Members of the R230 society,
After 3 years of ownership and only spending money on fuel, oil and filters on my SL500, finally the moment of digging for a failure root cause has arrived. Last Friday after a 20 miles drive, just when entering my garage I smelled an unfamiliar burning smell inside the cabin and at the same time the battery red light came on. I immediately open the trunk disconnected the rear battery and touched the BCM which was at normal temperature. Have not had time during the weekend to start gathering information from the diagnostic process and will start checking the alternator soon. I wonder about that unusual smell (plastic burning) inside the cabin which may be unrelated to the alternator fail mode. I wonder if anyone have experienced this type of issue.!

Looking forward for a hint!

Imack

 

I'd say you're still pretty lucky to have caught it right away and while pulling into your garage (vs. being stranded somewhere).

As far as next steps:

-Scan for codes.
-Remove the lid on the BCM and inspect for damage.

It's also possible the burning smell is not the BCM. I've encountered alternators that failed catastrophically, seizing completely and causing a burning smell.

 

(1) I wouldn't put the car inside of my garage if it has a burning smell! (2) Get a fire extinguisher and keep it nearby. Not a chemical or dry powder extinguisher. Those chemicals will ruin the electrical equipment in the car and any delicate mechanical items that the powder gets on. Use Halon (if you can afford it!) or a large CO2 extinguisher. Even a garden hose hooked up, laid out and ready to go is better than nothing.

I would open up the BCM and inspect the inside of it and look closely at the wires going in and out of it and their screw terminals before I reconnected the battery or attempted to scan for codes. You might want to pull both batteries out and have them tested before doing anything else. Or test them in the car if you have a battery tester. With any luck it might just be a bad battery.

 

Gentlemen, thanks for the feedback..! Yesterday I re-connected the rear battery and watched all around for any signs of smoke or smell, and found nothing. Then I proceeded to measure voltage on the rear battery, 12 VDC and as I cranked the engine, it went to 14 VDC, meaning alternator is OK. Then I repeated the same on the front battery and was 11.7 VDC and after cranking, the same, meaning no connection to the charging system. Somewhat, somewhere there is an interruption between the BCM, the fuses and the front battery. Scanned for codes and found miscellaneous codes involving the CAN controllers, but none indicating a charging connection. I plan to uncover the BCM and inspect, then check continuity from the cable connecting to the front battery to discard a blown fuse. Hope that BCM is still alive and well!

 

That's great! Hopefully (?) the problem is somewhere in the CAN system. But I think that you've been warned, and if you haven't installed Tom Manning's fuse mod for the BCM then you should do that ASAP.

11.7 volts on the charging battery doesn't necessarily mean that there is a problem. It might just be that that battery hasn't fully charged yet. The only way to immediately tell what's going on is to use a clamp on DC amp meter or to open one of the connections to that battery and temporarily install an amp meter. It's kind of tricky to use an amp meter with the front battery though. You have put the amp meter in one of the circuits that isn't used for the starting since that draws far more amperage that most amp meters can handle. And also it's hard to disconnect any starting battery circuit to connect an non-clamp on style amp meter since you're supposed to disconnect the system battery first and then reconnect it last. If done properly, the process would be: Disconnect the system battery, disconnect the starter battery, open up what ever connection there you want to install the amp meter and add the meter, then reconnect the starter battery and then reconnect to system battery. Then you can take your readings, Then you have to reverse all of the steps to disconnect the amp meter. You can use a clamp on amp meter but any of those that will work with DC currant are expensive and very sensitive to the earth's magnetic field and other distortions. Also most of the clamp on meter on the market are actually designed for 60 Hz AC power and they are frequently not very accurate for DC, particularly with small DC currents. One clamp one meter that does work well is the Fluke 80i-110s but the retail price for it is over $1000 US.

If I were you, I wouldn't worry about the starter battery yet. Since it's only used for starting you can leave it alone until you get everything else fixed and then drive the car for a bit and see if the battery does charge up. if it doesn't then you can go back and dig into the charging circuit for it.

In theory you could even remove the fuses that feed that battery or disconnect the wire at the BCM that feeds that battery. Badgator reported the he removed or disconnected K57 that connected to that battery and his car didn't suffer any ill effects. The relay K57 controls a 100 amp circuit that I THINK provides a quick high rate charge (5 minute?) to the starter battery from the alternator when you first start the car (and K57 also provides an emergency backup to the system battery but that is another story). However, I think that the BCM also directly charges the starter battery but for a longer time and at a lower rate. But the point here is that I'm pretty certain there are two different charging circuits for the starter battery.

 

Sunshine friend,

Thanks a lot for your enlighten...!
I have not dived deep into the issue by lack of time, but by studying the schematic I can tell that from the trunk I could test the fuses connecting to the starter battery by disconnecting the corresponding cable leading to the 100 amp fuse. I can see that CAN-B as appeared in my code reader, has a role on the activation or deactivation of the BCM..I will get there as soon as time permits.
There are systems out there using VSR's (Voltage Sensitive Relays) when using dual batteries in boats, campers, and other applications where a second battery is in the system. The VSR does basically the same function as the BCM by isolating the two batteries and charging the starter battery when the voltage drops under certain value and no fire risk!
If worse go to worse and my BCM is bad, considering the scarcity of this component, I will consider replacing the system with on VSR which is much less complex than the Mercedes system !
I'll keep posting as I penetrate into this troubleshooting journey..!

 

only big downside of doing this is you will have permanent warnings on the dashboard and no built in backup for a discharged rear battery, which could prevent the SBC working should the rear battery not charge up while driving.

 

I was going to say basically the same thing. Designing a replacement for the BCM shouldn't be too difficult but making it play nicely with the rest of the car systems via Can Bus would be a huge can of worms. For example, there are several basic operations of the BCM that we don't know if the BCM preforms on it's own or if something in the EIS or one of the other systems triggers them. One example: the engagement of K57 to supply starter battery power to the systems battery in the event of very low system battery voltage.

Traditionally, the voltage regulator in a car controls the voltage and current going to the battery via pure hardware But in Ford F150s, software in the Body Control Module operates everything and it even directly controls the voltage regulator and governs the charging voltage and current to the battery. MB may do something similar. And there could be many other functions that are performed by the BCM but controlled by other modules via the Can bus.

It's already known that when the system battery gets weak in the R-230 that it triggers a number of the "Convenience systems" to shut down. The radio is one, the heater system is another and window operations is another and I think also seat adjustment. And I also suspect that operation of the ABC and the pump that operates the roof opening and closure would also be affected. All of those are almost certainly done by commands from the BCM, or possibly by the EIS but based on BCM status data as reported via the Can bus.

I guess that as a first step to fully understanding the BCM, someone needs to take a fully working car and disconnect the can bus from the BCM and then see what errors are generated and if both batteries are still fully recharged and what other systems are affected both with a good battery voltage and with a low battery voltage. That would at least tell us what the BCM is doing on it's own and what functions are controlled by other modules.

 

My friend, you are right, there are some other sideway functions and conveniences, but in case of a burnt BCM and no options to get a replacement, I would ignore all these other conveniences and will replace with a VSR. The VSR basically will keep the two batteries disconnected and will connect when one of them drops the voltage over a threshold value..! As I said before they are commonly used on applications where two batteries are used..! Anyway, let's keep the fingers crossed so we don't have to go to that extend of failure and god willing the culprit may be a fuse or some cheap component available anywhere!

 

THIS MIGHT INTEREST YOU

 

Candjb, thanks for sharing, the problem is that the video does not explain what kind of insert are those...may be fuses..! After reading posts about BCM catching on fire I have been worried about it and for the time being, I have placed a Ceramic Fiber blanket over the unit and adjacent cables, but I really think that will help adding fuses to the connections for the front and rear batteries of a value that will not exceed the Max amps required by the consumers (excepting the starter) and the alternator amps. At least if there is an internal BCM failure that shorts any of these + cables to ground, the fuse will go before the cables will melt and catch on fire...!

In the schematic below, you can see that a 100 amps is already in place for the starter battery. Adding another 100 amp fuse at the accessory battery point of attachment to the BCM and K57 relay will eliminate the chance of these cables to get extremelly hot in case of a short to ground..!

 

Adding a 100 Amp fuse where you show probably won't protect very much. 100 Amps at 12 VDC is still a LOT of power (2400 Watts) and will still do a lot of damage if there is a short. Tom Manning's fuse design is better IMO since it limits the maximum current through the BCM (and the associated wiring) to 30 amps under all circumstances. Add it adds protect from power coming from G1 and G2 (like your design) and it gives protection from power coming from G1/4. IMO one of the problems with the original MB design is that the f52f1 and f52f2 fuses are so far from both the alternator, the batteries and the BCM and a short in any of those long wires will still cause major problems. It would have been less convenient for the car owners and/or the part changing monkeys but the fuses should have been located as near as possible to the power sources, i.e. the batteries and the alternator, so that as much of the wiring as possible would be protected.

I like your idea of adding a 100 Amp fuse but I would put it between where you show (terminal 30) and the contacts in K57 (in the line directly above where you show it.) Right now, even though there is a 200 amp fuse between terminal 30 and the alternator, there is still virtually unlimited amperage available from G1 so if there is a short in K57 or the wire going to it, something is going to get burned up!

 

My friend,you are right but for me, after I get over this headache causing red battery light, I will find ways to be safe..!
After the red battery light came on, I disconnected the auxiliary battery, but after 6 days, the starter battery was drained which is highly unusual. When disconnecting and connecting the starter battery there is current going through....is that normal?

Where else is this starter battery connected when the vehicle is off?

I ran my scanner (Creader elite) and gave me a lot of faults related to the CAN modules communications. But no codes related to battery connections!
I wonder if this is a cause of the battery connections issue. ?

 

No, I don't THINK that there shouldn't be any current draining from the starter battery but there are people that claim that the BCM does try to charge the starter battery when it's low so it's possible that that is the current that you're seeing. OTOH you're supposed to disconnect the system battery before you disconnect the starter battery so if you did that then it's impossible that the system battery and the BCM are attempting to charge the starter battery. If you disconnect one lead of the starter battery and then put an amp meter across the open junction, that will tell you how much current is flowing and in what direction.

If you have a helper what you can do is to connect one lead of the amp mater to the battery clamp and have your helper hold the other amp meter lead directly onto the same battery post. Then loosen and remove the clamp and all of the current will then flow through the meter and you can measure it. Also amp meters draw very little power by themselves so the system can't tell that you have disconnected the battery so you would not be violating the "disconnect the system battery first rule". Keep the amp meter leads on the post and the clamp the entire time and then reattach the clamp to the post when you're done and only then can you break the amp meter connections. It's hard to guess show much current might be flowing through the battery if something is shorted so I would try to use an amp meter with at least 40 amp or so capacity (you can turn to a lower scale if you need to) and make sure that it has an internal fuse so that you don't burn up the meter if the circuit draws more power than what the meter can handle.

BTW as long as you don't engage the starter, you can measure the battery current through either the positive terminal of the battery or the negative side. But if you measure the current on the positive side make darn sure that you don't let one of the amp meter leads touch any metal in the car except where you intend or you will short the battery out!

There aren't many things that connect to the starter battery so it shouldn't be hard to isolate them one at a time and see where the power is going. I would start by disconnecting K57. It should not be energized and it should not be drawing any power but it's possible that it has a fault. You could also pull that 100 Amp fuse and see if the current is flowing through it.

"after I get over this headache causing red battery light, I will find ways to be safe..!"

Yeah, me too. They say that hindsight is always 20-20 and I try to never have to solve the same problem twice!

 

The only connections for the starter battery are the starter armature and the BCM. Since you can start the car with a totally dead consumer battery, it is logical that the starter battery is supplying at least some power for the BCM and also powering the K57 and K75 relays. It is normal to have some current draw (spark) when you reattach the ground cable. Note that there is a wake up wire from the EIS to the BCM, so the BCM does not need to be active at all times and front battery drain is (should be) minimal.

 

Mike, when I can escape from the wife honey do's I try to determine where the problem is before I start disassembling the footwell and the BCM cover. I put the scanner and got several codes related to the CAN modules. But, that isn't telling me too much. Next, with the rear battery disconnected, I charged the starter battery completely isolated from the can system and connected the + to a light bulb (between battery pole and connector). That bulb is not fully bright, but indicates that there is a leak something drawing power from it. I stated removing fuses and relays one by one. So far the light bulb, still on. Need to go to the rest of fuses and then will go to the BCM since I believe the 100 amp fuse is still good. By then I'll access the BCM will check continuity to the + of the starter battery! I also will check that K57 and may disconnect some of the inputs to see the outcome!

You are right, the starter battery may have some normal drain from somewhere, but after I got the RED battery light, with the starter battery connected, in 5 days it so drained that couldn't turn the engine starter..! That tells me that there is something there making a short!

I appreciate your feedback guys since I am right now like in an unknown jungle trying to find a way out! I know that sooner or later with ingenuity, perseverance and help from the forum gurus, I will get there!

 

Guys, one more clue that is pointing out to a screwed up BCM...! When the accessory battery is disconnected and the starter battery is connected, the windows move up or down when the door is opened or closed. I never tested that with the system in normal operation but sounds illogical to me since the starter battery should be isolated from the rest of the system...! Any experiences of this behavior?

I also wonder...are any other Mercedes models using dual battery system? If so, do they use a similar BCM? Or a redesigned BCM?

Thanking you again for your kind feedback!

 

That would make me strongly suspect the K57 relay. In order for the windows to work, they need to get power from the front battery and that can only be done (assuming no wiring mods) by the K57 relay. However, with the rear battery disconnected, the car should only wake when you insert the key. Also, with a really low, or no rear battery, the window short stroke function should be disabled. Leave the car alone for 15 min. with the rear battery disconnected. If it wakes up on door opening, you have power to terminal 30 of the BCM. This should not be the case. Quick test would be to pull the K57 relay and retry. Also test K57 resistance from pin 30 to pin 87. Should be open circuit when unplugged.

 

Thank you Mike...I'll go that path!

 

Guys, I finally found the source of the burnt smell...! Unfortunately it is the BCM. Now I need to start searching for a replacement without being ripped off...!
The condition of mine deceased BCM is beyond repair...it is toasted..!
The capacitor shown in one of the pics was the source of the disaster!
Any hints on who, where can I find one?
I will start practicing with a VSR relay....! That will connect the two batteries to charge, but no other functions!

 

I think it was one of the surface mount caps in the middle of the first picture. On the plus side, the fire was contained in the module and didn't total the car. I have an 0945 from a low mileage '03, but I don't really want to sell it because I will probably need it eventually for one of mine.

 

Not just the BCM, but troubleshooting needs to find the cause if any, leading to that unit overheating and burning.

 

MB2timer, thanks for the suggestion, unfortunately my field of knowledge is not to the extend of analyzing the board and determine the cause of the burnt, but sure I can tell it originated in the capacitor located where the hole is..! But, if anyone is interested in having this board and trying to figure it out you are welcome to have it, and if the research leads to add protection to this sorry system, I'll ship it free of charge!

The R230 dual battery control system is so poor, that the germans decided not to use it anymore on later models!

I'll keep on digging on ways to override its use and hope the electronic geniuses in this forum could have good solutions on hand since this is the future of their BCM's..!

Patience and perseverance will lead you to conquer the impossible..!

 

After digging for a while I am going to do experimentation by connecting 30 and 30a terminals with a VSR and a DC-DC smart charger. What would happen with the peripherals connected to the BCM? That is what I 'll find out, but I can tell that the car started and engine ran with the burnt BCM. With the override, the VSR will maintain the two batteries separated when ignition is off, and the DC-DC smart charger will charge the starter battery in a way that will supply variable amps depending on the battery saturation. Kind of the same that the BCM does.

I'll keep you posted on the results as I progress in my journey to get rid of the BCM necessity.!