911 has some good info for you in his post. Was the 12.5v with the motor running checked at a range of RPM's ? I think there are places you can go to & have your alt checked for free, that would be my next suggestion. You could test the blue wire at the alt with the ignition on. You should get a reading of anything more than about 3v ~ 12v with it disconnected from the alt If not, you have a problem in your wiring.

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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.
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