when it happened to me the alternator was bad and the regulator was fine.

I replaced it with a 143a unit and all is well.

Not sure ont he diesel if you can go to that extreme, but I would suspect the charging system.

First telltale sign is the voltage. at idle it should read around 13 volts to 13.5 volts. under load it should read about the same, maybe a little higher.

That is typically how most cars I have seen are at with voltage.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon