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2006 W221 S500 M273 – Rear Battery Voltage Drop & Slow Electronics
Hey everyone,
I’m having a weird voltage issue with my 2006 W221 S500 and wanted to see if anyone else has experienced this.
Issue:
When I connect the rear main battery, voltage drops from 12.6 V → 12.1 V, and the dash shows ~11.9 V.
Battery is healthy (tested, swapped with another good one).
Current draw when connecting is around 0.5–1 A, which shouldn’t cause such a big drop. When the car is sleeping, it draws around 0.3–0.4 A, which isn’t ideal, but this small draw should not cause the voltage to drop as much as it does.
What I’ve checked:
Rear SAM and fuse box are clean and dry from what I can see.
Rear ground near battery is tight and corrosion-free.
Positive distribution block looks fine.
Important detail:
The factory ground strap between the transmission and chassis is missing because the nut broke off. I am not sure if this can affect this.
I also added an extra ground cable from the chassis strut to the top of the engine to see if it helps.
Additional observation:
Electronics now work noticeably slower, including seat adjustments, soft-close doors, and other comfort features when engine is off.
Has anyone experienced similar voltage drops or slow electronics caused by a missing transmission-to-chassis ground strap on a W221? Any advice or experiences would be greatly appreciated.
there are plenty of bolts to refit an earth strap down by the gearbox - its ridiculous any fool did this - plus its a 400bhp car and the engine and gearbox need to be attached safely !!!
expecting the critical driveline systems to be happy without earthing the engine is crazy - not because it will have any effect on the battery or voltages where you are looking - but the engine ECU and gearbox will hate it and might blow up their ECUs
This morning I installed the missing ground strap between the chassis and transmission, but the issue still persists.
When I check the Vehicle Data on the dash, it shows a voltage of 11.8V, while measuring directly at both the rear battery and the front jump posts/engine bay shows 12.4V when connected to the car.
Is it normal to have this much of a voltage drop between the system and what the dash reports? What should I check next?
I’ve installed a brand new battery, and the result is exactly the same as with the old one.
Both the old and new batteries were tested at my local car parts store for CCA and internal resistance, and both tested good.
Ok, so I went to the car after it had been sitting for about an hour. Without turning the ignition on (no key in position 1,2), my diagnostic tool showed:
Starter battery: 14.9 V
Main (rear) battery: 11.9 V
Then I turned the ignition on — the readings stayed the same.
As soon as I turned the ignition off again, both batteries immediately showed around 12.4 V.
What could cause this kind of behavior? Is this normal?
none of the voltages you are speaking about sound unusual - these eat so many volts with just the door open you'll soon tip a slightly tired battery in to low volts - turn the key and its draining 37 amps
you do not get the same voltage readings around all areas of the car - it has so much crazy electrical systems it just doesn't work like that - never did
get a decent load test done on the batteries - likely need a battery or two
all Mercs have always got more and more voltage hungry as they age - maybe its a solder job on a module you could waste your life and money swapping out 60 ECU trying to find to bring it back the the std massive drain it had when new - or just have a new battery every 5 years and drive the car every two weeks
none of the voltages you are speaking about sound unusual - these eat so many volts with just the door open you'll soon tip a slightly tired battery in to low volts - turn the key and its draining 37 amps
get a decent load test done on the batteries - likely need a battery or two
all Mercs have always got more and more voltage hungry as they age - maybe its a solder job on a module you could waste your life and money swapping out 60 ECU trying to find to bring it back the the std massive drain it had when new - or just have a new battery every 5 years and drive the car every two weeks
Yeah, I get that these cars eat power when awake — I’ve seen the current spike up to around 30–40 A with the ignition on or doors open. But what confuses me is that strange 14.9 V reading on the starter battery while the engine is off and ACC/Ignition Off. Once I turn the ignition on and off again, it instantly drops to around 12.4 V, same as the main battery.
Could it be the F32k1 relay getting stuck, or is it something else?
I also noticed that the Internal Charge Transformer shows as ON even when the ignition and engine are both off (key in position 0). F32k1 shows *** Open but what causes the Starter battery to be at this voltage and Main to drop?
if you really got 14.9v engine off - thats unusual - but batteries can go silly now and then - test the batteries off the car and see what condition they are in
- the other bit I have never heard of - but lots of stuff will stay away for up to 7 mins after you turn off and remove the key - all very normal on modern cars
if you really got 14.9v engine off - thats unusual - but batteries can go silly now and then - test the batteries off the car and see what condition they are in
- the other bit I have never heard of - but lots of stuff will stay away for up to 7 mins after you turn off and remove the key - all very normal on modern cars
Both batteries — starter and main — were tested at a car parts store with a battery tester, and both are in good condition.
It looks like there’s an intermittent problem with a module that, for some reason, is sending all the voltage to the starter battery. I suspect it’s the Internal Charge Transformer, because when I saw the 14.9 V reading on the starter battery (engine off, ignition off), the diagnostic tool indicated that the transformer was ON.
Now, the starter battery voltage is 12.8 V, and the diagnostic tool shows the Internal Charge Transformer is OFF.
I think this is most likely why all the electronics feel low on power — the starter battery has almost all the voltage, leaving the main battery and systems underpowered.
Internal Charge Transformer is integrated to the BMS module or is it a separate one?
I have never heard of the term Internal Charge Transformer
these dual battery cars are wired up stupid - the front ONLY starts the car - all the rest is done off the toys battery in the boot - above the rear battery is a stupid module that can play up - its job is to help decide whats going on... the rear battery can support the front if it wont crank (I think)
I have never heard of the term Internal Charge Transformer
these dual battery cars are wired up stupid - the front ONLY starts the car - all the rest is done off the toys battery in the boot - above the rear battery is a stupid module that can play up - its job is to help decide whats going on... the rear battery can support the front if it wont crank (I think)
read this hard to follow junk
I’ve already read through that doc when I had a random issue where the starter battery would not charge. At that time, I used Xentry to perform actuations, and after that, the starter battery started charging properly. Most likely some type of relay was stuck inside that block.
I think my diagnostic tool refers to the Internal Charge Transformer as the N82 module, which controls when to charge the starter battery and when not, but I could be wrong.
Sadly, I don’t have access to DAS/Xentry at the moment, otherwise I could at least perform actuations to see what it’s doing.
To be honest, at this point I’m not going to chase this problem further, as the car starts and works fine. I need to focus on changing the steering rack, since it has started getting stiff when cold and the steering sticks, which is a more pressing issue. I haven’t found any info on this forum about someone performing this change, but from looking at the WIS documentation, it seems fairly straightforward. Apparently, you don’t even need to drop the subframe, which is a nice bonus.
Turns out the issue was software-related at the N82 module. After borrowing an Autel from a friend and rewriting the software, everything is back to normal — voltage is now stable through both batteries.
Apparently, it was somehow charging the starter battery from the rear main battery even when the engine was off, which caused the slow voltage drain and drop.
interesting - the box of junk above the rear battery ?
I wasn't aware it had software or was codable - just checked my the list of likes to fail modules - and it lists BNS - Vehicle power supply control module MB2215458232 - and mine lists FW as 2214420655 from week 20 2006
Just found one on fleabay running FW 2214420455 from week 39 of 2005
blynaslt - please could you scan the car and see if it now reports that module having later SW / FW part number?
date and part number please
Will check that later, but I think that the software number was the same, or it had _002 or something like that. Maybe something got bugged in the software, and just reinstalling it got it to behave normally?..
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