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The BCM has no communication which I dont believe a bad battery would cause, opened up the module and found 1 little chip with corrosion around it but not burnt
@merzlover can you post voltage at front and rear batteries before start and after start please.
Do you have any other messages? Convenience functions unavailable?
Any fault codes? (a Foxwell NT530 is almost essential to support these vehicles unless you have an unlimited budget to replace things because someone here said so or to pay your local MB specialist)
@dave_n_s I have an autel scanner that supports mercedes very well, it has no communication with the module and the cluster is stating no communication with BNS as well. Terminal 61 was on and batteries were charging as they should fuses are also good. No communication to BNS so I opened it up and found no burning just a little chip with corrosion. I have brought the module to a local PCB repair shop and they are checking it over now under a microscope, so far the chip with corrosion indeed is not sending a signal he will check over the rest of the board and see if he can acquire parts necessary. I have also seen online repair shops that claim to rebuild BNS for anywhere from 300-800 USD which worse case scenario will be my next step. In anyone’s experience is there something else that would not let the BNS communicate? Can bus bar looks all good and all other modules communicate fine
If you have a scanner with manufacture-specific software, it should be fine - for instance can it display ABC system pressure?
I would be cautious about having a shop that does not have a schematic for the BCM trying to troubleshoot it. Although we have the external pinout for the BCM, I'm not aware of a schematic "escaping" from MB.
If the starter battery is reading over 14V with the engine running, either the system is in emergency mode (relay K57 closed) or the BCM is functional and there is a CANbus comms problem.
If you have a scanner with manufacture-specific software, it should be fine - for instance can it display ABC system pressure?
I would be cautious about having a shop that does not have a schematic for the BCM trying to troubleshoot it. Although we have the external pinout for the BCM, I'm not aware of a schematic "escaping" from MB.
If the starter battery is reading over 14V with the engine running, either the system is in emergency mode (relay K57 closed) or the BCM is functional and there is a CANbus comms problem.
last time I scanned i believe front battery was reading 12.4 volts running and rear about 14.4, i have since removed the BNS to have it checked out. corrosion on the chip along with no communication is most definitely pointing towards issue with the BNS board, will get board back this week and see what happened
Be aware though that if the front battery is charged and healthy you may not read a charging voltage across it. The BCM is a complicated module but in effect serves a very simple purpose: to charge the front battery when needed; it's effectively an intelligent charger. When the front battery comes up to spec it will stop charging so measuring no voltage across the front battery does not necessarily indicate a faulty BCM.
I'm a bit suss on the 'corrosion on a chip' diagnosis. I'd be happier to see a more systematic diagnosis here. A failing rear battery will throw the red code - replace the rear battery and clear the code with a scan tool.
I don't have any documentation of BCM function down to that level. If you have some, please share, however at system level, each start discharges the starter battery enough for it to be topped up by the BCM.
I do agree that a weak rear battery could potential generate a red message, because during the start before the alternator is online the rear may not be able to supply enough current to meet the demands of the BCM to top up the front.
In fact I see that on my SL occasionally and intend to instrument both charging circuits to collect hard data on what voltage / current actually triggers the red warning.
If only MB shared the full logic for each message!
You will usually measure a charging voltage on the front, although if you're restarting warm you may not - I have seen that. Safe to say though that if starting from cold and if you don't measure charging on the front, then that circuit is not working. To complicate matters more, the front battery has a very easy life, all it has to do is crank the car so it can do many starts before it winds down, however by that time you'll have the red warning telling you that something is wrong.
I received my BNS back today from the circuit board repair, the two little chips that had corrosion were indeed bad not sending signal and causing a short on the board, he cleaned them up replaced the pads and solder and then he got a good reading. I Went back to my shop and installed, crossed fingers and started it up… NO RED BATTERY LIGHT! Then proceeded to see if i had communication again with the BNS and I indeed do. Got lucky this time, I have had 3 burtn modules on 3 different SL550s in the past. Ive owed over 50 sl500 2003-2006 and this was my first BNS issue on one… seems like more prone to burn on the 550s in my experience
Hi does any know where i can get the a2305400945 schematic?
Originally Posted by dave_n_s
The old girl 05/SL500, owned for 15 years) has taken to intemittently waking up with a red battery fault. I've measured volts at the battery previously in this case, but not attached the scanner till today and when I got the scanner updated and connected, there was no red message, but there would be a stored BCM fault, wouldn't there?
So battery voltage pre-start 12.7, post start 14.2.
BCM Fault - NIL stored or active with red message not present
BCM Live data with engine running: BCM terminal 30 14.1V, 30a 14.6V
Terminal 30 is Alternator to BCM and consumer battery
Terminal 30a is BCM charging supply to starter battery
I'm OK with 30a voltage being higher than 30 voltage as that's what DC-DC converters do, providing those are the right voltages, which I have no documentation on.
Does anyone have any data on what the red battery message actual means and any other troubleshooting advice, apart from getting the %&*)ing scanner connected while the red message is present?
In the absence of any more data and (I hope I was not imagining it) a bit of a suspicious smell in the trunk, I've decided that I need to inspect the BCM internally, given the history of these and DC-DC converters in general, which can fail by burning when too much current is drawn due to a component failure. Electronic components do fail - that's my world - but it's unfortunate if there is a design that allows a single component failure to result in a cascading failure that ends up with a fire.
So I'll remove the BCM, inspect it internally and carry out any necessary work to get it back in action, providing there isn't significant PCB damage. We're equipped to do that and it may spread some useful knowledge on preventative action.
I intend to document this as I haven't found even an honest documentation on how to remove the BCM without taking a knife to the carpet. The one YouTuble video on removal recommends not putting the lid into service mode, which is plain wrong unless your hard plastic trim is already damaged.
Please all dive in with corrections / additions / links to good documentation of at least BCM removal / refit that I've not found.
And finally, yes, I did disconnect the rear battery as soon as I decided I had a justifiable suspicion of the BCM being in the process of failing as it's doing its thing, charging the starter battery and powering allsorts while the car is parked / switched off.
With all the problems with the BCM and that strange smell, I'd definitely check the module for any internal damage. Like you said, if the DC-DC converters go bad, it can cause more damage or even start a fire.
02-02-2024, 06:09 PM
