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Old 04-30-2015, 09:49 AM   #1
RyanPR
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Bad Alternator?

A question about an e46 M3 battery light.

The battery light came on a week ago. I went home and checked the battery (12 months old, from BMW) and the voltage was fine. I then ran the diagnostic test I found here (

). And it was showing only about 12.5V running. So I pulled my alternator and had it tested. It tested OK in the local parts store. I checked continuity between the alternator positive wire and the positive terminal on the battery (that took a LONG wire) and that was fine. I threw the alternator back in the car to do some more testing and the battery light went away.

Then it came back yesterday. I then took this video (

) of me running the car with the diagnostic mode running. Sorry for the lighting, but I ran with lights off and on with and without giving it gas. As you can see, sometimes the voltage is 12.0V and sometimes as high as 13.9V.

I'm guessing the alternator is flaky. Any thoughts before I pull it (again) and buy a new one?

Thanks,
Ryan_a_

Last edited by RyanPR; 05-04-2015 at 08:34 AM.
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Old 05-04-2015, 08:34 AM   #2
RyanPR
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Anyone?
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Old 05-04-2015, 09:12 AM   #3
forzam
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I don't know much about electronics, but maybe your voltage regulator is bad.
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Old 05-04-2015, 09:24 AM   #4
vandyboy
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I when through this scenario three years ago. The alternator checked out fine at Advance Auto parts so I bought a new battery (mine was old and probably the original.) After three days, the battery got weak and I knew it was the alternator. I would retest the battery to be 100% certain it was tip top. Otherwise, you can replace the voltage regulator and brushes or just pony up for a remanufactured Bosch (who remans them with both the Valeo and the Bosch style connectors.) After 3 years with my remanufactured alternator, still no problems.
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Old 05-04-2015, 09:34 AM   #5
RyanPR
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Thanks for the responses, I was wondering about the regulator as well. If you buy a reman alternator does it include the regulator? I'm wondering because AZ sells the alternator for $127 and the regulator for $200. Seems like the alternator would be more if it included the regulator...

Thanks,
Ryan
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Old 05-04-2015, 09:56 AM   #6
forzam
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As far as the regulator goes, it depends on what kind of alternator you have. There is a bosch and valeo with different regulators and prices.

EDIT: And I'm fairly certain the alternator comes with the voltage regulator.

Last edited by forzam; 05-04-2015 at 09:57 AM.
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Old 05-04-2015, 10:01 AM   #7
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It's pretty common for the alternator to start failing intermittantly. The brushes and commutator get worn and dirty. Voltage regulators have different failure modes, but the most common is just going dead. Get a reman alternator, which includes a VR.
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Old 05-04-2015, 11:30 AM   #8
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If you want to save some money, just replace your voltage regulator. If you can get the whole alternator for a few more dollars, go for it.



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Old 05-05-2015, 09:51 AM   #9
markusmarkus
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First, determine which brand of alternator is in your car. An M3 will most likely have a Bosch 120amp alternator. The voltage regulator (VR) is easy to replace on a Bosch alternator.
- disconnect the ground/earth/negative cable from the battery
- remove the air cleaner box and the first large air hose
- remove the serpentine belt from the alternator
- remove the connector on the rear of the alternator
- remove the big 12VDC cable from the alternator
- unbolt and remove the alternator
- remove the rear plastic cover (caution: it's plastic that's had s rough life so it'll be brittle; recommend you buy a new one)
- remove the VR (and its brushes; they come out as a unit)
- install the new VR
- installation is removal in reverse order.
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Old 05-06-2015, 08:38 AM   #10
RyanPR
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Hi, ended up replacing the alternator, did it in a record 45 minutes. I drove to work today in diagnostic mode and stayed above 14V the entire time.

One thing worth noting. The one pulley was a little noisy when I pulled it so I ended up replacing both pulleys (part #s below). The difference is amazing as the car is much quieter than before, definitely worth doing for $75 while replacing your alternator.

E46 M3 Alternator Pulley Part Numbers:
11287841228
11281748131

Thanks,
Ryan
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Old 05-06-2015, 08:42 AM   #11
forzam
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Nice job!
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Old 05-06-2015, 09:57 AM   #12
BaliDawg
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Quote:
Originally Posted by RyanPR View Post
Hi, ended up replacing the alternator, did it in a record 45 minutes. I drove to work today in diagnostic mode and stayed above 14V the entire time.

One thing worth noting. The one pulley was a little noisy when I pulled it so I ended up replacing both pulleys (part #s below). The difference is amazing as the car is much quieter than before, definitely worth doing for $75 while replacing your alternator.

E46 M3 Alternator Pulley Part Numbers:
11287841228
11281748131

Thanks,
Ryan
Welcome to the DIY world. Now, how's your cooling system?
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Old 05-06-2015, 11:27 AM   #13
RyanPR
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Quote:
Originally Posted by BaliDawg View Post
Welcome to the DIY world. Now, how's your cooling system?
Cooling system is fine. Power steering hoses are next along with the rear springs. And before that, a good wash.
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Old 05-07-2015, 07:40 AM   #14
RyanPR
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FYI: The DIY on this forum says you need a T50 to get the belt off. That was wrong on my car. On mine (2003 BMW M3) it was an Allen 8MM.
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