Battery light is on, but alternator is charging - Mercedes-Benz Forum
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#1 (permalink) Old 07-31-2009, 10:18 PM
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Battery light is on, but alternator is charging

Hi,
My battery light has started coming on lately while I've been driving, but my current tester shows that my alternator is charging. With the engine off, it reads 12.5V, with it at idle, it reads somewhere between 13 and 15V (it has kind of a vague scale).

Nonetheless, even with just headlights on, the battery light comes on-- and when I test the current with headlights, fog lights, and stereo on, it still reads above 12.5V.

What the heck? I've replaced both my alternator and battery in the past few months. Thoughts? Thanks!

Last edited by Lordofporridge; 08-01-2009 at 12:25 PM.
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#2 (permalink) Old 08-02-2009, 10:37 PM
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*bump*

I did some more test drives, found that the battery light only comes on at speed, I can run everything at idle without a problem.

I tightened the alternator belt today, just in case. I'm pretty sure it isn't the problem-- its tight enough that I can't turn the alternator pulley manually at all. All wires coming from the alternator/battery look good.
Could it be the voltage regulator? As I said, the whole alternator unit is pretty new...

The only other thing I can think of is that I just put a new starter in, but I can't think of any way that could possibly be sucking power out.

Come on fellas, I'm clueless!
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#3 (permalink) Old 08-02-2009, 11:29 PM
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If the Alternator is still under a warranty take it back and explain the problem.
At the same time bring your fully charged Battery and have them do a free load test.

If you have one near by go to Harbor Freight and get an inexpensive digital Volt/ohm meter (multimeter) and re-test. When they are on sale then can be as low as $3 each. I have several and keep one in each vehicle and have 2 in my tool boxes.

Re-check your wires at that Starter; may be the big nut came loose and one of the Fat Red Alternator wires attaches there.

Also make sure your Batter to Body and Engine to Frame Ground Straps are making good contact. You Alternator Regulator controls the output via an internal ground inside of the Regulator.

If you are actually charging at 15 volts sometimes and it goes on long enough it can ruin a good battery.

Also you might do a temp swap a Battery from another Vehicle. Even newest Batteries can have internal defects.

Last edited by 300Dman; 08-02-2009 at 11:33 PM.
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#4 (permalink) Old 08-03-2009, 06:35 AM
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Thanks for the advice! I was exhausted when my first alternator crapped out on me a few months back, so I let a mechanic put the new one in. I overheard him saying, "lifetime warranty? like thats gonna matter on a 28 year-old car!" The alternator he put in physically broke up on the road the next day. If this one's failed, too...
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#5 (permalink) Old 08-09-2009, 07:26 PM
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It was my 2-month old alternator that broke, what a POS! It kind of died a kind of slow death as I drove from washington-michigan this weekend: started out not charging intermittently, then only charging at low rpms, now not charging at all and making awful grinding noises! I ended up driving 1000+ miles today without anything electrical but my gauges and brake lights because I was too rushed to replace it.

So this is the 2nd O'reilley alternator to fall apart on me in about as few months. I went for the 100$ off-brand ones because of the lifetime warranty-- I'm thinking now I might cash in the warranty and spring for a Bosch, just a 2-year guarantee but at least I won't have to fear constantly for my charging system...

Anyone have thoughts?
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#6 (permalink) Old 08-09-2009, 07:47 PM
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My Daddy once told me,"Never be afraid to buy quality, you'll never be disappointed in it's performance."
I would return it under warranty and spend the bucks on the upgrade and rest assured you spent the money wisely with the confidence gained.

1981 240D---234K - They said she was too far gone to be brought back ......... she lives!!! http://www.benzworld.org/forums/w123...completed.html
1983 300TD--103K
1995 E300D--162K
Yea these cars will last a million miles but what with terrorists, natural disasters and the H1N1 swine flu pandemic... God help us there's anyone left here to drive them.
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#7 (permalink) Old 08-09-2009, 07:53 PM
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Did a MB shop install the alt? I would have yelled at the guy if he said that about my car, LOL.

Oh, get a Bosch unit and be done with it.

'79 280CE with 226,000 miles
'76 240D 4 speed with 190,000
'85 300D Turbo with 266,000 miles


GONE but not forgotten
'76 300D with 195,300 miles
"83 300D with 174,000 miles

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#8 (permalink) Old 08-10-2009, 11:08 AM
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Nah, it wasn't an MB shop... just some towing place AAA pointed me at. In addition to insulting my car, they broke the lower alternator pivot bolt (I found the snapped pieces in my battery tray later), and replaced it with one that was too short, so that the nut didn't even go on more than halfway. Of course, it all started falling out on the road, which was exciting-- and that was just a few hours before that particular alternator started physically breaking apart.

I also overheard them going through pictures of accidents they had been contracted to clear and chuckling at where/how some of people who didn't survive ended up...

Great guys, let me tell you.

In other news, sent for a bosch alternator today. I dunno how off-brand manufacturers stay in business putting lifetime warranties on such crappy products.
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#9 (permalink) Old 11-01-2012, 04:59 PM
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*major bump*

mine comes on when i start the car and then turns off when i give it just a tiny bit of throttle while in neutral

never understood why



1984 Mercedes-Benz 300D (Euro)
5-Speed 'Getrag' Manual
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#10 (permalink) Old 11-01-2012, 06:12 PM
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I had a bearing go bad in My Alternator. When I removed it it had a rebuilt by Bosch North America sticker on it.
The bad bearing was made in China.

I replaced it with another Chinese Bearing because they were cheap on eBay like $2.50 with the shipping cost included; I bought 2 bearings. It has been like 3 years now with no Bearing Failure.
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