Replace the voltage regulator !!! Mine did the same thing about 8 weeks ago. It had enough output to keep the alternator off but it couldn't keep the battery charged. Now the Voltage measured at the cigarette light is around 14.0 Volts instead of 12.2 . Cheap and esy to Fix

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1978 300 D 258,000 + miles sold 1995
1983 300 D Turbo 152,000 miles
Thank God for the man who put the white lines on the highway