Red battery light after working in trunk
Thread Starter
Newbie
Joined: Jun 2023
Posts: 13
Likes: 4
2008 S63
Red battery light after working in trunk
Weird issue with my 2008 S63. My backup camera doesn't work, so I pulled the trunk apart last night. I took out the rear plastic piece, center floor section, and left side liner to check the wiring. All looked good, so I pulled the liner on the underside of the trunk (boot) lid. Since all the wiring looked good, I thought I'd just leave the trim out and wiring to the inside trunk stuff (lights, auto-close button, interior pop button) unplugged while I waited on a new camera to get here. While I was at it, I had a new rear accumulator, so I replaced it. All good, right?
Then I went to start the car this morning and had a red battery light on the dash. I checked voltage at the underhood battery and found that it's only 12.7V with the car running. Seems to be that my alternator suddenly isn't charging. I did not even see the rear battery (not sure exactly where it is, actually). Is it possible that something being unplugged (or inadvertently broken??) in the rear could be causing the issue? The previous owner replaced the rear battery. Even then, that shouldn't cause the low 12.7V with the car running. My thinking is that it has to be the alternator, but it's strange that would happen immediately after unplugging things in the trunk. I just want to make sure there isn't some weird subtlety with the M-B system before I spend $400 and several hours replacing the alternator.
Thanks.
Then I went to start the car this morning and had a red battery light on the dash. I checked voltage at the underhood battery and found that it's only 12.7V with the car running. Seems to be that my alternator suddenly isn't charging. I did not even see the rear battery (not sure exactly where it is, actually). Is it possible that something being unplugged (or inadvertently broken??) in the rear could be causing the issue? The previous owner replaced the rear battery. Even then, that shouldn't cause the low 12.7V with the car running. My thinking is that it has to be the alternator, but it's strange that would happen immediately after unplugging things in the trunk. I just want to make sure there isn't some weird subtlety with the M-B system before I spend $400 and several hours replacing the alternator.
Thanks.
Super Member
Joined: Dec 2018
Posts: 857
Likes: 501
From: Washington, DC
2007 S600, 2007 Chrysler 300 SRT8, 2000 C5 Corvette, 2017 Mustang GT, 2023 300C, and a 2019 Lincoln
I would only comment that a non-running S class consumes enough power to drain the consumer battery in the trunk in under two hours. My bet is you drained that big rear battery. I would put a battery charger on it and charge it up before I replaced that alternator. If after getting that rear battery charged, you still show lower than 14 volts while the engine is running, then it does sound like your alternator is failing.
Do you have any codes?
Do you have any codes?
Super Member
Joined: Dec 2006
Posts: 972
Likes: 191
From: Atlanta
W222 AMG S63
If you are looking at the trunk, the back wall (against the rear seats) is a false wall. It's held in by friction. If you can get a finger or a trim tool in there, it will open up from the left side (maybe right, too). There you'll find the battery sitting behind the rear-right seat.
There is a ground cable in the spare wheel well (if memory serves me) on the right. Very obvious. Maybe it's loose?
There is a ground cable in the spare wheel well (if memory serves me) on the right. Very obvious. Maybe it's loose?
Thread Starter
Newbie
Joined: Jun 2023
Posts: 13
Likes: 4
2008 S63
No codes, vettebk. Would a problem with the rear consumer battery cause insufficient voltage at the front battery? I'm not sure how the wiring works in these. I would assume each was independent. But maybe not?
Super Member
Joined: Dec 2006
Posts: 972
Likes: 191
From: Atlanta
W222 AMG S63
Know that the front battery is the "Starter Battery" and the rear battery is the "Electrical System Battery" (about 50% larger than the starter battery). Here are the part numbers on the receipts for my car. I bought them at RBM in Atlanta (big MB dealer):
- "Starter Battery": 001-982-82-08, $265 (now it's $310: https://mbparts.mbusa.com/oem-parts/...ttery-19828208 )
- "Starter Battery": 001-982-79-08, $374 (now it's 396: https://mbparts.mbusa.com/oem-parts/...ery-1982790826 )
I'm not deeply familiar with how these batteries share work in the car, but my guess is that the starter battery is only used to start the car and the other battery is used to power all of the electronics in the car.
My dealer will match the prices on the official MB Parts website: https://mbparts.mbusa.com I suspect yours will too, if you ask. If they don't want to give you the MB internet price, simply get the part number you need from the parts department, order it online from MB and pick it up at that very same local MB parts counter, hahaha...seriously.
Senior Member
Joined: May 2019
Posts: 305
Likes: 114
2007 S550
You can check the voltage and charge the rear battery from the engine compartment. Just in front of the starter battery is a little panel with 2 posts in it. The short post goes directly to your starter battery. The tall post goes directly to your accessory battery. Any ground will do for trickle charging.
If either battery does need to be replaced, you don't have to go with the expensive Mercedes ones. The accessory battery is an H8 AGM battery. They sell them at Walmart for about $180. The starter battery is an oddball size. I upgraded mine to a larger, more common group size so I could use another AGM battery. They tend to last longer than flooded batteries in really hot climates. Your battery tray in the engine compartment has hold downs for multiple sizes of batteries.
If either battery does need to be replaced, you don't have to go with the expensive Mercedes ones. The accessory battery is an H8 AGM battery. They sell them at Walmart for about $180. The starter battery is an oddball size. I upgraded mine to a larger, more common group size so I could use another AGM battery. They tend to last longer than flooded batteries in really hot climates. Your battery tray in the engine compartment has hold downs for multiple sizes of batteries.
Super Member
Joined: Dec 2018
Posts: 857
Likes: 501
From: Washington, DC
2007 S600, 2007 Chrysler 300 SRT8, 2000 C5 Corvette, 2017 Mustang GT, 2023 300C, and a 2019 Lincoln
Check voltage per Shadly1 instructions. We need to know at rest voltage on the rear consumer battery before starting in the morning. Should be around 12 volts. Then start the engine and drive for 20 minutes and check voltage while engine is running. Should be 14 volts or more. If less than 14 volts while engine is running, check serpentine belt for tension and if belt is good, consider replacing alternator.
Keep us posted.
Keep us posted.
Trending Topics
Thread Starter
Newbie
Joined: Jun 2023
Posts: 13
Likes: 4
2008 S63
Great stuff in here. I pulled the rear panel to start chasing things and started the car. No red light. I've started and driven it twice and it has apparently healed itself... I still learned a lot about how they operate, though, and did some digging. If the issue reappears, I'll be ready. Until then, it's on to fixing Distronic, Active Seats, and the pano roof...
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
Super Member
Joined: Dec 2018
Posts: 857
Likes: 501
From: Washington, DC
2007 S600, 2007 Chrysler 300 SRT8, 2000 C5 Corvette, 2017 Mustang GT, 2023 300C, and a 2019 Lincoln
Great news. It really sounds like you discharged the consumer battery and it just took a little driving for that alternator to charge it back up. Monitor voltage periodically and you should be good to go.
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
back up camera not working.... might be broken wires to the boot lid
on some merc models they design it so the thicker brown earth wire snaps like a carrot due to movement of the boot lid
I expect the red light is a separate fault
quite interestingly - red light on the dash is normally on the facelift where something goes odd - still waiting to hear the complete cure for that one
on some merc models they design it so the thicker brown earth wire snaps like a carrot due to movement of the boot lid
I expect the red light is a separate fault
quite interestingly - red light on the dash is normally on the facelift where something goes odd - still waiting to hear the complete cure for that one
Member
Joined: Sep 2017
Posts: 80
Likes: 16
2011 S550 / 2001 S430 - RIP
Great stuff in here. I pulled the rear panel to start chasing things and started the car. No red light. I've started and driven it twice and it has apparently healed itself... I still learned a lot about how they operate, though, and did some digging. If the issue reappears, I'll be ready. Until then, it's on to fixing Distronic, Active Seats, and the pano roof...
Super Member
Joined: Dec 2006
Posts: 972
Likes: 191
From: Atlanta
W222 AMG S63
back up camera not working.... might be broken wires to the boot lid
on some merc models they design it so the thicker brown earth wire snaps like a carrot due to movement of the boot lid
I expect the red light is a separate fault
quite interestingly - red light on the dash is normally on the facelift where something goes odd - still waiting to hear the complete cure for that one
on some merc models they design it so the thicker brown earth wire snaps like a carrot due to movement of the boot lid
I expect the red light is a separate fault
quite interestingly - red light on the dash is normally on the facelift where something goes odd - still waiting to hear the complete cure for that one
Here's what I found when I went in there. I couldn't believe it:
I was gearing up to either solder or crimp the wires and repair them. It would have been a bear for me being that I don't have a lot of experience with such. Then I had the bright idea to ask a stereo shop if they would do it. For around $35 (+ tip) they did a job better than I ever would have. Very professional and did it in 30 minutes while I had a coffee next door.
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
on my father's car the trim design to "help" the cable follow a path and ALL the manu fitted cabled ties were PERFECTLY fitted to ensure ONLY one section took the entire stress upon opening - the best bit was the trim design with a nasty sharp stress point exactly where they broke
mod 1) cut the hidden trim bit to provide a smooth arc not a nasty stress point
mod 2) remove 9 pointless cable ties, so I could free up the spare 6" of loom to better route and follow the smother arc I created
mod 1) cut the hidden trim bit to provide a smooth arc not a nasty stress point
mod 2) remove 9 pointless cable ties, so I could free up the spare 6" of loom to better route and follow the smother arc I created
Super Member
Joined: Dec 2006
Posts: 972
Likes: 191
From: Atlanta
W222 AMG S63
on my father's car the trim design to "help" the cable follow a path and ALL the manu fitted cabled ties were PERFECTLY fitted to ensure ONLY one section took the entire stress upon opening - the best bit was the trim design with a nasty sharp stress point exactly where they broke
mod 1) cut the hidden trim bit to provide a smooth arc not a nasty stress point
mod 2) remove 9 pointless cable ties, so I could free up the spare 6" of loom to better route and follow the smother arc I created
mod 1) cut the hidden trim bit to provide a smooth arc not a nasty stress point
mod 2) remove 9 pointless cable ties, so I could free up the spare 6" of loom to better route and follow the smother arc I created
