If you break down the components of a typical alternator...

- Bearings x2: these last some 200K-250K.

- Windings > 200K. Rarely ever goes out.

- Inside the so-called VR is actually 2 parts:

* Brushes: look in bimmerfest E39, but after 180K, they are worn down.
At 200K, they are basically gone. The copper rings are also worn at 200K.

* The actual VR itself: being an electronic component, this lasts anywhere between 30K-500K. Completely unpredictable. This is exactly where the problem is...

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT