When I first got my 300D, I was driving it to a friend's house about 20 minutes away. The radio started getting fuzzy and the headlights were incredibly dim, but the car didn't miss a beat. As soon as I shut it off, dead. Next day, jumped it and drove it on a dead battery to my mechanic who diagnosed the issue.

It's most likely the alternator, or the internal voltage regulator brushes are too worn.

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Never be ashamed to ask for help.

1985 300D
1987 300SDL
1970 Jaguar E-Type OTS
2002 Dodge Intrepid (Traded In)
2011 Ford Crown Victoria