Charging system issue?

2005 MERCEDES BENZ SL500

87,000 MILES • V8 • 2WD • AUTOMATIC

BHAMDOC1973

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Since i had the car, the battery light has been on. Two batteries were replaced, one in trunk and a new one from Benz under the hood. When i check the voltage in the trunk it shows 14 some under the hood 11 some, is that possible? If i leave the car running, no issues, then when i turn it off and come back 2 days later it starts. Then after that if i leave it linger it is dead. If i jump it i will start but only if i jump it from the rear battery.

Nov 24, 2024 at 10:06 AM

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I would start by checking the fuses, here are the wiring diagrams for the charging system, for some reason they are not good quality so you will need to print them out to piece them together to read them.

This guide can help:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the images (below). Please let us know what happens.

Images (Click to enlarge)

Nov 24, 2024 at 12:51 PM

BHAMDOC1973

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This car has thousands of fuses. Is there no way to tell where the ones related to the charging system are located?

Nov 24, 2024 at 12:57 PM

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They just give us the wiring diagrams, which is the only way to tell which fuses are involved.

Nov 25, 2024 at 8:51 AM

BHAMDOC1973

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Also, i thought the front was the starting battery, but it has 11.5, but it's the rear i have to jump off to start the car cause its dead when i try to start the car. I thought the rear was for the electronics.

Nov 26, 2024 at 8:45 AM

BHAMDOC1973

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I checked the power cable from behind the alternator and it has power. Checked the 2-pin connector and has no power or ground whether engine off or on. Seems like someone has already tried to check it as the insulation on the wires on top are partially removed. What are these connectors supposed to have? Ground and power? And anyways u can find out which fuse is responsible for it?

Nov 26, 2024 at 10:04 AM

BHAMDOC1973

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Images (Click to enlarge)

Nov 26, 2024 at 10:10 AM

BHAMDOC1973

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Checked 200 amo fuse on passenger side and it's good.

Nov 26, 2024 at 10:41 AM

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My information for this car is limited for some reason. Mercedes does this sometimes on their high-end cars. Here are the fuse and relay locations and what they do. Check out the images (below). Please let us know what happens.

Images (Click to enlarge)

Nov 26, 2024 at 11:38 AM

BHAMDOC1973

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Turned out there was a 100-amp fuse underneath the 200-amp fuse. 200 charges the rear, 100 charges the front, removed it but can't tell if its blown or not. But with my tester light there was power to one side but not the other.

Images (Click to enlarge)

Nov 26, 2024 at 11:58 AM

BHAMDOC1973

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I tested continuity with voltmeter and didn't beep. Hopefully its done. Hard to see if its blown. Just out of curiosity why would something like this blow? Voltage too low? Battery weak?

Nov 26, 2024 at 1:00 PM

BHAMDOC1973

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Also, do you think something like this when blown could affect other electronics not working in the car?

Nov 26, 2024 at 1:02 PM

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Yes, the 100 amp fuse is blown which can be caused by a weak or shorted battery. Lets replace it and put the car back together to see what happens.

Nov 27, 2024 at 10:58 AM

BHAMDOC1973

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The fuse fixed the charging issue and battery light. Thanks

Nov 29, 2024 at 4:04 PM

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Glad you could get it fixed, thanks for letting us know. Please use 2CarPros anytime we are here to help.

Nov 30, 2024 at 10:54 AM