Dan,
Can you post a photo of the rear of your alternator? Is it a single wire, or a three-wire set-up? I had a charging issue on another "color" tractor recently. I had 13.5 volts at the battery terminal and the feed terminal on the alternator, but I was only getting 6 volts on the "activation" terminal. That was not enough to tell trigger the charging mode. I got a kit for the "resistor wire" to the activation terminal, and that fixed my problem.
I aimed to get out to the barn and see what was on my 1130, but didn't make it out there during today's blustery conditions. If you might be able to post a photo of the wiring set-up on back of your alternator, or even tell me how may wire system it is, that might enable me to give you some other ideas.
Cheers,
JB