I agree with that Mark.

I remember fumbling my way through starter motor diagnosis's & often getting it wrong, then I read a chapter in a Haynes manual on diesel engines. Titled "Starter motor diagnosis", it explained quite simply how to proceed through a logical process & pin point the fault.

Once I had understood it, I became aware of how I was previously working with my eyes closed. I would say that volt drop testing & the use of a scope, have been the two most valuable things that I learned along the way.