. Where they go and what they do once inside those units I have not tried to figure out yet but may dive into that at some point in the future.

The schematic is labelled with a code 4U7, which decodes to Start/Stop voltage dip protection, so I'm sure you are correct in that respect. But the battery also apparently has some function with the transmission, since it's been shown that shifting won't happen if the battery is bad (and there's the "Park Pawl Battery" label too).

Chefcct

So, I just replaced my original 7 year old main battery and aux battery preemptively also!
FYI, I paid $550 for both installed…
so I asked my Mercedes master mechanic (now independent shop owner) what the aux battery is for since I don’t have the auto start/stop feature. He said it controls the electronic shifting because they did away with the mechanical shifting…
Just thought I’d add something to the general knowledge base. Be well everyone!

GLE-Curious

Yeah, I think I saw a post by someone in another thread about being stuck in first gear when that battery died on them. interesting thing to know.

kajtek1

Don't know how it was played in GLE class, but W212 sedans come with mid-age lift and older models do have small battery mounted in the dash, just for shifting backup.
But later models do have ECO stops and they have bigger battery that had to be installed in trunk.
We can guess what those bigger aux batteries actually do, but seems to me that the system is bulletproof as nobody had to take it apart to date.

kajtek1

The job finally "come to me" and none of the above could help me on my 2016 model.
No need to remove seat cushion, no need to remove lower seat, just till it forward. You need to remove trunk luggage front anchors to get access to back rest lower bolts and after removing 4 bolts, where 2 are torx and 2 E, the rear back rest pull up (it still has pegs) and I slid it to the trunk area.
Then pull the carpeting from the area and here comes the battery.
It is small, 1.2 Ah and my ECO stop did work even when I was getting message about battery malfunction. Only brake hold did not work.
I replaced the main battery last year ($180 at Costco) and small Mighty Max battery is $14 on amazon. It is more powerful with 1.3 Ah in the same size.

kajtek1

UPDATE.
My aux battery come down to 6V when I had car sitting for 6 months and did not disconnect it.
But having digital charger with desulfating mode, I gave it a try. So after 6 hr procedure, the battery come back to life and even it drops the voltage with 50 amp load tester, no more warning light.
Looks to me that car charging module, once it sense low battery voltage is not attempting to charge it anymore.
Now I am thinking that cutting the carpet for easy aux battery access is not such bad idea .
It is done on main battery.