It's either seriously out of adjustment or the microswitch in the motor has failed and isn't stopping the motor at the correct position.

Does it open all the way or just a crack? If just a small amount, it can probably be adjusted, but if it doesn't stop until full open, you probably need a new motor.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!