Would it work as you suggest Jim if installed the way I found it? It appears based on my ohm test to be faulty. I haven't run the engine with the alternator working yet for fear of damage to some other component without it.
I was told it was used to protect from lighting and transient voltage (surge / spike protection) which should have it connected to D- rather than D+. If how I found it installed is correct it appears as Lubener suggested as a voltage regulator between B+ output and D+ exciter.
That was my suspicion that it had something to do with voltage regulation but Hatz doesn't call for it. I spoke with Hatz service and they couldn't find any reference for it to be there. Hatz suggested having one of their techs look at the system which sounds like an awfully expensive shot in...
I have a Hatz 2L41C diesel engine on a Haulotte 3388 scissor lift that the alternator isn't charging due to an open diode in the exciter circuit. While diagnosing the problem I discovered that it has a zener diode between B+ and D+ at the alternator. Machine wasn't charging when I received it so...