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I have replaced the drive gear on this door opener and tested it 15 to 20 times afterwards. For the next 3 days I used the door opener several times and it worked perfect. Then one morning I opened the door and the motor continued to run. By the time I was able to pull the electrical plug, the new drive gear was ruined. I can replace the drive gear but I need to understand why this happened or this will happen again. Any ideas?

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I am not absolutely sure, but I think that model may use a mechanical limit switch that is mounted on the trolley rail. The upper and lower limit switches are mounted on brackets that are adjustable on the rail to fine tune the travel of the door. It is not uncommon for these brackets to get loose and move. If this switch moves too far, then your door will strike the floor or top end of travel and stop, but the switch will not activate stopping the motor. Result is motor continues to run, but chain will not move, so there goes your gear drives. Check the location of these limit switches, test the switches themselves to assure they are working. (Turn off power, unplug and test switch with an OHM meter.)

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