[georgerazvan]

Originally Posted by Oksamit78

The main cable from front (+) terminal is this one (item 1):
https://www.realoem.com/bmw/enUS/sho...diagId=12_2042
But something else could be at fault...

Since you get +14V on alternator direct and depending how old is your F34, as precaution you should check the ground cable between engine/transmission and the body of the car. It could be old and corroded and thus causing abnormal electrical resistance that could cause the +12V at the front and rear terminals.
Example based on F34 340iX (Items 2 and 3):
https://www.realoem.com/bmw/enUS/sho...42#12427614083

Then, next thing to check (because you can start the car by boosting through front terminals):
The positive cable that goes to the back (Item 2):
https://www.realoem.com/bmw/enUS/sho...diagId=61_2891

Thank you. My F34 is from 2015.

I checked the ground strap but it looks good, but I will ask when I will contact the service in the next days.

One new thing I found is that the terminal wire is too hot, it almost fried my skin