Generally, what fails first on these alternators is the voltage regulator brushes. You can buy a new VR (or even just the brushes) for a fraction the cost of a new alternator and probably buy yourself a lot of miles. Andrew Wynn has a thread showing how to replace the slip ring if you want to really do it right, but it's probably more work than most people want to tackle. I just put new brushes in my Valeo when it failed. I don't expect to own the vehicle by the time it fails again (maybe in 50-75K miles?)

AM.

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E70 2010 3.0 M57 AT Titanium Silver with Black Leather
E53 2004 3.0 M54 AT Toledo Blue with Gray Leather
E83 2008 3.0 N52 AT Silver Gray Metallic with Black Vinyl
E46 1999 2.8 M52TU 5MT Black with Tan Leather 250K+ miles (Sold, but not forgotten)