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.

Frederick NL

Here's a thread that you'll likely find interesting.

https://mbworld.org/forums/sl-class-...aulty-bcm.html

dave_n_s

Frederick NL, thank you for that link.

I've seen a few photos of the results of a component failure in the BCM. These are useful.

The debate about why and how to change the design is rather too late though as changing one or two components to "improve" something is typically adding risk unless we have all the design data, including the schematic, voltage test levels to test new units coming off the line and all the failure mode testing they did to confirm the reversionary modes are engaged as they should, without causing further failures.

I'd hoped that at least a full schematic would have "escaped" from MB by now, so that at least we would know the correct values of all the components and the connections through what I assume is a multilayer board, so we can thoroughly test a board before and after installing replacement components.

With more than a little experience in electronics design, certification and reliability, this issue looks like a case for some preventative maintenance, perhaps some secondary external module to provide added protection, and an inspection and specific component replacement at x years or y kms whichever comes sooner, together with some clearer advice on how to manage the batteries during active use and winter storage.

So anyone with a schematic please?

DavidCA

Here is the schematic and docs for module N82/1, as requested.

dave_n_s

DavidCA,

First, thank you. There is some info I've not encountered before, primarily the service-tech oriented system logic as opposed to the informative but incomplete training doc on the two battery system.

One nugget in there is at least the partial logic behind the red battery message - a failure of the system to go into auto mode. Can anyone confirm if that is the ONLY reason? Note that I didn't write "failure of K57"

Unfortunately I'd forgotten that Schematic means one thing on this side of the pond (where I live now) and something else on the other side (where I used to live).

The 4 page system diagram is more useful info, but doesn't go below module level. What I need is a good old fashioned circuit diagram, identifying every component on the PCB and how they are connected.

Dave

Frederick NL

Although it is no answer to your question this thread is incomplete without the pdf that outlines the functionality of the BCM. Easy and informative reading. For reasons unexplained uploading a pdf is forbidden for me here.

g0rsq

I think you mean this document.

dave_n_s

I've now had a look inside the BCM and it looks pristine to me. Also no odour. One observation is that there is no gasket below the cover. That seems an omission and water ingress on this PCB can cause havoc - I'm 99% certain the PCB is not conformally coated, which allows a moderate amount of humidity to collect on the PCB surface without causing harm. Has anyone opened one of these on an 05 and found a gasket? I'm now reassured I haven't got a BCM that was in the process of failing with potentially catastrophic results. Even if a BCM issue is responsible for the intermittent red battery message, it's not drawing sufficient current to provide visual evidence. I'm therefore comfortable operating the vehicle, with the carpet removed for now and will try to get some codes next time it misbehaves and do some more research on how to progress troubleshooting without swapping things that might be the cause.

Can any of the members who've worked on this system please advise on next troubleshooting steps.

The BCM was the priority, although not easy to access, because it can cause the red message, due to a failure causing it to drop the system into emergency mode, or also do that in the event of an Alternator failure. Correct?

I can measure the alternator output most easily at the rear battery terminals. Are there any specific values that the BCM is looking for, which would cause an intermittent message if the alternator was marginal? This is a 110k miles vehicle, so the alternator has every right to be tired at this stage in its life, but we should be able to measure what tired means!

cdk4219

I have a bcm that was faulty with a batttery light, no burning, no smoke, but was triggering the battery light. If you would like it I would gladly send it to you for further investigation. Pm me your address and I will box it up and get it to you. If you are interested. Thank you for your help in this investigation.

dave_n_s

Hi,

Thank you, yes we have the resources to analyse these boards. I will send you a PM later today with some questions etc

Frederick NL

Perhaps join forces with calibenzdriver ? Without being a technician (meaning - not inhibited by knowledge) I think his remarks about spikes are worth taking in the equation.

dave_n_s

It would certainly be good to have folk experienced in the world of power electronics collaborate on this, especially if one of them had a full schematic (circuit diagram) for the LRU and associated external circuitry.

At system level this is a very good design, but MB haven't put the effort into improving reliability that they could have, which is disappointing.

jvakos

​​​​​​@cdk4219 am sorting out a 2004 SL55 battery charging issue for some time now. I show zero codes on my foxwell scanner, but my rear battery doesnt charge while driving and I have the red battery light on. I can fully charge the rear battery where it will show 12.5-7 when the car first starts and then it slowly over around 1 hour of driving drops to 11.0 and below that the car freaks out and shuts down all consumers etc.

I pulled out all the front passenger footwell layers to check the 100 amp and 200 amp fuses which were fine. I am assuming this is either an alternator problem or BCM so my question is wouldn't a non-functioning BCM throw a code of some sort???

I've had alternators go bad on MB cars in the past when the voltage regulator brushes wore down and never got codes for those...

MikeJ65

The BCM has nothing to do with charging the rear battery.

Tom Manning

I would go straight to the regulator.

Put your voltmeter across the rear battery and check the charge after starting the car.

If the rear is not charging, or charging below optimum then the front will not be charging correctly either; you can get many starts out of a slowly discharging front battery which is why you aren't noticing it - yet.

Rear is charged by the alternator. Front is charged via the BCM. One of its functions is effectively an on-demand battery charger for the front.

The regulator can be replaced without removing the alternator

Tom Manning

Also, there was a known fault with the regulators. Mine was replaced within a month of original purchase.

dave_n_s

There are a lot of scare stories on this system and the BCM can fail with serious consequences, however troubleshooting most problems is straightforward if you treat it as a simple system.

Even before you go looking for codes, check both batteries, car not running. !2.5V or so? OK. Rear battery <10.5V, the front battery will lose charge over time and generate the red message when pulled down during start.

Then remember the front battery is charged from the BCM, using power from the alternator or rear battery (which is charged by the alternator)

So start the motor and check voltages at the front and rear batteries.14.5V or so? Nothing major going on, although if you get the red message with a good running voltage at both batteries, it's likely that one of them is ageing, potentially the rear not being able to supply enough current during the start cycle as the front battery droops during the start. Base next steps in that case on the age of the batteries. Anything over 3 years, perhaps worth a change.

Motor running and rear battery not hitting 14.5 or so? either a bad cell pulling down the regulator output or alternator / regulator issue.

Motor running and front battery not hitting 14.5V or so but rear battery good: BCM or fuse issue.

Tom Manning

You may, and probably will measure charging voltage on the front battery, but even if you don't, the system is probably still working properly. The BCM is not always charging the front battery; it's effectively an intelligent charger that charges on demand.

Before condemning the BCM, and after fixing the underlying issue, do several charging checks on the front battery until you get a charging reading of around 14.5, but know that you won't necessarily measure it all the time, particularly if the front battery is in healthy shape.

g0rsq

How did you test the fuses?

Test with OHM meter not voltage, as even when blown you will see voltage on both sides.

Best remove fuses to test.

jvakos

​​​​​​Hello @dave_n_s @Tom Manning great info thank you as it sounds like my rear battery not charging is narrowed down to the Voltage regulator and/or Alternator issues... I did have a oil filter housing leak that soaked a decent amount of oil on my accessory belt and I found that the pulley on the alternator was slightly oily.. not sure the oil made its way to the voltage regulator but will check. I replaced the voltage regulator 6 months ago with a Bosch unit from FCP and did order a new one that I'll start with to replace and if that doesnt work then I'm yanking out the alternator and going through that process.

super appreciate the help and education on the BCM which I am no longer freaking out about here

merzlover

Hello I cannot communicate with BCM (BNS) module red battery light on so I opened it up and nothing is burnt but i do see slight corrosion on one little chip on the pcb, is this something you can repair?

peterkay

I think your BCM is fine. Likely you have developed a parasitic current drain that leads to the accelerated discharge of your rear battery. I recommend putting a amp-meter inline with your rear battery and watching the current as the car settles to sleep. It may take 30-45 min during which the current will decline. The good sleeping current is less than 0.05Amp. In my case, the current remains high ~0.4amp which leads to accelerated discharge and red battery light.

cdk4219

Battery light doesn’t come on when consumer battery is dead. Instrument cluster shows convenience options not available. Replacing both batteries is recommended, and disconnect when stored for over 3 weeks or ctek charger for the rear “consumer “ battery.

peterkay

Are you sure about this? My red battery light used to come up daily because of the large parasitic drains that dropped the voltage of the rear battery overnight.

cdk4219

I don’t recall the battery light coming on when the rear battery was dead in my 2003 car. Soon after replacing the battery it came on and was the BCM. Could be mistaken though. Regardless if this BCM issue about Mercedes not producing them doesn’t get fixed, it will make these cars worth nothing. Used ones are going for $2000 up.