NOT sure what you mean by "charging diode", but I don"t think it has one, outside of the ones in the alternator.
What issue are you having?
If you go to the FIAT dba CNH parts site, I believe the wiring diagram will come up in the parts manual. I don"t have time right now to go look, but I will later.
Also, I will look as as I have SOME Versatile shop manuals from that era.