When it's not starting it's the perfect time to do some testing. Learn how to do the voltage tests or better yet understand how to test across a circuit for voltage drop under load. If you draw yourself a bit of a diagram and understand how the start circuit works (check wiring diagram) you'll have it figured out in no time.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)