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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Mon, Jun-25-2012, 10:23:43 PM   #1
hemmy
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Default Good alternator, new battery = low voltage output.. Stumped!

I searched and searched... So the alternator went out and the first two were DOA. Got a third one and tested it. Good. I put it in and tested the voltage. Battery voltage is 12.4V. When the car is on it's sitting at 12.17V at idle.

I'm suspecting a bad ground somewhere. Ground strap? We tested at the charging post as well and it put out a 12.17V too at idle. Even at 2K rpm it still was at 12.17V.

Anyone else have this problem?
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Old Mon, Jun-25-2012, 10:30:38 PM   #2
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

I don't know what you mean when you say you tested the 3rd alternator. If they bench tested it, maybe it passed but is failing under a real load; in fact, if your battery voltage was 12.4 before you started it, and is now 12.17 at idle, i'd expect this alternator is junk too.
You could have a defective new battery too, but if you can charge it fully, then start the car a couple times and - with alternator disconnected - it is still over 12 volts, it is probably fine.
How was this alternator tested?

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Old Mon, Jun-25-2012, 10:34:48 PM   #3
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

I think that was useless information. The first two alternators were from pep boys and were dead out of the box. I took the alternator to Napa to have them test it and tested fine.
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Old Mon, Jun-25-2012, 11:17:30 PM   #4
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

Interesting response to someone trying to help you out. You wouldn't be the first to have 3 bad alternators, and as I mentioned, they can pass a bench test and NOT work under load. I'll let you go back to checking your ground wires, like that will fix it. You'll be on my ignore list about 5 seconds after I post his. Good luck.

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Old Mon, Jun-25-2012, 11:18:34 PM   #5
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Have you tried testing the output directly from the alternator?

How certain are you that the alternator and battery are good? Were they both thoroughly load tested?

Know how to do a voltage drop test?

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Old Tue, Jun-26-2012, 01:01:53 AM   #6
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

Sorry Red, I meant to say I posted useless info about the two dead alternators, not your info was useless. Sorry for the confusion.

I took the alternator to another alternator shop and they are getting correct voltage out of it under load. I also took the battery back to have it retested (bought a couple weeks ago) and they said it was good.

I do not know how to do a voltage drop test, but I do know a couple people who might.
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Old Tue, Jun-26-2012, 01:03:38 AM   #7
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

renault makes some wonderful alternators.
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Old Tue, Jun-26-2012, 02:25:00 AM   #8
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

the new Alternator is it re-manufactured? save your self the time and headache and go buy a brand new OEM one and call it good.
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Old Tue, Jun-26-2012, 05:13:33 AM   #9
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Default Re: Good alternator, new battery = low voltage output.. Stumped!

Problem resolved... Good to have an electronics guy as a friend. The starter power wire was loose at the starter. Tightened it up and was pushing 13.6-13.8V. Thanks guys!
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Old Tue, Jun-26-2012, 08:27:59 AM   #10
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Originally Posted by hemmy View Post
Problem resolved... Good to have an electronics guy as a friend. The starter power wire was loose at the starter. Tightened it up and was pushing 13.6-13.8V. Thanks guys!
Bet you he found that out with a voltage drop test. :thumbup:

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Discussing Good alternator, new battery = low voltage output.. Stumped! in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)