Replaced Alternator???

I replaced my alternator this week and when i started my car my battery light was on. I checked the voltage at the post on the back of the alternator and it was only 12 volts when i first started the car and the more i ran it the lower it got. I checked the pulley and it was turning fine, and i have a brand new battery. the only thing i can think of is that i got a bad alternator from Napa?!?! any other ideas? I'd rather not yard out the alternator again just to find out its not the problem.

thanks for the help

You should have 12.4 to 12.6 volts on your battery with engine off or your battery is not fully charged. You should have 13.6 to 14.0 volts once you start the car.

Check the voltage at your battery post, check all chassis grounds too. If they are good make sure you do not have parasitic drain/ a short somewhere pulling all the voltage to ground.

You have to start over on the diagnostics:
- DIY how to test a BMW battery & alternator

I feel your pain about removing the alternator again. I had to replace a new one I got from Advance because the bearing was bad. Take it out and have them test it, the weird thing about mine was the battery light never came on, even with the voltage low enough for the car to not crank.

Originally Posted by jmo69

the weird thing about mine was the battery light never came on, even with the voltage low enough for the car to not crank.

Same thing happened to me.

By the time the battery light came on, the car had already died in the middle of the road.

Full details (as always) here:
- One users' example of total electrical failure (AAA towed away) alternator repair (1)
[Read post #66 & post #107 & especially post #146 for M54 DIYs]

Don't care where you got the alternator from, but Bosch or Valeo replacement is a must. Anything else is going to fail faster.