the test to isolate those two pieces is called a full field test, on an old Bosch it requires the DF and D/B to be shorted.

It is quite easy on japanese and american alternators, a bit involved on german. I would suggest you stick these terms in google and search for it, that way you can learn the test and its results.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)