I suspect it might be a transient throught that also. A small wire might not take the "surge" and consquently the amperage would "spike"? (I know it woudn't e from "heat" increasing resistance.)

Anyway, running the ground back away from the engine is the right way to do it.

I'm still shocked (no pun intended) that one gets a back surge. I understand that the Alterntor's exciter gets over stimulated (I didn't think it was that fast).