Check the back of the B+ jumper point. That is the primary disconnect for positive.

Twice a year that ends up being the cause of such on xoutpost. It's the easiest thing to fix (just be very careful not to short tools you will turn a wrench into an arc welder quick).

Second most likely especially since engine replaced is the main engine ground. That ends up behind the problem at least once a year on the forum.

Meausure the voltage from the vs. Block to the B- jumper on the fender while running some heavy electrical loads like the seat heaters and the rear defrost then have somebody rev the engine.

The voltage should only be millivolts not like half a volt.

Also: measure the voltage directly from engine block to B+ and B- to B+

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