Otis, I had the same charging problem with my '77 280 including the day I picked it up 40 years ago. Went through 3 oem voltage regulators over the years until I replaced it with a solid state unit from Advance Auto Parts. No problems since then. Easy replacement, relatively cheap. Good luck!

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Here's the part

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Available at Rock Auto on closeout for $7.26

https://www.rockauto.com/en/catalog/nissan,1977,280z,2.8l+l6,1209248,electrical,voltage+regulator,4884


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