Mercedes-Benz Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
1,149 Posts
Discussion Starter #1
Woke up this morning to 5 inches snow, got it all moved and then went to start my car. I pushed the button on the remote and nothing...
Now the battery has been getting weak and I knew I would be replacing it soon but now I can't even get the door open. I'll use the "extra" key inside the remote to gain access but this got me thinking. I know there are2 or 3 batteries in this particular ride and I'm sure the main battery under the hood is needing to be replaced due to the way it pulls down when it starts but which battery controls the door locks? I'm curious if the one under the back seat is weak as well. Thanks for your help, Paul
 

·
Registered
Joined
·
1,149 Posts
Discussion Starter #2
Really, no one knows which battery controls the door locks? I would look it up in the owners manual but it's locked up in the glove box. I can call the dealer in the AM but thought I could circumvent them and just buy and install. Autozone has a Duralast Platinum for bout 2 bills.
 

·
W221 & Audio Moderator
Joined
·
10,863 Posts
I doubt you will find the information in the owner's manual, but you can download an electronic version at https://www.manualslib.com/products/Mercedes-Benz-2007-S-Class-1936976.html (Mercedes seems to have deleted manuals for models older than 2012 from its site).

You knew you had a weak starter battery. Replace it, and see if it fixes your door lock problem. If not, you know what to do. And that battery is behind a panel in the trunk behind the back seat; it is not under the seat. Check the DIY & FAQs for a discussion of the battery locations.
 

·
Registered
Joined
·
791 Posts
Woke up this morning to 5 inches snow, got it all moved and then went to start my car. I pushed the button on the remote and nothing...
Now the battery has been getting weak and I knew I would be replacing it soon but now I can't even get the door open. I'll use the "extra" key inside the remote to gain access but this got me thinking. I know there are2 or 3 batteries in this particular ride and I'm sure the main battery under the hood is needing to be replaced due to the way it pulls down when it starts but which battery controls the door locks? I'm curious if the one under the back seat is weak as well. Thanks for your help, Paul
The doors are controlled by the big battery in the drunk behind the cover wall.

When rear battery dies you will lose access to the doors. If it's close to dying, you might get to open the doors but you will notice things like the clock rolling back to regular time once you start up the car and raising the vehicle will not work unless you turn off the car and turn it back on.

When you turn the ignition on, both batteries kick in. So the engine starts with the small battery under the hood, but the rear battery also starts to get charged by the small battery and when the car starts, it will charge both batteries.

When you have bad trunk battery, you might notice no lights once you enter the car but as soon as the ignition is turned on, you will get all the lights back on.

I have dealt with you dead trunk batteries and they symptoms are all the same. When you can't get access to the doors, that battery is completely done. Usually they hold charge for 2-3 hours then go completely dead.
 

·
Registered
Joined
·
1,149 Posts
Discussion Starter #5
Thanks Arlin, I put a charger on the battery under the hood just for good measure this morning and noticed that it does not indicate that it was dead so I surmised the battery in back was controlling the doors. Do you know if by placing the charger on the front battery will that also help to bump up the battery in back?... Just curious, Paul
 

·
Registered
Joined
·
1,149 Posts
Discussion Starter #6
I got into the car tonight when I got home from work and was able to get it started, clock started spinning. I let it run for a good 15 minutes and was able to get into the trunk to see where I was going to disassemble when the weather gets better (it's 1.8 degree right now, supposed to be in the 40's on Sunday). Is there a procedure for replacing the trunk battery so as not to have problems with codes and such? Do you have an allotted amount of time to get the old one out before you put the replacement in?

Warren, I did check the DIY section on battery replacement but didn't see my question addressed, thanks Paul
 

·
W221 & Audio Moderator
Joined
·
10,863 Posts
Yes, the only writeups we have would tell you what type and how many batteries you have, their specifications, the locations and how to get to them, and stress the importance of matching your batteries to your charging system. Unfortunately, we don't have much on just what is powered by each - "electrical consumers" is an amorphous term for what a rear battery powers, and doesn't answer your question very well.

I don't know of a good procedure to keep power on the busses when removing a battery - I can speculate, but don't know if it is safe. However, the "RESETS" writeup in the DIY & FAQs will let you quickly and easily restore anything that needs to be reset (and you probably won't have to reset most of what is addressed there). https://www.benzworld.org/forums/w221-s-class/2377497-resets.html#post12223513

It would be best if you had the new battery on hand for immediate installation, rather than leaving the old one out for much time (i.e., buy the replacement first; don't take the old one out to take it to the store to buy the new one). However, because the battery was so low, it is likely that whatever would need resetting already needs it, and would not be affected by the disconnect time.
 

·
Registered
Joined
·
791 Posts
Thanks Arlin, I put a charger on the battery under the hood just for good measure this morning and noticed that it does not indicate that it was dead so I surmised the battery in back was controlling the doors. Do you know if by placing the charger on the front battery will that also help to bump up the battery in back?... Just curious, Paul

I replaced both of mine in the walmart parkinglot. I did not get any codes. Just the usual dead battery where the doors can't be open.

Also, you can use the trickle charger from the trunk to charge the rear battery.

on the passenger side under the hood , close to the front battery, You will see the 2 connectors.
You can follow the POSITIVE cable from the front battery which connects to a post upfront. Next to it, you will see another cable that runs behind the firewall. That is the positive cable that connects to the battery in the trunk.
 

·
Registered
Joined
·
1,149 Posts
Discussion Starter #10
Just a quick follow up. Went to the dealer and got the battery for 250 bucks, it weighs 60 pounds. Just got it installed, I used a pizza box to place the old battery on and slid it out of the trunk to where I could lift it out then reversed the procedure to put the new one in. The one thing that threw me is I didn't realize until I had removed the battery that there was a vent tube attached to the positive side that feeds out though the bottom of the trunk. I was able to fit the tube into the new battery once I had it almost all the way into the shelf where it resides and I also figured out that the red cover flap on the positive side of the battery has a small vent tube plug attached that I broke off and plugged the open vent hole on the negative (left) side of the battery.
It is amazing how much better the car operates now... all the little gremlins are gone! This car was new to me late July 2018 and as I was getting used to it's wonderful features I started noticing some things that just were off at times, the steering wheel wouldn't adjust when I put the key in, the passenger side mirror wouldn't lower when I put it in reverse, lights wouldn't come on when I opened the door.... I had no idea it was battery related until the remote refused to open the doors. Then I posted this thread. :)

Now, right after I replaced the battery I got in and started it up and ran through a few resets and now everything works like a charm, absolute opulence. Thank you so much for your responses Warren and Arlin, this forum is the best.... the best or nothing. Paul
 
1 - 10 of 10 Posts
Top