SL/R230: Parked again BCM?
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Joined: Feb 2023
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05 SL500
Parked again BCM?
Hello everyone, I finally got my 05 running. I does drive really nice. I have come across another issue. On a test drive rear battery got low enough for the circuit to parallel. I made it home, barely. Charged the batteries back up, started it up again to monitor the charging. It seems to be stuck in parallel. Neither battery is charging. Turned it off to save what charge I had. Then I tested the voltage at bcm.
The left terminal (rear battery) read the same as the battery, the right terminal reads 24v. I figure it's a bad bcm. Youcanic is not picking up any codes from bcm. Fuses and relays all good. Any thoughts? I appreciate any input in advance.
The left terminal (rear battery) read the same as the battery, the right terminal reads 24v. I figure it's a bad bcm. Youcanic is not picking up any codes from bcm. Fuses and relays all good. Any thoughts? I appreciate any input in advance.
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From: Gretna, NE
'03 SL55 Black, '03 SL500's Aegean, Mars Red, Tazanite, Alabaster, '11 S550 Flint Gray
Alternator should be charging the rear battery, so start there. Right terminal on BCM is front battery. If you have 24v there, you don't have a connection to the front battery. Check the 100A firewall fuse.
Thread Starter
Junior Member
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05 SL500
I took both fuses out and checked them, both were good. I tried something a little off. I thought maybe the relay was stuck, so I started it without the relay, then the charge went to the front battery at 14.7
isn't the bcm responsible for energizing the relay to switch front to rear? How do you suggest I check the alternator? It seems to be working.
thank you for the quick response
isn't the bcm responsible for energizing the relay to switch front to rear? How do you suggest I check the alternator? It seems to be working.
thank you for the quick response
MBWorld Fanatic!
Joined: Aug 2020
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From: Gretna, NE
'03 SL55 Black, '03 SL500's Aegean, Mars Red, Tazanite, Alabaster, '11 S550 Flint Gray
The alternator is connected (indirectly) to the left post of the BCM. With the engine running, you should have > battery voltage at that terminal. Seeing 14.7V at the front battery does not mean that the alternator is charging. The front battery is charged by a DC-DC converter in the BCM. The BCM will energize K57 if the rear battery is below a certain voltage, which just jumpers the two batteries, same result as bridging the two large BCM posts.
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05 SL500
Ok, that makes sense. The only way I know to check the alternator is by the battery voltage. So as soon as I get a chance I will go over all the connections and test it again. What you explained will help in diagnosis. Thanks again, and I'll let you know how it goes.
Thread Starter
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05 SL500
Hello again, I am trouble shooting at the moment, and I have noticed when I disconnect rear battery, (to charge). The bcm post on the right goes to 24v. When the rear battery is connected it shows Normal voltage. Do you know if that is normal, or do I have an issue?
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Thread Starter
Junior Member
Joined: Feb 2023
Posts: 36
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05 SL500
Cdk4219, thank you for your help.
I got everything cleaned up, and tight.
Got it started, and monitored the charging for a while. Seems to working good now. I checked this morning, both voltages were good.
I got everything cleaned up, and tight.
Got it started, and monitored the charging for a while. Seems to working good now. I checked this morning, both voltages were good.
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95 SL600
It’s time for 2 new batteries. These cars do not like old batteries, and things can get expensive if old batteries, or weak batteries are used. Mercedes cars don’t do well with weak batteries, and it is especially true with this car.
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From: Rochester, NY
2007 SL550 AMG Sports Package
Replace your Voltage regulator , just had the same issue on my 2007 SL550, Had to replace the 100amp fuse in the passenger footwell. Then I noticed my radio wouldn't start and my consumer functions would take a while to come back online. Then a generator code popped up, which confirmed the issue, replaced it and runs like new !
