Are you thinking of a flyback diode -
Flyback diode - Wikipedia, the free encyclopedia From what I see the diode is there to protect the contacts in the relay from arcing, not from pushing power into the system vs being driven. I'm a gearhead vs a spark chaser, and that generator thing just doesn't make sense to me. Then again, on big power generators (electric utility size) one of the most important pieces in the generation controls is the backfeed prevention to keep from motoring the generator with grid power if the turbine trips..... I think you've got some other gremlin to chase, and good luck with it!! Keep us in the loop as you try/find things. Interesting, interesting......