kiddpi

I am brand new to this site, and desperately need some help. I have a 2008 Mercedes SL550 convertible. Recently the red battery indicator light came on/off sporadically, then finally came on and stayed on. I took the car to locate Advance Auto, who said battery was fine, but the alternator was only charging at about 12.8 instead of more than 14. I purchased a new aftermarket alternator and belt, took it to my local Mercedes dealer to have them install it. They called and said the battery and alternator was fine, but the battery control Module A2305401045 was bad. Further, they said the part was no longer available, and they offered no solutions. I found one on Ebay for $1500which matched exactly what I need, but the dealer says the BCM is "VIN specific". They said they could only program the used BCM if it was wiped clean, and my used BCM is not wiped clean. Dealer says they have no way to wipe it clean. They recommended I find an online hacker, which sounds shady. I have not returned the used BCM yet. Any suggestions would be very helpful.

FYI, I am not a mechanic, nor do we have any savvy Mercedes/import locations other than the one Mercedes dealership in WV. Right now the car will start and function fine, except the red warning light being constantly on. HELP!

elMacko

I do not think the BCM is car specific, but that will be easy to confirm by installing the $1500 BCM, which may not be good since it is used..>! But install is easy and you can find out quickly. You probably may read the thread addressing the BCM replacement. I did have a burnt BCM and replaced with a DC-DC charger and a Microprocessor from Michael Pitts..! Work just fine!

MikeJ65

As usual, the dealer is an idiot. Plug and play on the BCM, assuming that they diagnosed it properly, which is also doubtful. Also, software can be updated like every other module on the car. There are several codes that will pop up in the BCM that don't indicate a need for replacement, but Mercedes techs like to do the easy/expensive thing first. If your city is big enough to have a Mercedes dealer, it will have an independent import shop that is way better and way cheaper.

Tom Manning

Sounds like you might have two issues:

Firstly an intermittent regulator in the alternator. These had a fault and recall from new! Since the alternator and rear battery are connected directly, a fault in the BCM will not affect the charging of the rear battery. I would change the regulator, since the first shop measured the 12.8 Volts which it too low to charge the battery. Maybe it came good when tested later.
Secondly when the rear battery goes low, you'll get the red battery light which can only be cleared with a scanner. Buy one and clear it out yourself - this is assuming the BCM is OK which I reckon it might be.
Thirdly pull the lid off the BCM and look for any burnt components.
To call the dealer an idiot is disrespectful to genuine idiots. BCM is definitely swappable.