As I understand it, a diode is a one way switch that allows electrons to flow in one direction but not the other. I had a diode in a harness that tested 100% good, but I was still frying coils on a B+S Vanguard commercial. I bypassed the diode and the relay it was attached to and ended up with both a key switch for starting and what amounted to a kill switch for stopping. Seems to have solved the problem, but it took me the better part of 3 years to diagnose and figure out.