You might at testing the plug wrong in the first place. With one meter on ground (frame or engine's crankcase) test each wire the Red one with the diode lump is the DC circuit for charging the battery. The black lead is the AC side for lighting.
Before you start check the battery standing voltage. Now start the engine and run at full throttle. Testing DC lead should have a voltage above the battery's standing voltage and be climbing a few tenths of a volt as the system charges the battery. This voltage reading should be the same cross the battery terminals. IF you got good at the red lead and not at the battery then there a problem with your wiring.