Just had the same problem last friday on the 300SD. It was the alternator. Easiest way to check is to use a voltmeter. Check battery voltage with the car not running. A fully charged battery should be around 12.8VDC. Anything less means the battery is not fully charged. Anyway, with the car running the battery voltage should be around 13.1~13.8VDC meaning the alternator and voltage regulator are working fine. If the battery voltage remains the same as when the car was off than the alternator is bad. Not worth the time to try to tell if its the voltage regulator or brushes, just replace the alternator. About $75 at Schucks, unless you have a spare lying around and nothing else to do. Also check for tight and clean connection at the battery posts and all other electrical connections. Not rocket science, just basic electrical troubleshooting. SKI

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Ski
"Love them diesels"

63 190D (sold)
84 300SD
97 Dodge Cummins
97 HD FLSTC(if Harley made a diesel I'd have one)