Need help with Red Battery Warning Light (2004 E240)
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Newbie
Joined: Dec 2025
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2004 E240
Need help with Red Battery Warning Light (2004 E240)
Long story short, I'm having intermittent red battery warning light for months. I have:
What I already did are:
I already spent quite a lot for BCM, K57/2 relay, and SBC service. If possible I'd avoid to spend money for something that isn't going to solve my problem. I mean these AGM batteries and alternator are not cheap, haha.
Any help will be appreciated, thank you and have a good day.
- Replaced aux battery (12V 12Ah 200A CCA, AGM)
- Replaced K57/2 relay, tested it using xentry, works fine.
- Replaced BCM (N82), also tested it using xentry, works as it should.
What I already did are:
- Checked front and rear prefuse box, looks good and dry
- Checked wire under the aux battery, looks good and no visible broken wire
- Checked BCM connection, it is dry.
- Monitoring the charging using UB function on instrument cluster (all with AC, radio, and headlights are on):
- At idle it is 13.8 - 14.1v.
- Driving 14.1-14.2v
- Occasionally drops to 13.6v for under a sec when braking, SBC motor pumping, or using front power window (both sides)
- Occasionally drops to 13.0v for under a sec, usually when braking (idk why)
- Has shorted wire on both mirrors. Forgot what the code was.
- Swap the main battery with AGM one (currently it is not AGM, but MF battery)
- Inspect the wing mirror, someone said he had a problem with water enters the car from holes on the side mirror (exposed when it is folded), causing CAN communication issues on his car
- Bad wiring between N82 (BCM) and K57/2 relay
- Bad BCM or K57/2 relay (if one of them is bad, shouldn't the warning always appear immediately?)
- Bad alternator or voltage regulator
I already spent quite a lot for BCM, K57/2 relay, and SBC service. If possible I'd avoid to spend money for something that isn't going to solve my problem. I mean these AGM batteries and alternator are not cheap, haha.
Any help will be appreciated, thank you and have a good day.
Newbie
Joined: Dec 2021
Posts: 2
Likes: 2
W211 E500 2004
Long story short, I'm having intermittent red battery warning light for months. I have:
What I already did are:
I already spent quite a lot for BCM, K57/2 relay, and SBC service. If possible I'd avoid to spend money for something that isn't going to solve my problem. I mean these AGM batteries and alternator are not cheap, haha.
Any help will be appreciated, thank you and have a good day.
- Replaced aux battery (12V 12Ah 200A CCA, AGM)
- Replaced K57/2 relay, tested it using xentry, works fine.
- Replaced BCM (N82), also tested it using xentry, works as it should.
What I already did are:
- Checked front and rear prefuse box, looks good and dry
- Checked wire under the aux battery, looks good and no visible broken wire
- Checked BCM connection, it is dry.
- Monitoring the charging using UB function on instrument cluster (all with AC, radio, and headlights are on):
- At idle it is 13.8 - 14.1v.
- Driving 14.1-14.2v
- Occasionally drops to 13.6v for under a sec when braking, SBC motor pumping, or using front power window (both sides)
- Occasionally drops to 13.0v for under a sec, usually when braking (idk why)
- Has shorted wire on both mirrors. Forgot what the code was.
- Swap the main battery with AGM one (currently it is not AGM, but MF battery)
- Inspect the wing mirror, someone said he had a problem with water enters the car from holes on the side mirror (exposed when it is folded), causing CAN communication issues on his car
- Bad wiring between N82 (BCM) and K57/2 relay
- Bad BCM or K57/2 relay (if one of them is bad, shouldn't the warning always appear immediately?)
- Bad alternator or voltage regulator
I already spent quite a lot for BCM, K57/2 relay, and SBC service. If possible I'd avoid to spend money for something that isn't going to solve my problem. I mean these AGM batteries and alternator are not cheap, haha.
Any help will be appreciated, thank you and have a good day.
On my 2002 E500 I also had intermittant battery warning. My small iCarsoft diag. tool mentioned K57/2 relay or short in wiring. Allthough I allready bought a replacement relay it was only 1 old wire repair gone bad: 1 of those pre soldered then insert wire ends and heat joiners. But: at a near impossible loaction:
its wherè the thin cable to the relay (connector) separates from a thick loom; in my European LHD below the aux. bat. and washer tank. As there was an earlier repair it might be that this was a weak point in the cable. Whatever: all fine after soldering in some new wire.
Thread Starter
Newbie
Joined: Dec 2025
Posts: 3
Likes: 1
2004 E240
Maybe too late and not applicable for your problem but anyway:
On my 2002 E500 I also had intermittant battery warning. My small iCarsoft diag. tool mentioned K57/2 relay or short in wiring. Allthough I allready bought a replacement relay it was only 1 old wire repair gone bad: 1 of those pre soldered then insert wire ends and heat joiners. But: at a near impossible loaction:
its wherè the thin cable to the relay (connector) separates from a thick loom; in my European LHD below the aux. bat. and washer tank. As there was an earlier repair it might be that this was a weak point in the cable. Whatever: all fine after soldering in some new wire.
On my 2002 E500 I also had intermittant battery warning. My small iCarsoft diag. tool mentioned K57/2 relay or short in wiring. Allthough I allready bought a replacement relay it was only 1 old wire repair gone bad: 1 of those pre soldered then insert wire ends and heat joiners. But: at a near impossible loaction:
its wherè the thin cable to the relay (connector) separates from a thick loom; in my European LHD below the aux. bat. and washer tank. As there was an earlier repair it might be that this was a weak point in the cable. Whatever: all fine after soldering in some new wire.
But on my case it was the cable that connects K57/2 relay and front pre-fuse box that got loose. It was loose quite bad that almost completely disconnected. After sort that thing out, the car is back to normal! I am super happy that it is such a cheap-straightforward fix.
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