SL 350 battery light on dash - can anyone help??

[Shoey]

Battery light came on today and I’ve checked both batteries and they’re all okay, although rear battery alternator is charging but isn’t charging the front battery. I have checked F52 fuse in passenger footwell and that’s all okay. Any ideas how to fix this? Is there a fuse or relay that you would suggest?

 

[AMGeed]

I just had a similar problem with my E55 although I think the batteries are in different places. My main battery is in the boot and the aux to supplement the SBC is under the pollen filter in the engine bay.
I would check the voltage of the main battery with a voltmeter. This should be at least 12.6V fully charged Then start the car and see if the voltage rises to 13.4V> 13.8V with the engine running. That will confirm if the alternator is doing its job properly.
Mine wasn't and I needed a new alternator.

http://forums.mercedesclub.org.uk/index.php?threads/e55-just-a-flat-battery.172782/

 

[LostKiwi]

As above but check both batteries. If one increases and maintains in excess of 13v with the engine running and the other doesn't the battery control module may be faulty.

 

[Shoey]

When I checked the voltage on the front battery it was 12.4v but didn’t change when I started the engine, battery in the boot was 13.8v and went upto 14.1v when I started the engine.
Where would I find the battery control module on the sl350?

 

[LostKiwi]

Right side of boot I believe:

 

[Shoey]

Ok thanks, any idea how much the battery control module would cost to replace it?

 

[flowrider]

Ok thanks, any idea how much the battery control module would cost to replace it?

Before throwing money at the problem I think you should get the car on STAR to find where the problem actually is. These cars a very adept at emptying your wallet.

 

[LostKiwi]

Before throwing money at the problem I think you should get the car on STAR to find where the problem actually is. These cars a very adept at emptying your wallet.

Agreed.

 

[ajlsl600]

Agreed.

ditto.... this seems like some controller, that one in boot ? maybe fuse that controls charge reaching starter battery, no ?

 

[j. Frederick]

No. That’s your battery control module (BCM). It has a built a nasty reputation for itself causing fires. If you notice a sulphuric smell coming from your trunk, disconnect back and front battery ground wires (in that order). It’s easy to open your BCM for a visual inspection. Factory new ones are not available, it seems.

What is does: https://mbworld.org/forums/attachme...-318-ho-r230-dual-battery-system-11-28-02.pdf

About foresaid trouble:
https://mbworld.org/forums/sl-class-r230/716209-sulfur-burning-smell-coming-trunk-battery.html

My source is obvious: mbworld.org, an American treasure trove.

 

[ajlsl600]

****** marvelous ,maybe we should keep the burgers in the boot !

 

[Michael F]

Battery light came on today and I’ve checked both batteries and they’re all okay, although rear battery alternator is charging but isn’t charging the front battery. I have checked F52 fuse in passenger footwell and that’s all okay. Any ideas how to fix this? Is there a fuse or relay that you would suggest?

Hi Shoey.
I wonder if you found a solution for this problem. I have been experiencing the same in my SL350. Car runs and rear battery gets recharged. Front battery doesn't. The way the problem manifests is that after approx 30 min the battery light in the dashboard turns on. I have checked the fuses (the two large ones behind the subwoofer) and they are both fine.
Currently I have learned to live with the problem: I recharge the front battery every two months - its a pain really.
I would hugely appreciate if you were successful and indicate what you did.
Best Regards. Michael

 

[peterws1957]

Hi Shoey.
I wonder if you found a solution for this problem. I have been experiencing the same in my SL350. Car runs and rear battery gets recharged. Front battery doesn't. The way the problem manifests is that after approx 30 min the battery light in the dashboard turns on. I have checked the fuses (the two large ones behind the subwoofer) and they are both fine.
Currently I have learned to live with the problem: I recharge the front battery every two months - its a pain really.
I would hugely appreciate if you were successful and indicate what you did.
Best Regards. Michael

Shoey hasn't been on the forum since he posted, so you may not get a reply from him. Don't throw money at the problem, go to an indie and get the codes read on Star - well worth the money. My guess would be an alternator or battery control module problem, but just guessing.

 

[LostKiwi]

^^^^this^^^^

You can guess and hope you hit the right part first time (rare) or get the system analysed on STAR and dramatically increase the chance if getting the right part first time.

 

[mattkh]

Currently I have learned to live with the problem: I recharge the front battery every two months - its a pain really.

Are the two batteries the same age..?
Is the front one older..?

 

[mioba]

Are the two batteries the same age..?
Is the front one older..?

Not really relevant of one is not getting juice from the alty

 

[Michael F]

Shoey hasn't been on the forum since he posted, so you may not get a reply from him. Don't throw money at the problem, go to an indie and get the codes read on Star - well worth the money. My guess would be an alternator or battery control module problem, but just guessing.

I did check the alternator and actually replaced the regulator - the graphite bars in the old one were still OK. That was definitely not the problem (the boot battery recharges beautifully - the problem is only with the starter battery, the one at the engine compartment.)
I replaced the starter battery as the old one was rather old anyways.

I don't have star system, but I use an iCarsoft i980. With this I can read the fault reading worth says "Starter battery defective. Please replace the battery". I imagine this is standard message - as said, I replied the starter battery and fitted a new one, which I recharge monthly with an external charger

I suppose it could be the control module. This module (boot rear right under the arm panels) looks visually OK (no burnout). I wonder if there is a way I could test it.

 

[Michael F]

^^^^this^^^^

You can guess and hope you hit the right part first time (rare) or get the system analysed on STAR and dramatically increase the chance if getting the right part first time.

Agree. Still I suppose iCarsoft i980 is providing me with some relevant information.
As I mentioned to peterws, I would love to be able to test the battery control module to ascertain whether air not it is the root cause of this problem.

Other problems I have found in the past with this car have been rootcaused to electrical connectors that can cause malfunctions because of pin film corrosion. I had for example a problem with the rear parking sensors. The problem went away when I disconnected and reconnected the plug located at the right side car boot near the air and hydraulic pumps.

Whilst still a hypothesis - it is a hard one to prove; which and where are the relevant connectors? I am inclined of some defect in the BCM or a relay... don't know

 

[Michael F]

Are the two batteries the same age..?
Is the front one older..?

Thanks. As mentioned earlier, I have re[lace and fitted new batteries. Both batteries recharge beautifully. The rear battery is charged by the car alternator during normal running. The front one I recharge externally every few weeks.

 

[Michael F]

Not really relevant of one is not getting juice from the alty

Agree. Still appreciate mattkh's input.