Check/refurbish grounding connections for Batt.-Chassis and Chassis-Engine. Also, make sure there is no corrosion at the alternator terminal strip on the passenger fenderwell. See if this makes a change.

If not, then the voltage regulator or alternator diodes may be fried. To check the alternator diodes, put your meter to Volts AC, with the engine running, and it should be less than 1 VAC.

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Sam

84 300SD 350K+ miles ( Blue Belle )