I agree w/Larry. Your battery and alternator is new so that can be eliminated as your source of problems. I would go as far as to replace the wires, at the very least check the connectors.

That mechanic you used was an idiot. Any mechanic who works on Mercedes should know that the alternator regulator is a separate part located in the alternator. Don't go back to him. He didn't tell you anything you didn't already know. Your money would have been better spent going to the dealers to diagnose the exact problem...it would have costed a little more but you would have gotten your money's worth.

Good luck.

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JR
1986 Gold 300E 4Dr automatic