I searched and searched, cant seem to find out if there is a part # for a new voltage regulator, or if it is even possble. Please help, I have done several of these but I can not find out if this one is possible. Thanks fellow Bimmerphiles!:mad
What is the year of your 540i? I'm also not finding a part# for the voltage regulator for the 540i. I went to real oem where they had a diagram of the parts for the alternator but there was no description of the parts, part#'s, or price which is very unusual because it's been in my experience that the website is usually spot on.
You may have to purchase an entirely new alternator unfortunately (or rebuilt). I found this on Turner Motorsport with a $100 core deposit. It might be your best bet unless you want to go junk yard diving and try to get lucky.