281C and 281D Alternator Charging Problem

[Jacob4691]

Good evening, everyone. I am having some trouble with my charging system that is very confusing to me. I just replaced my alternator on my 2005 BMW 330ci, as the original had over 150k and failed (no voltage output and started to screech and overheat). I replaced it with a refurbished Bosch unit, exactly like the original, and it seemed to work fine the first day, but now it's acting up... I can start it just fine, and it will maintain 14.1v, normal, but as soon as I start to drive it will fluctuate between 12-16v before leveling off at 12v. I rechecked the connections behind the alternator and it is secured, battery terminals seem to be secured. I had no problems before the original alternator failed. Any ideas? Thanks in advance!

[NewB2Beemer]

Sounds like a bad voltage regulator on the new alternator. Can you swap the regulators between the old and new?

[E39 Newbie]

Couple questions. When the alt died, did it drain the battery before you replaced the alt? And, before you replaced the alt did you fully recharge the battery? Running a new alt with a dead battery will often cause problems.

So, have your battery tested, just in case.

Where did you get your rebuilt alternator? Who rebuilt it? The only rebuilt unit I'd trust would be one rebuilt by Bosch, and even then I'd opt for new if available. Anything from Autozone, or NAPA, or whoever, is immediately suspect. For whatever reason, rebuilt parts quality has really gone into the crapper over the last few years. Best guess is that your new alt has a bad regulator.

Worst case is your bad battery caused the new alt to fry and you'll need to replace both.

[Jacob4691]

The battery only drained due to starting the car a few times on the dead alternator. I took the battery out and charged it up, it's only a year old. The alternator was bought from OEMBimmer parts with a Bosch remanufactured unit from Bosch themselves. But what's weird is that both times when the oridginal alternator was dead and the new one isn't charging, the red battery lamp on the dash hasn't gone on. I was thinking of the regulator on the back of the alternator, but at the same time, it does regulate on startup, even if I Rev it in neutral, it still regulates. It's only in drive does it start to go wonky 😂