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The question would be if there is ever going to be over 30A on that wire without the ignition on. My fear is that when a door is opened and the car wakes up, you might get several modules drawing a few amps and then maybe the SBC or PSE pump kicks on and takes it over 30. Also, if the rear battery is low, the BCM needs to activate emergency power before you will have circuit 15 voltage. When that happens, the front battery connects to the rear one at the BCM. I would not be surprised to see >30A load in that case. If we route the K57 relay to the battery side of the fuse as I mentioned above, we allow for 100A emergency power, 30A DC-DC converter power for the BCM, and everything is protected. The dual battery information says that the max. charging rate for the front battery is 15A. Even with the loses in the DC-DC converter, 30A should be more than adequate. You could also remove the front battery supply from the BCM and add a 30A fused lead. That would limit the BCM to 30A from the rear battery and 30A from the front.
In this scenario, the BCM could "consume" 360 watts of energy without blowing a fuse. This is more than enough to cause overheating and potentially a fire. It would run at nearly a Kilowatt for 10 seconds before the fuse blew!
Gentlemen, I have on my hands the low drain electric golf cart relay. All I need now is to find a positive key on/key off connection somewhere in the trunk area where to connect the relay coil. I am sure some of you have done such a search that have the answer right in your memory. Please let me know where to find that green "+".
What powers the EIS? Is it the battery you have disconnected? Would you get an "ignition" voltage anywhere if rear battery is disconnected.
Maybe K57 would energise from front battery power, but what if the issues were due to excessive operation of K57, burning the contacts, causing them to overheat, and limit current flow?
How would you get in the car, as the PSE pump wouldn't be connected.
You would need to use the key every time you got in the car.
The emergency key lock mechanisms are not designed for heavy usage, so would wear out.
In this scenario, the BCM could "consume" 360 watts of energy without blowing a fuse. This is more than enough to cause overheating and potentially a fire. It would run at nearly a Kilowatt for 10 seconds before the fuse blew!
With the current configuration the battery can output around 900A for at least 30 sec. I would feel better if the car in my garage had 30A fuses.
Mike, I don’t think that fuses in both sides of K57 will protect the BCM since it is known that the fires start when the vehicle is parked, which means K57 is open and both batteries are isolated.
I really think the issues appear when battery power is grounded inside the BCM creating an overload over the connecting cables. That overload may even be caused by any of the consumers hitting a short ground and heating up the power line..!
I would think that isolating the rear battery without disconnecting the consumers will offer a good alternative for protection. I could even place 30 amps fuses on each line to the consumers at points C and B. In that way we are fully protecting the lines going to the consumers, which may be the initiators of the overloads nearby the BCM!
Here is what I am thinking for possible solution. The purpose is to disconnect the accessory battery without disconnecting the accessories. We could do that placing relay in C and Relay in B. When the vehicle is parked, these relays are de-energized and connecting the battery + to the consumers (Load 1 and Load 2). The battery will be disconnected from the BCM. So, when the vehicle need to be started, the Key will energize Relays in A, B, and C. Relay in A will connect the battery to the BCM and relays B and C will connect the loads to nodes E and F, therefore connecting the accessories as in normal operation. When the relays B and C are transitioning to the NO pole, a millisecond of no power to the Loads may occur, we will install capacitors U to supply voltage to the loads for that Millisecond!
Mike, I don’t think that fuses in both sides of K57 will protect the BCM since it is known that the fires start when the vehicle is parked, which means K57 is open and both batteries are isolated.
I really think the issues appear when battery power is grounded inside the BCM creating an overload over the connecting cables. That overload may even be caused by any of the consumers hitting a short ground and heating up the power line..!
I would think that isolating the rear battery without disconnecting the consumers will offer a good alternative for protection. I could even place 30 amps fuses on each line to the consumers at points C and B. In that way we are fully protecting the lines going to the consumers, which may be the initiators of the overloads nearby the BCM!
The fuses are not on both sides of K57, they are on the front battery connection to the BCM and the rear battery connection to the BCM. These are the only energy sources to the BCM except for the K57 and K75 coils, which the BCM grounds to activate. Those currents are limited by the coil resistance of the relays, so they shouldn't be an issue.
If the issue is internal grounding in the BCM, the fuses will greatly reduce the power that can be consumed in the BCM and should blow a fuse before it gets to the point of ignition.
As far as your diagram, you lost me a little bit with relays B and C. Why not just eliminate the C-E wire and the F-30 wire and add wires from C-D and F-D. That leaves your accessories connected to the rear battery and the BCM is still disconnected with relay A. However, I think you are still going to be in emergency mode for startup because the BCM will see no voltage at the rear battery. That's a problem if you want to drop the roof without waiting 15 minutes.
Ahhhhhh, I got you now Mike..! Below is what you meant..! As far as the current will not exceed 30 amps, that will be a much simple way to go! Will K57 have enough juice to interconnect both batteries in emergency mode?
Thinking rationally, the diagram below is better to keep the alternator and consumer through K57 and fusing the BCM!
Now, that alternator shows a 200 amp fuse and on the other side a 100 amp fuse..! Where all that current go?
Thinking rationally, the diagram below is better to keep the alternator and consumer through K57 and fusing the BCM!
Now, that alternator shows a 200 amp fuse and on the other side a 100 amp fuse..! Where all that current go?
Yes, this is what I was proposing. As you know, the rear battery runs everything but the starter and BCM under normal operation.
Load 1 is the rear prefuse that feeds the rear SAM/fuse block and provides one feed to the front passenger SAM/fuse block.
Load 2 is the front prefuse that has feeds for both front SAMs, EIS, cooling fan, AC fan, and circulation pump.
The BCM is just a convenient point to connect the battery cable to the heavy cable running up to the front prefuse. Despite the appearance, the BCM does not normally consume or pass large currents. It needs maybe 20-25A rear battery/alternator power to provide a 15A max charge to the front battery and it uses a few amps from the front battery to power the module. When it detects low rear battery voltage, it engages K57 to provide emergency power. This emergency power is limited by the 100A firewall fuse. In order to not exceed 100A, the BCM sends CAN messages to many modules to limit power consumption. For example, it does not allow the roof to open, seat heat, rear defrost, etc. It also engages K75 to drop power to the cigar lighter and the rear 12V lighter port in the trunk. These are not controlled by a module, so power is removed by the relay.
The side effect of having this battery to front prefuse connection at the BCM post is that it makes full battery potential available to the BCM. An internal short from the left post (rear battery) to ground in the BCM will have at least 900A available to potentially start a fire.
While most of us are familiar with the K57 'consumers unavailable' operation when the rear battery runs down during storage, the BCM will also activate it if voltage drops during a drive. This supplies front battery power to the system and limits consumption, mainly to preserve enough voltage for brake pump operation and give warning to discontinue driving.
The new 30A fuses will not affect the K57 operation, it is limited to 100A either way. Of the two fuses, the one of the right side of your diagram is far more important as it reduces a full unfused battery source to a 30A fused supply. The one on the left reduces 100A potential to 30A.
I think I have a very easy way to do the fuse on the right side of the diagram (left terminal on the BCM). I am going to order parts and test it first and then I will share.
This looks to be a solution to the spontaneous ignition. How would one bench test this to determine if it’s ignition proof? Will adding components to a system generate unintended consequences? Like more resistance.
Great work and tip of the great hat to you all!
Mike, what other models of Mercedes use the same configuration for two batteries. Are there any episodes of BCM burn in other models.? Has the BCM re-designed at any point in time for the newer SL models? Perhaps investigating the re-designed system may shine some light on us...!
BTW, what would be the easier way to fully access the BCM. A while back I just cut a small section on the carpet covering it, but for any rewiring and fuse location, I will need more accessibility!
I have identified the connections for the rear battery +, but not for the front battery+, !
The 2007 to 2009 Sclass W221 had similar 2 battery system. I have been on these forums for 4+ years and have not seen any reports of BCM fires in those cars. My 2012 W221 has a one battery system , which I think came in 2010, not totally sure on precise model years.
One more update on the car that was the initial subject of this post. I have this car at our second home in FL, so I have not had much time available to work on it. However, the electrical repair work is now 95% complete, the car runs and drives fine, no BCM or battery related codes.
For anyone considering a BCM fire repair, here is what I have replaced so far: complete trunk harness, BCM, BCM bracket, alternator cable and front battery cable that run from firewall fuse to BCM, feed wire from F61 to K75, BCM CAN pair, remaining BCM partial harness with K57/K75 relays spliced in (soldered and heat shrink sealed), complete right frame lock with cylinder, most attachment parts on the right roof hinge (roller, rub piece, roof open switch, etc), trunk lid liner, right roof flap, right rear strut plunger sensor and harness, 30A fuse installed on BCM terminal 30. I also replaced both batteries. I think that the rear battery was destroyed by the BCM short, front was just weak from age (2014 date).
Still need to replace right load assist cylinder, right main roof cylinder, Keyless Go right rear antenna, pneumatic line for fuel door, TPMS right rear antenna, trunk divider, front trunk lining, right side trunk lining, wrap wire and secure affected wiring.
I have a parts car to work with, but without that, it would be $3-4k in parts and at least 40 hours labor. Even this very nice 30k mile SL55 would have been considered a total loss due to the fire. Unless a car has a big sentimental value, I don't see many base SL500s that would make sense to repair. Nice SL55, SL600, or SL65 models would be better candidates.
I pulled wires intact where practical. The large cables from firewall to BCM, CAN wires, and K75 feed wires were all done without splicing. Wiring like the strut plunger was soldered and sealed with heat shrink tubing. Replacing this intact would require complete replacement of the floor harness which requires pretty much complete disassembly of the full interior and dash pad and would at least double the repair time. In addition, there are many variations of the wiring harness, so it would be difficult to find a harness that is 100% correct and 100% intact.
As a side note, the firewall fuses were both intact. F61 which feeds the K75 relay was blown. I suspect that the feed wire shorted when the harness melted down.
It's hard to believe (but true) that Mercedes has never acknowledged this occasional, but very real, Battery Control Module (BCM) design problem and has never developed a solution for this obvious bad design. I guess someone may have to die in a BCM-caused house fire and their heirs obtain a multi-million dollar liability judgment from MB before the company addresses and corrects this situation. Since about 50% of my residence (the bedroom wing, naturally) is directly above my garaged 2004 SL600, I manually disconnect the negative terminal of the consumer/systems battery from my SL's chassis ground when the SL is parked more than a day or two in the garage. At least in my car's case, I don't have to reset any computer circuits after re-connecting the trunk-mounted battery's negative terminal to my chassis ground.
I am not aware of any burned cables or components that have occurred near the engine compartment's starter battery as a result of this BCM design flaw, so hopefully my house and its occupants are safe.
I would be happy to purchase and install any retrofit kit that our forum members can devise to correct this MB design problem. It seems that we have some very knowledgeable engineers currently looking into this situation. I thank them for their work and I hope they succeed with developing a solution.
One more update on the car that was the initial subject of this post. I have this car at our second home in FL, so I have not had much time available to work on it. However, the electrical repair work is now 95% complete, the car runs and drives fine, no BCM or battery related codes.
For anyone considering a BCM fire repair, here is what I have replaced so far: complete trunk harness, BCM, BCM bracket, alternator cable and front battery cable that run from firewall fuse to BCM, feed wire from F61 to K75, BCM CAN pair, remaining BCM partial harness with K57/K75 relays spliced in (soldered and heat shrink sealed), complete right frame lock with cylinder, most attachment parts on the right roof hinge (roller, rub piece, roof open switch, etc), trunk lid liner, right roof flap, right rear strut plunger sensor and harness, 30A fuse installed on BCM terminal 30. I also replaced both batteries. I think that the rear battery was destroyed by the BCM short, front was just weak from age (2014 date).
Still need to replace right load assist cylinder, right main roof cylinder, Keyless Go right rear antenna, pneumatic line for fuel door, TPMS right rear antenna, trunk divider, front trunk lining, right side trunk lining, wrap wire and secure affected wiring.
I have a parts car to work with, but without that, it would be $3-4k in parts and at least 40 hours labor. Even this very nice 30k mile SL55 would have been considered a total loss due to the fire. Unless a car has a big sentimental value, I don't see many base SL500s that would make sense to repair. Nice SL55, SL600, or SL65 models would be better candidates.
I pulled wires intact where practical. The large cables from firewall to BCM, CAN wires, and K75 feed wires were all done without splicing. Wiring like the strut plunger was soldered and sealed with heat shrink tubing. Replacing this intact would require complete replacement of the floor harness which requires pretty much complete disassembly of the full interior and dash pad and would at least double the repair time. In addition, there are many variations of the wiring harness, so it would be difficult to find a harness that is 100% correct and 100% intact.
As a side note, the firewall fuses were both intact. F61 which feeds the K75 relay was blown. I suspect that the feed wire shorted when the harness melted down.
While I applaud you for putting this car back together, I question why if nobody has determined what specifically made it happen, and more importantly a tested solution to this issue. The only solution as of right now is to disconnect both batteries.
g0rsq, I have been tracking the old BCM related topics to find answers and found your post. If you like I can give you my BCM board that is partially burnt. Perhaps you are the man who finds a way to retrofit this poor and unreliable piece of electronics. !
08-14-2023, 06:26 PM
