Low voltage for 1/2 a second will trip and/r4x4 but also brake light should have come on. If alternator is weak long enough you will also get engine failsafe and transmission failsafe.

Clear all the codes and bring up real time voltage on the hidden OBC menu (test 7 or 9 I think), if the voltage drops more than 1/2v when you release the gas pedal e.g. coasting before a stop the alternator needs work.

On both of my X5 that just meant replacing the brushes (at about 130,000mi for me and 180,000 for wife's) was all that was needed. My repair stopped working after about 40,000 miles but it turned out to just be carbon dust buildup so I kept my old alternator as a spare I can bring with on long trips.

Look for my alternator repair threads. I describe how to reverse the polarity on the armature winding and nearly double the lifespan of the slip rings (one gets 80% of the wear). New brushes and flip the polarity should get you 80-100,000 more miles.

My engine bay is suffering from an oil leak right now that apparently made the carbon dust from normal wear clump up and made the alternator malfunction, so I replaced it only to discover a shot of brake cleaner in the brushes would have brought it right back to life.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)