Welcome to the SS forums you two!
Sounds like the 12V alt is functioning correctly, but the 24V one is not charging.
First question - are there two alternators? The
Roscommon 12V modification often includes pulling the 12-24V (high 12V battery's) alternator, leaving only the GND-12V (low 12V battery's) alternator. That's how my '86 M1009 came from the last owner.
The factory wiring diagram is in section 4 of the
xxxxxx-20 Technical Manual. It's all there, and for free. Beats a Chilton's any day
The Gen2 (12-24V) alternator is triggered from a relay powered by the ignition as well as the 24V voltmeter in the dash. If that circuit has been altered or removed during a 12V conversion, you'll have to repair that to make the 24V portion work correctly. You should also check the wiring to the 24V terminal block on the firewall (passenger side). If it's measuring out at 12V then it's been rewired too.
Pictures help when you can't describe something:
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