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Old 12-02-2012, 11:45 PM   #1
MyNewRag
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Join Date: Jul 2010
Location: Vancouver USA
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My Ride: 05 ///M Convertible
Battery Light

I just had the alternator and belts replaced. Drove it for a couple days and tonight, I got on the gas a little extra a couple of times, lol, and the light came back at high rpms but went off at idle.
I checked via the OBC and at idle it is charging at 14.0, when I rev it over 4k the light comes on and the voltage drops down into the low 12's. go back to idle and the voltage climbs back to 14.0.
So it seems backwards to me. Maybe a loose belt? I don't know. Any thoughts would be appreciated....

I have searched, and most of the threads I find are about the opposite... Light on at low rpm and goes off when you give it some gas. That is how mine started, until it totally died. That is why I got the alternator replaced to begin with.

Again, any thoughts would be appreciated...thanks

05 ///M3 with 35k miles
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Old 12-02-2012, 11:59 PM   #2
2003 325i
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Belt slipping is my first guess. Take it to a competent shop if you can't diagnose on your own.
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Old 12-05-2012, 02:59 PM   #3
MyNewRag
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Took it back to the shop that did the original work, they diagnosed an "out of box quick failure" of the new alternator. It would be fine when the car was cold, but after driving about 10 miles, it would start to act up.

They warrantied the item and the install of course. Had me back on the road in a few hours and it has been fine for the past few days. Hopefully, that took care of it and I don't have a bigger issue causing me to blow through alternators.
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Old 12-05-2012, 03:08 PM   #4
jdstrickland
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Originally Posted by MyNewRag View Post
Took it back to the shop that did the original work, they diagnosed an "out of box quick failure" of the new alternator. It would be fine when the car was cold, but after driving about 10 miles, it would start to act up.

They warrantied the item and the install of course. Had me back on the road in a few hours and it has been fine for the past few days. Hopefully, that took care of it and I don't have a bigger issue causing me to blow through alternators.
There is nothing that will cause you to "blow through alternators." Alternators either do the job they are supposed to do, or they don't. There is nothing in the car that will causethe alternator to fail.

Having said that, you could install a bizillion-watt amplifier and continuously draw more power than the alternator is designed to furnish, but that would cause other nasty things to happen before the alternator went belly up.
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