1/2v drop on the ground strap seems excessive other than at start but if the voltage ever drops below 13 that's a sure sign of VR failure.

Confirm by measuring the voltage from B+ terminal to the engine block. If you get similar drops in voltage than the alternator is the fault.

When mine died it was the slip ring/ brushes and I replaced them with a &20 kit. Other than bearings those are the only parts that really ever wear. The stator windings are locked in enamel and can easily last a million miles.

Measure the ground strap voltage at start and confirm the 0.5v was just normal running. I found my problem before I got that far didn't need to test my ground strap so don't have a baseline. Within the past year somebody on xo had a loser ground strap causing his similar problems

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