Battery light on, need help.

This is a rebuilt alternator from copperstate alternators (https://phoenix-alternator.com/MPartDetails.aspx?IDNum=51034) and it's only 6 months old. (2007 MS3 $140.11 alternator)

 

That's my guess and hope, belt was just replaced too, got a new one but tried with all our might to get it on and it was just too tight, so I am not sure if there's a trick for these gen 1 speeds but had to leave the old belt on there.

Well if the alternator solves the problem, I will probably get a new battery shortly after as well, it was good to test it with the truck battery though, which did the same thing as the night prior (this is with the speed battery 2 nights ago: charge from cold until operating temp, then stop charging). Hoping to pick up an alternator today, any suggestions on what to get?

 

According to the alternator manufacturer, the bench tests do test the regulators when being bench tested, but should I buy a new alternator and just hope for the best??

 

Your alternator has a life time warranty. Did you test the unit where you bought it?

Edit: if the bench test isnt able to recreate the environment while failing, you will need to run some tests while on the car, running and failing to get a guaranteed answer.

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Mine has a year, warranty and yes I tested it at oreilly and the place I bought it. Both said it passed. The alternator shop where I bought it from tested it and said it was outputting 15v, but couldn't test the regulation part (bc its ecu regulated). So I am not sure. We tested yesterday on the car, alternator works until motor is hot. Tested voltage at alt and battery, both were 13.8 when alt was working, after car heats up and it stops both were down under 12v.

 

I believe if the ecu monitors charging you will have a 3 wire connector plugging into the alternator. If so, you cant test that unit with a multimeter. 2 wire connectors you can.
If you want a definitive right answer, I think you need to run tests on the harness, which now requires a wiring diagram. Once you have verified that is all good, I would return to the store with that info and ask for a warranty replacement.
Other than that, popping in a used known working alt would be easiest if you know someone who is willing to lend.

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I've been searching for a diagram, can't find much...

edit: found this

 

That manual shows the 2 wire alternator. If your car has a 3 wire, that is not for you.

You could spend some time chasing wiring for continuity for which you will need some periodicals for an 07 MS3 , or you can go purchase a new alternator. I would verify that you have an issue with the company you bought the current alternator from. If your issue is resolved with the new alternator, I would return the current alternator as warranty and get your money back. If the problem persists...you will be spherical kahunas deep into ecm issues.

 

I have yet to trust rebuilt alternators. I always go used OEM or NEW aftermarket, in that order.

Replace with a used one from the yard, if it still doesn't work, it will be a small loss. It looks like those alternators go for $75-100 from yards in the states. Check Car-part.com.

If the shop you got the rebuilt alternator from isn't able to properly test your alternator, how much do you trust their rebuild? What if it was rebuilt due to a failing regulator, and now it is acting up again?

 

Currently heading to go get an alternator. I can bite the restocking fee if it doesn't fix it.

 

Also dropped off battery to get fully charged and then tested, have new (well rebuilt) alt in hand.

Edit: tested on bench with the "autozone tool." battery is fully charged and test said it was fine.

 

New alt is in, fully charged battery was at 12.59V when car was off, and 13.9v running with load. Then drove around for about 20 min, 12.95V when parked back at home. Got it up to temp, everything seems to be good as of right now. Will keep posted after driving more today and tomorrow.

 

It is quite satisfying when you finally find the problem!

Your feedback and results are appreciated! Hopefully someone can use this information in the future.

 

Thank you! I always seemed to find dead end threads with no solutions, so I thought I would keep posting. I hope it's the final solution, but I'll stay posting!

 

So drove it around, so far so good, buttttt stupid mistake forgot to transfer the plastic shroud that pushes that air into the other shroud to keep alternator cool..... Soooo might have to pull it, again.

 

Nah. Not 100% needed; I have an aftermarket alt that's 270 amp and it's fine.

 

You're fine, nobody that is big turbo runs that shroud because it gets in the way. I have been running without it for over 2 years of DD life.

 

Which shroud are you talking about? I am talking about the one that bolts directly onto the back of the alternator.

 

That's the one. Used to direct air into area shroud covers/surround. Has a snorkel like protuberance, and half clamshell backside attached to alternator.

 

And people run without that? Just a "raw" alternator?