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850T-3 Not Charging

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Antietam Classic Cycle:
New rotor:
https://www.euromotoelectrics.com/product-p/boalt-rotor642oe.htm
https://www.mgcycle.com/index.php?main_page=product_info&cPath=37_161&products_id=760
https://www.crustycycle.com/products/copy-of-moto-guzzi-bosch-alternator-brush-holder-17712452-t3-convert-le-mans-bmw

This is a worthwhile upgrade while you're in there:
https://www.euromotoelectrics.com/product-p/vr-extadj.htm
https://www.mgcycle.com/index.php?main_page=product_info&cPath=37_161&products_id=1110

n3303j:
A few things just don't add up here. First if you measure more than an a few ohms between the slip rings (brushes pulled clear) then the rotor is open circuit and failed. But you said the charge light stayed on while the engine was running and the charge light is used as a limiting resistor to apply initial current to the rotor. If the rotor is open circuit the charge light has no source of continuity and will not light. So no red light at cruise could mean all is well or your rotor or your brushes are toast. That is the reason that at 125K miles after being bitten in the butt by my second rotor I installed a voltmeter. Now I know if my system even hiccups.
But if it were my bike I'd clean and tighten every connection in the circuit. There's a few at the regulator, more at the diode board, a bunch more at the alternator, one at the key. And check all the ground paths. There is the main negative cable, a wire from frame to headlight shell and another one from diode board to frame. Connections can get a bit wonky after 50 years.  That's why I installed a Bender harness last winter.

John A:
It isn’t causing the problem but when you put it back together the stator should be clocked so that the brushes contact the rotor from the side rather than from the top so the brushes are less likely to bounce and cause a low charging rate on rough roads. They put out a service bulletin on it.

moto-uno:
  Concerning rewinding the rotor , getting the 2 outer flange pieces off is a bitch , they are pressed onto the splined main-shaft with an extreme interference fit .
Once those are off the entire winding pulls off . The wire wrap and shellac (and wire) were available from an electronics store here in Vancouver (Lee's Electronics)
I originally started with 22 gauge wire , ended up at 19 gauge which gave me ~3.4 ohms , the correct reading and it charged well with the lights off , not enough with lights on.
That's when I realized that although I'd wound it well , I hadn't built up the base of the main-shaft enough with the wrap to locate the coils closer to the flanges .
After doing this 3x , spring arrived and I put off the next one waiting for a few rainy days .
The supplied shellac was heat resistant and kept all the wire windings in place.
And to simplify the charging wiring , get rid of everything except for the 3 rectifier leads and the +ve lead from the regulator and add a voltmeter(as suggested earlier) and be done
with all that other crap that can add it's own special problems , has served me well for decades on my LeMans2 .

n3303j:
Looking hard at the pictures (nice detail) I'd take everything apart and clean it thoroughly. If the rotor truly measures infinity then replace it. The brushes seem to be sitting a bit low in the holders. Best time as any to solder in new ones. And I'd be really suspect that dampness has gotten into the regulator and put it out of service. MG Cycle stocks a solid state one for about $25. Don't bother with adjustable as the potentiometer is just a spot for water intrusion. The fixed one outputs 14.4 VDC which is good for any battery the T3 likes. The fixed one is fully potted and as a bonus takes less space in the same mount. Lasts forever (or 100K miles, whichever comes first).

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