He just doesn't understand and with how much details are thrown back and forth it's easy to miss. How it "soft failed" (where it worked some times) is very strange.

My test for BST is: use a jumper cable from the car battery to the B+ jump point under the hood. If the car starts it's almost certain the BST has opened.

There is an automatic disconnect that opens (think fuse) in the main power cable at the battery + terminal.

BMW makes a repair kit search for that but it's pricy.

The cheaper solution is to get one from a wreck (but confirm with Ω reading it didn't blow during the wreck).

You would have to cut out the section of bat+ cable from the battery to under the back seat. Find the directions online for repair BST. It shows exactly where to cut the cable.

With a heavy duty butt splice and a working section of cable it's not a difficult repair.

You cannot drive the car once you start with a jump because it also disconnects the alternator no bueno. (if I was stranded somewhere I would just leave the jumper cable from the trunk to the hood to make the car work until I got somewhere safe).

I would be very interested in seeing that old BST to do a proper post mortem. Was the car ever in an accident that you know of? If the BST opens on purpose there is a set of barbs designed to keep it from closing it's a one shot device.

I want to figure out the exact mechanics of how the thing works and if it somehow opened without the explosive charge it's very likely I could restore proper function and could in-effect make a BST repair kit somebody could use

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