If shes settin there not running and you turn lights on and the ammeter swings to - discharge, the ammeter must be wired at the correct polarity, and even if the genny shop had the genny at Neg ground once you installed it on the tractor at Pos ground but then polarized prior to start up, it ought to be okay.
In case the ammeter isnt working to see if its charging or not put a voltmeter on the batetry and when setting not running Id expect like 6.3 volts but if running at fast RPM if the genny is okay Id expect the battery voltage to rise to 6.5 to 7+ volts DOES IT?????? If so shes charging.
If not, when running momentarily dead ground the gennys FLD post n see if she charges then?????? If so BUT NOT OTHERWISE::::;
LHBD switch dont have a good ground,,,,,the resistor on the LHBD is bad/open,,,,,,,the wires open from FLD on genny up to LHBD......
Try this for a complete procedure