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2003 SL55 AMG
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Discussion Starter · #1 ·
Car is 2003 SL55 with about 140k miles.

Red Battery Warning message comes on the dashboard as soon as I start the engine.
Resetting the fault code (B1828) via STAR software doesn't help. It comes right back.

Within the last 2 weeks, I have replaced:

  1. Both batteries are brand new
  2. 100Amp fuse in the foot well is brand new
  3. K57 Relay - brand-new OEM relay installed

Both batteries are charging well when car is running (13.8-14.5V)

Alternator voltage regulator was replaced 3 months ago, using BOSCH regulator. Alternator is only 2 yrs old.

After all this, the red battery warning message still comes on, every single time I start the car !!


Here is the interesting finding I am trying to find an answer for - WHY is the K57 relay next to the BCM always warm, even when the car is off? It looks like the relay is constantly energized.

Per the documentation, this happens only when the charging system is in 'emergency' or 'limp' mode. If one battery is bad or not charged, then the relay energizes and stays closed (and will be warm).
On my car, both batteries are new and both are charging well ....so why is this relay constantly energized and warm?

Is this a sure sign of bad BCM module, since this module controls the K57 relay? Anything else I may be missing?

The BCM module is a $600+ part. I don't want to replace it for no reason if the issue could be hiding somewhere else, but honestly, I am really running out of ideas. I've replaced all the other possible weak areas.

Thanks in advance!!
 

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CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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It could be a bad BCM, but it could also be that something is draining the battery, such as if the PSE pump run continuously due to pneumatic leaks in the trunk latch.
 

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2003 SL55 AMG
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Discussion Starter · #3 ·
It could be a bad BCM, but it could also be that something is draining the battery, such as if the PSE pump run continuously due to pneumatic leaks in the trunk latch.
The issue is that the K57 relays is ALWAYS energized, even when both batteries are fully charged (they are both brand new) and showing 12.8V.
The relay acts as if the charging system is in "emergency" mode, per the official documentation.
The BCM is the only thing that can be triggering "emergency" mode and if there is no reason for it (bad battery, no charge, low voltage) then the BCM must be acting up.

I spent $680 on a new BCM. :( Waiting for it to arrive. Hopefully this will fix this very stubborn issue. If it doesn't, i am fully out of other ideas.
 

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2003 SL55 AMG
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Discussion Starter · #4 ·
Here is a new twist .... i just removed the K57 relay and now the warning light is completely gone! I started the car multiple times - no error! No warning! Miracle!
Both batteries are charging just fine when the car is running.

How is this possible? Why removing the K57 relay not only doesn't generate an error, but it actually cleared the red warning light? This makes no sense.
And before you say "you have a bad relay", i have tried with 3 different relays - 2 of them were brand NEW, OEM i just bought from dealer. Any of the relays were getting warm when plugged in, even with the car OFF.

Any ideas?
 

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2007 SL550 (R230) and 2012 SClass Designo (W221)
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Here are 2 ideas
1. Stop spending on parts and labor without deep dive diagnostic work
2. Use the proper tools for diagnostics , DAS Xentry. In your case it appears to me you have a parasitic battery drain. I would use a The Power Probe 4 is a multi-use Circuit Tester with built-in diagnostic power. This tool simplifies the testing of fuse boxes, verifies correct operation of motors, lights, relays, sensors, wires, communications circuits and computer drivers. The large LCD color display makes viewing live test information and mode selection simplified in even direct sunlight or complete darkness.https://www.pp-tek.com/store/product/power-probe-4/
 

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2003 SL55 AMG
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Discussion Starter · #6 ·
Here are 2 ideas
1. Stop spending on parts and labor without deep dive diagnostic work
2. Use the proper tools for diagnostics , DAS Xentry. In your case it appears to me you have a parasitic battery drain. I would use a The Power Probe 4 is a multi-use Circuit Tester with built-in diagnostic power. This tool simplifies the testing of fuse boxes, verifies correct operation of motors, lights, relays, sensors, wires, communications circuits and computer drivers. The large LCD color display makes viewing live test information and mode selection simplified in even direct sunlight or complete darkness.https://www.pp-tek.com/store/product/power-probe-4/
I have DAS Xentry and the diagnostic steps there are pointing to "bad K57" which I replaced 3 times. :) Did not help. Yet, when I removed the relay today - the red light went away.

I just came back for an hour-long drive. Car drove great. Both batteries charging fine. However, I stopped by the store to buy something, came back, started the car, and bam! - red battery light came back on! 🤬🤯 This happened after driving the car for an hour. Batteries were fully charged when i tested them - both of them. One was showing 12.8V and the other 12.71V , just like i would expect after the car was driven they were well charged.
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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My car will throw a red battery warning light like that. I believe in my case, it's a problem with the voltage regulator on the alternator. F\or some reason, it takes just a little bit too long to start putting out full voltage. This often happens when the engine is hot and restarted after a very brief shutdown. I have a USB adapter in the cigarette lighter that shows voltage and I can watch it go from something low, like 11v up to 14v, but ti takes a minute and in that time, I get the warning.
 

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2009 SL550
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Rudeney...agree. My alternator change removed some funky codes...and I think it was the VR.
200 bucks and 2 hours. I had fun changing it and the problem is gone. (WIN/WIN)
 

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2009 SL550
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407 Posts
Here are 2 ideas
1. Stop spending on parts and labor without deep dive diagnostic work
2. Use the proper tools for diagnostics , DAS Xentry. In your case it appears to me you have a parasitic battery drain. I would use a The Power Probe 4 is a multi-use Circuit Tester with built-in diagnostic power. This tool simplifies the testing of fuse boxes, verifies correct operation of motors, lights, relays, sensors, wires, communications circuits and computer drivers. The large LCD color display makes viewing live test information and mode selection simplified in even direct sunlight or complete darkness.https://www.pp-tek.com/store/product/power-probe-4/
parasitic draw is NOT on the starting battery and the red icon is from the starting battery. (MB using a starting battery started with avoiding parasitic draw keeping the car from starting, so the extra battery for everything else, FYI)

It points to the BCM or the voltage regulator in the alternator.
Further discussion about this helps the community, so tell us more, like your opinion and feelings because we are listening.

Side note: Lewis Hamilton is bad-ass...anybody who watched the race in Brazil today knows what Im saying. WOW!
Additional Side note...looking at a private seller - 2011 S63 tomorrow, think I'm in, thoughts? (asking is 24900 at 76K miles)
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550 FORMER: C300, ML350, CLK550 Cabriolet, C240, ML320, 300TD
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parasitic draw is NOT on the starting battery and the red icon is from the starting battery. (MB using a starting battery started with avoiding parasitic draw keeping the car from starting, so the extra battery for everything else, FYI)
Just to clarify, the reason there are two batteries is because of SBC. Without electrical power, SBC-equipped cars have no brakes except for a nearly useless backup hydraulic master cylinder that's slightly more effective dragging your foot to stop the car. The front (starter) battery is used only to start the engine. The rear (consumer) battery powers everything else in the car. This prevents a discharged battery from long/difficult starts causing a loss of power for SBC. With the ignition on, if the voltage on the rear (consumer) battery drops too low (under 10.6v, IIRC) the BCM will engage the relay and put the front (starter) battery to supplement power.

This describes the two-battery system for pre-MY2009 cars:

 

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If k57 is warm and measuring battery volts across it that does not sound correct When car off.

as you have identified if it’s only the BMS supplying the feed, then it sounds like a fault within this module. ( shorted fet etc … )

as to why it clears the fault code, it could be ……. k57 is effectively joining the two batteries together ( when the BMS is probably not expecting this condition ) , so charging currents will not be as expected, hence removing k57 will allow the dc dc convertor in the bms to control it’s charging etc …

you say you’ve ordered a new bms ?
 

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2007 SL550 (R230) and 2012 SClass Designo (W221)
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parasitic draw is NOT on the starting battery and the red icon is from the starting battery. (MB using a starting battery started with avoiding parasitic draw keeping the car from starting, so the extra battery for everything else, FYI)

It points to the BCM or the voltage regulator in the alternator.
Further discussion about this helps the community, so tell us more, like your opinion and feelings because we are listening.

Side note: Lewis Hamilton is bad-ass...anybody who watched the race in Brazil today knows what Im saying. WOW!
Additional Side note...looking at a private seller - 2011 S63 tomorrow, think I'm in, thoughts? (asking is 24900 at 76K miles)
Seems like a decent price. Have you checked w221 forums?
 

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2007 SL550 (R230) and 2012 SClass Designo (W221)
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Not to boast about a tester but pp4 could help understand voltage, current, ground, resitance situation ahead and behind current BCM in various battery, ignition position, running situations, etc. Can send current too if you want to test devices.
If you were my neighbor I would loan and help!
Good luck and keep us posted.

I
 

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2003 SL55 AMG
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46 Posts
Discussion Starter · #14 ·
Update:

New BCM arrived today. I disconnected both batteries (rear first, front after) and I swapped the modules. Then i connected the batteries - front first, rear last.

The new BCM one is identical, except the HW date is 35/11 and the HW date on the original module is 44/00. Looks like the new module is about 11 years newer. :) Inside, the components are visually the same, but the chips have different part numbers. I can post some pictures for comparison.

The good:
  • No Red Battery warning on the dash, even after i started the car 5-6 times, in short sequence, after a 10-min drive, after sitting idling, etc. No more red light. No errors. No fault codes.
  • The K57 relay does not get warm anymore.
  • Rear battery is charging great - getting 14.2-14.3 Volts at idle.

The bad:
- Front battery is getting no charge or very little charge. It stays at around 12.7 volts when the car is at idle. I measured it 5 times, and it's always around 12.6-12.7V.....didn't go down, even after i restarted the engine 5 times. This is also a brand-new battery, so it's strong.


The 100amp fuse in the foot well was replaced 2 weeks ago, so why is the front battery not getting more volts now?
Or is this expected behavior since the BCM now works properly? Does the front battery start charging ONLY if it goes under a certain voltage? I need to review the documentation again. I am getting confused after weeks of troubleshooting. :oops:

So, overall, I see some progress after I spent $670 on a new BCM. I just need to figure out the front battery charging situation now. Likely I will check the 100amp fuse again tomorrow. If it's blown, the BIG question is, what caused it to fail again? This is a big fuse. It's not easy to melt a 100amp rated fuse. And I was extra careful to disconnect the batteries before i touched anything.
 

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The front battery only gets charged when needed. 12.6v is a fully charged car battery. it can actually output a higher voltage, but 12.6v is perfect.
 

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1998 MB E300TD, 1997 MB E36 AMG, 2001 MB E55 AMG. 2011 BMW 335d
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The car does not charge the front battery unless it falls below 12.5 or 12.4v.
 

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2003 SL55 AMG
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Discussion Starter · #17 ·
OK, thank you, gents. I am happy to read that the system is functioning as it should, finally!!!

So it sounds like, before, with the old module, I was constantly getting 14V to the front battery due to some sort of BCM malfunction.

If i could only figure out what component of the old BCM actually failed. Visually, everything inside it looks clean and brand new.
 

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I don't see any blown capacitors, so it may be a weak solder than finally gave up.
 

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2005 R230 SL350 (M112)
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Here's a wild guess: I just had an amplifier repaired (home hifi). Denon used to secure the electrolytic capacitors against some kind of echoing effect, by glueing them onto their pcb boards. The glue in time may decompose and corrode underlaying copper strands (if that's what you call them). So those amps irritatingly need to be checked under their caps when they fail and nothing else seems wrong. Perhaps...
 
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