Sounds like a stop in there is broken or not working properly. At least you can't burn your motor out. My motor has been replaced twice, once since I've owned it the PO burnt it out the last time. I trashed the gear box trying to do a manual close. I've read that the sunroof is too complex for the normal person to tackle, but if you know what you're doing take it apart and see. I'd just take it to mercedes and pay the 500 bucks.