James, that may well NOT be the original Generator that tractor came equipped with. My literature shows Delco Remy Generators that were used on Model A Tractors were:
1100955
1101356
1101371
1101385
Best I can tell (NO warranty) that Generator was used on some Case Crawler Tractors or IH Tractors.
HOWEVER as long as its for a Class A Charging System and the Voltage Regulator (also for an A system) matches it and it was properly polarized, there's no reason it cant charge a battery that just happens to be on a John Deere or any other brand of tractor, it don't know the color or brand of the tractor does it lol
NOTE the output of a Genny is across the ARM post and its case frame (ground) NOT Field and Armature as you measured!!!!!!!!
NOTE that tractor was originally Positive ground, how is it wired now???? It can still work and charge at Neg ground provided the genny was polarized (plus a few other minor wiring changes)
OKAY if it doesn't charge the genny may be at fault or the wiring or improper polarization messed up the VR or the VR is at fault.
The first thing I would try is to Polarize (see my link below) the genny and see if that helps but if it were incorrect the VR could have been damaged.
TROUBLESHOOTING
If you follow my Troubleshooting Procedure linked below it can help diagnose the cause of non charging for a Class A system, I have copied and pasted one of its basic short tests whereby you Full Field the genny (by grounding FLD post) and by pass any Voltage Regulator functions to see if non charging is the fault of the genny itself or something else.
NOTE the genny and VR must have a good frame ground to work, always try that and simple things first like good grounds and good connections.
Try my Troubleshooting Procedure, a part of which is pasted below:
TO DETERMINE IF ITS A GEN OR VR PROBLEM
5. With the tractor running, temporarily ground the Gens Field post to case. If she charges then, the VR may be bad, or a wires missing from VR"s Field post to the Field terminal on the VR, or the VR isnt well grounded.
6. If she still dont charge, leave Field grounded and jump a wire across from the VR"s BAT and ARM terminals (jump by passes the cutout relay) and see if she charges. If then but not otherwise, the VR"s cutout relay isnt working correct (maybe points burned/carboned/resistive)
7. With the 2 steps above, you have basically by passed the VR functions, so if she still dont charge, you're left with a bad battery or wiring or the Gen itself.
8. MOTOR TEST. You can Motor test the Gen. If its grounded and you remove the belt and apply hot battery voltage direct to its ARM Post, it should motor n run well (Armature n Brushes and Commutator okay). Then if you ground the Field and it slows down some, the Fileds probably good. If it passes both those tests, it should charge, and if not, it may be a wiring or battery or grounding problem.
Let us know
Best wishes, John T
John Ts Troubleshooting Procedure