Garage Door Opener Model 139.18415sr

 I just changed the plastic worm gear on my 1/2 hp, 19-year-old Craftsman garage door opener, and now the motor will only spin for a couple of seconds in one direction, then a couple of seconds in the other direction. When the chain is hooked up to the sprocket, it moves the garage door perhaps a foot, but the motor operates in this way whether it's hooked up to the door or not. I've adjusted the downward and upward pressure in every variation, and there's no difference. Similarly, I've adjusted the distance sensors on the plastic screws every which way so that the contacts touch or don't even touch, but there's no difference. This is exactly as if the door is moving, finds an obstruction and returns to its original position, except that everything seems to be exactly as it should be. Can anyone offer a suggestion as to what I'm not doing right here?