I believe two functions going on: I think this starter uses magnet to engage the drive pinion vs. just helix. If the ground not solid for the coil but is for the motor you'll get a spin no engagement.

I could be wrong on the starter engagement, and if it is just helical gear engagement, if the voltage is not high enough you could get your symptom.

Check your B+ jump terminal. If that gets loose it can cause you symptoms.

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