[Wbulk]

Ok, I have been thinking about this. I don't think that is the problem. When you shut off the engine there is residual current that can flow back through the I terminal, but not when the engine is running. The diode is to stop that residual current that only last for a second or so. I looked at some old 60s regulator diagrams and they didn't add those diodes until later. You should still put that diode in. But it's purpose was just to stop run on after the ignition switch is turned off.

Wayne