Question about Sears Craftsman LiftMaster Chamberlain Garage Door Opener Wireless Keypad Model 976LM Security+

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25 yr+ old Sears opener runs past lower limit

1/2 HP chain drive. Ran past lower limit and jammed the door shut. Had to dissasemble door bracket to open door. Motor hums every few minutes but will not run. Pressing a remote makes it hum louder for a second. I removed the cover and the chain. Gears are good. Motor was replaced 1-2 years ago. Feels warm, not hot. Lower limit contact was made but it over-traveled. I backed it away but motor did not run. Then it started by itself and ran past the lower limit again. Maybe it cooled enough to run? I'm not sure if the shaft rotation sensor is OK. Should it be lit? I usually have to replace the capacitor every few years but this lower limit thing seems like a different problem.

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Update: I remembered that I had a problem with the door reversing too much a while back. It was chattering in the tracks and I could not fix it with lubrication and changing spring tension. So I dialed up the down reversing force. This contributed to the jam-up I had when the lower limit was ignored. If I had a lighter force, it may have at least reversed.

After futzing with it I left it alone for several hours. The motor would hum every few minutes. I was considering cutting power to it overnight. I pressed the remote one more time and the motor ran normally to a limit and stopped. I tested it a few times then reconnected the chain and door. Seems fine now. I suspect an intermitten logic board problem. I'll try lightening up the down force and using it for a while.

Posted on Jan 07, 2009

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Posted on Jan 02, 2017

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