When opening the garage door, instead of stopping automatically, the opener keeps going until the thing-a-ma-jig that rides on the rail slams into the main unit. It then becomes lodged against the main unit and can take 15-20 minutes (or more) of repeated button pushing to dislodge. I don't have the instruction manual because the opener was purchased and installed by the previous owner! I also searched sears.com and found nothing. I have adjusted the up and down force on numerous occasions without luck. Could it be related to the tension of the chain??? Any suggestions appreciated... other than "buy a new one" unless that is absolutely necessary!