By that model number it is fairly old. Maybe 15 YO. And it is pre - safety eye.
Sears is made by Chammberlain/Liftmaster and use the same parts.
This is the manual for models that have use the same replacement control board as your model.
But it does not have trouble shooting information.
This is a manual for current units. While there have been lots of changes it still uses the same basic techology with the RPM sensor, the gear train, and limit switch assembly.
So the trouble shooting section might be of some help.
But first disconnect the door and operate it manually. It should work smoothly and be balanced so that if you open it 1/2 way the door will stay in that position.
If the door spring(s) are broken you will get similar symptoms.
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