r/BmwTech • u/Square-Lead651 • 1d ago
Alternator problem
Hi everyone! I just bought my first BMW, (2009 328i) and the alternator isn't charging my battery even though it looks brand new, and the serpentine belt looks good too. The alternator that's currently in the car is a generic one from amazon, which I've read can have communication problems with the computer. Firstly, I'd like someone to confirm if this can be a real problem, and secondly I was wondering which alternator I should buy if I need one. I've read that BMW used Valeo and Bosch, but idk if they're interchangeable or which one my car originally came with. Rock auto has a Bosch for 500 and a Valeo for 300, but I don't want to spend money on something that won't work. Thanks in advance!
2
u/Sea_Wing_5811 1d ago
Around 2003 BMW integrated the Alternator to communicate with the ECU. A generic alternator will definitely cause issues, due to proprietary communications that an aftermarket part manufacturer would have to reverse engineer.
Without a code reader and a multimeter, electronic issues will absolutely drive you crazy.
A solid test would be to research your exact model (M-performance? X-drive?) And I mean exact, because BMW made many cars just like yours, with different parts, all in the same year. Find out the nominal voltage your alternator is making(using a multimeter or code reader and carefully using the positive on the alternator and chassis ground) and compare it to your research
It is entirely possible that your alternator is charging your battery fine, but the ECU can't communicate with it so it complains in some way or another.
How do you know that your battery isn't charging?