It sounds like you have a connectivity problem. When you did your voltage checks where did you connect the black lead of the volt meter? If it was on the case of the alt & you have low voltage @ the battery , you may have a bad connection some place between the alt & the battery + terminal. Go clean & tighten every joint. If you have 13 0r 14 volts when you do this & only 12 when you have the black probe on the - of the battery then you have an earth connection problem. You need to be doing the test with the lights on & the motor at fast idle.

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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