I have a Wisconsin THD 18hp gas engine. It has serial number 2942748. In the manual it says, "Beginning with engine serial number 3988441, the standard wiring circuits of all 12 volt electrical equipment for Models TH, THD is negative ground polarity instead of the previously furnished positive ground."
My engine is equipped with a generator (not alternator). It starts right up and runs reasonably well (I'm about to do a tuneup) but the ammeter shows no charging going on, and in fact I have to charge the 12V battery to run the thing. The original mechanical voltage regulator has been replaced by a solid-state kit which can be wired for 6V or 12V, negative or positive ground. It is currently wired up for negative ground, and the negative battery cable indeed goes to ground.
I am wondering a couple of things. First, according to the manual, my engine should have a positive ground, not a negative ground. If a positive-ground engine were connected as a negative ground, would the starter motor turn it over backwards? If the field magnet were an electromagnet then it seems possible that the polarity on the starter motor is irrelevant. I'm working on the assumption that someone replaced the battery cables and rewired the thing for negative ground by mistake, and that might be what's wrong with my charging system.
Also, assuming the manual is wrong about the ground polarity, is it possible to bypass the (2 terminal) voltage regulator by simply jumping its terminals, to see if the generator puts out current? If I'm thinking it through correctly, this should apply full battery power to the generator's field windings, so it should put out max power. I have learned the hard way to not just jump in blindly and start shorting things, though - sometimes that leads to the magic smoke escaping, and it's sooo hard to put back in. :-)