That nut will isolate the power feed between battery and starter/alternator.

Their is a small connection to the alternator and the starter but they aren't live with the key off (in fact testing for any voltage on those small wires to starter and alternator would be a good test for parasitic drain).

On the m54, the power cable goes from b+ to starter then to alternator.

When changing the starter, it's at least 4x easier to fit the starter if you disconnect the alternator power lead to get some slack! Store that one away for future use!

I don't disconnect the battery ground when changing alternator or starter I just make sure no key in the ignition. Just remove the B+ jumper nut.

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