Is that the fuel pump side? Should be four wires two big two small.

If your tank reads and the pump works I have a feeling it's an extra wire to what I have only guesses like PO hooked up a test lead for something.

Haven't had e53 pump apart in quite a while so it's possible I'm mis remembering.

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