When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Hi all - have a strange issue that I could use some help with - my front starter battery keeps dying on me leaving me unable to start the car.
I've had the battery replaced so have ruled that out as an issue, no lights on the dash and the rear battery seems fine (and seems to charge fine). When measuring the voltage across the starter battery with the engine running it seems to be fluctuating quite a bit (12.2v - 13.3v) or just steady and not charging.
I've scanned the battery control module and get the following code - anyone familiar with this?
B1838 - Terminal 30 or terminal 30a short
I've not yet checked the f52/1/2 fuses but I suspect they're fine if the starter battery voltage is fluctuating when engine on on (showing that the BCM is charging it somewhat and a connection is being made?)
Any ideas?
Last edited by BeanMeansCLS; 03-31-2024 at 05:51 PM.
I've not yet checked the f52/1/2 fuses but I suspect they're fine if the starter battery voltage is fluctuating when engine on on (showing that the BCM is charging it somewhat and a connection is being made?)
Any ideas?
On my car at least (a 2009 SL500) pin 30 of the BCM is connected directly (unless you've done the fuse mod discussed elsewhere) directly to the positive terminal of the consumer battery. Pin 30a is the other side of the DC/DC converter inside the BCM that charges the starter battery. However, that connection goes to the starter battery via a 100 amp fuse which is situated in front of the passenger's feet. I'd be checking around those terminals to see how things look. If you're not aware there's a lot of discussion online about the BCM dying and shorting out one of the batteries and thereby causing a fire. If I were you I'd take off the side panelling in the boot (or that might be the trunk for you!) and have a careful look at the BCM wiring.
Just in case it's useful, I've attached the BCM wiring diagram (admittedly for my car) to this message.
Thanks, so the best place to first start would be inspecting the BCM? When you say pin 30 do you mean the pin on the connector going into the BCM? (sorry I'm a bit of a noob with this stuff)
I'll check it all out first thing tomorrow - if it looks like the BCM is toast (have seen the other threads where it effectively burns itself out) can they be repaired? (I've seen the price of some 2nd hand used ones )
Thanks, so the best place to first start would be inspecting the BCM? When you say pin 30 do you mean the pin on the connector going into the BCM? (sorry I'm a bit of a noob with this stuff)
I'll check it all out first thing tomorrow - if it looks like the BCM is toast (have seen the other threads where it effectively burns itself out) can they be repaired? (I've seen the price of some 2nd hand used ones )
Pins 30 and 30A aren't actually pins at all, their post style terminals. IIRC, one is an 8mm post and the other's 6mm. They're kind of obvious in the photo of my BCM:
Terminal 30 is the one on the left with 3 wires going to it; terminal 30a is the one on the right with four mostly skinnier wires going to it.
Very likely a fault with the BCM - its main function is a smart charger for the front battery.
I'd be pulling the lid to check for any burning. If you're getting an unreliable voltage at 30A then it's virtually certain that the BCM is faulty. If not, as mentioned then you'll have to trace it through.
Gotcha - thanks. Out of interest did you opt to go for the fuse mod? Seems like a no brainer for the sake of potentially avoiding a catastrophic fire
I did, seems like a no-brainer. Mind you, the car's been off the road all winter and disconnected while I fixed a self-inflicted problem with the parking brake. As of today, it being April, I'm going to take it out...
Hmm, that MOSFET does look properly fried. (I'm an ex electron herder, I know all the jargon.) Problem is, semiconductor devices rarely die of their own accord so it's much more likely that something else on that board has died which has fried that transistor. Old capacitors are very likely the root cause. I'd suggest that your only real approach is to find a new/used BCM, although I understand that they're not exactly easily acquired and only then for silly money, or find someone who could repair it. There certainly are people out there who claim to remanufacture the BCMs. Mind you, the one I'm looking at says it's a mere £500 to repair one.
It is BUZ111S by Siemens, the refurbishment service changes all the capacitors and problematic resistors as well. I might get away with just changing the MOSFET (I'm pretty confident soldering PCBs) but who knows what else might be wrong with it that caused it to blow in the first place. There's no cheap way out of this one I don't think
Yes true. Thinking about it any competent PCB repair shop should be able to replace the MOSFET, capacitors and check other components whilst they're in there...(for much less than than £500!)
That's very reasonable, especially considering the cost of a used module. Most failures are beyond fixable, but it would worth the money to replace the caps as a PM in a fully functional module.
So today after 45 mins of driving I get the dreaded red battery light "visit workshop" along with the following error:
The starter battery is showing good voltage and looks to be charging OK. If I clear the code it goes away but then returns after 45 minutes with the red dash error....any ideas?
Hi all - have a strange issue that I could use some help with - my front starter battery keeps dying on me leaving me unable to start the car.
I've had the battery replaced so have ruled that out as an issue, no lights on the dash and the rear battery seems fine (and seems to charge fine). When measuring the voltage across the starter battery with the engine running it seems to be fluctuating quite a bit (12.2v - 13.3v) or just steady and not charging.
Just because it is a new battery doesn't mean that it's good! I've seen a LOT of bad new batteries since Johnson Controls bought out most of the other battery manufacturers in the US. Get the battery tested! For crying out loud; a new battery costs less than the one hour minimum labor that Mercedes charges! Most of the auto parts suppliers will test them for free if you take the battery to them and I think that they can test them without removing them from the car.
A couple of added notes; The name on your battery means absolutely nothing. Johnson Controls and the other companies will put any name on the battery that you want , including Mercedes Benz, Bosch, AC Delco, etc just to name a few. None of those companies actually manufacturers their own batteries. But you can look on the bottom of the battery and the initials of the actual manufacturer are usually stamped on the bottom. A second note, retail companies like Sam's Club and Discount Auto parts often sell batteries made by one manufacturer in one part of the country and batteries made by someone else in a different part so the hear-say that applies in one part of the country may not be true else where. By the same token, resellers frequently change suppliers so they may use one supplier last year but a different one this year. With the heavy weight of lead acid batteries; a major factor is the shipping and the logistics of moving them from one part of the world to another or from one part of the country to another. As well as how much each manufacturer is willing to cut their wholesale price each year in order to get the business of the big retail stores; so the quality and/or the manufacturer of the batteries out there is constantly changing.
You could also put an an amp meter in series (NOT across!) with the cranking battery and see what's really going on and if it is being charged or not or if it's continuously drawing power, i.e. shorted or partially shorted and never fully charging. An amp meter will often time tell you more than a simple voltmeter can. You can insert the amp meter between the BCM and the battery or between the battery - terminal and chassis ground. Just don't try to start the car with the meter in the circuit or you will fry it!
"The starter battery is showing good voltage and looks to be charging OK. If I clear the code it goes away but then returns after 45 minutes with the red dash error....any ideas?"
Well since the BCM is keeping the battery charged it should always look like it's charged even when it's not because what you're really measuring is the charging voltage coming out of the BCM. Disconnect the starting battery and then check it's voltage and then recheck the voltage over the next 24 hours or so. I think that you'll find that it's not holding a full charge. It's very common for one cell in a battery to short out and the battery voltage will drop to roughly 10 volts. 5x 2 volts per cell instead of 6x 2 volts per cell.
03-31-2024, 05:44 PM
)
reinstalled with fuse mod
