If your alternator has the little brush/regulator unit on the rear of it ....might want to swap that device out to see if it makes any change in charging operation........Quick & easy.... I think you can change it withthout removing the alt....I have a couple of those if you need me to send you one to try....or if you need one fast... junkyards or any parts store can get pretty quick....I assume you have tried another ground cable between frame, engine block & and negative post....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)