You might also run a good frame ground to BOTH the VR andddddd the Genny just to be safe.
Then with her running dead ground the Gennys FLD post n see if she charges then?????? If so but NOT otherwise the VR is bad or not well grounded or the wires open from FLD on VR to FLD on genny......
If still no charge even when running with gennys FLD post dead grounded,,,,,,,run a jmper wire to jump BAT and GEN/ARM on the VR together (that by passes the internal cutout relay in case its bad) n see if she charges then?? If still not, the VR is out of the equation so the genny must be badddddddddddd
How you know if its charging or not??????? If you put a volt meter on the battery it should read around 6.3 when shes setting but when running at fast RPM a good genny should raise battery voltage from 6.3 to at least 6.5 to 7 or so volts ASSUMING THE BATTERY IS GOOD ENOUGH TO ACCEPT A CHARGE ???? Sure the battery is okay???
The genny cant charge the battery unless it has a path to it, thats from GEN/ARM to BAT on VR via the closed cutout relay........to and through the ammeter,,,,,,,,to the ungrounded battery post often where the big battery cable attaches to starter switch/solenoid.... But if you show battery voltage getting to BAT on the VR that must be okay
The motor test, it ought to spin fairly fast not just drag or barely turn which means the brushes and armature are at least decent. If you run the VR by checks above and regardless of a grounded genny FLD and VR by passed she dont charge and she spins slowwwwwwww on the motor test I suspect the genny. Typical problems are brushes worn down too short,,,,,,,,,,,,Commutator is greasy or carboned up (clean it),,,,,,,,,brush hold down springys are sticking (clean and exercise them) ,,,,,field windings are shorted or open,,,,,,,,,bushings allow armature to drag thats realy really badddddddddd