If the alt is charging 13.5 could be expected. I would be concerned about connections first. When the engine is on a fast idle, is the voltage any different?
You need to make sure you have voltage at the alt end of the small wire with the ignition on. If you dont, the alt will not start charging.
when you put the new reg in did you make sure you sort of wiggled it in and didnt break the brushes?

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving