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

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Stuck garage door - Sears Craftsman 1/2 HP GDO, Model 139.18876

I just installed a new gear & sprocket assembly (Part# 41C4220A) along with the worm gear and all associated parts (bearings, etc.) since the old drive gear was totally chewed up. I didn’t know until afterward that they sell just the part that I needed; instead I bought the whole assembly and assumed I had to change it all. The problem I'm having is that when I run the opener, the sprocket will make a clockwise cycle but will immediately do a counter-clockwise cycle in the same distance. I've done it without the chain and with the chain attached and it's the exact same thing either way. That is what happened initially before I changed the gear & sprocket assembly; the door just opened 3/4 of the way and then wouldn't move at all (because of the chewed up drive gear). Now with the chain attached to the sprocket, the door moves a few inches up and then back down to where it started from, making the overall movement zero. The sensors are aligned and nothing is obstructing them because the lights don't blink when this happens and when I deliberately step into the sensor, the lights do blink. I tried adjusting the force and amount of travel but nothing happens. Nothing else is broken or stripped anywhere throughout. All wires were fine & plugged back into their proper spots. After the repair, the door is moving whereas it wasn’t before; it's stuck in the same position that it broke in. Since it happens with or without the door, it can’t be anything to do with the door or rails. I even tried running it without the light switch assembly and RPM sensor plugged in and it’s the same thing. Any help/suggestions would be greatly appreciated. Thanks.

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  • andylanders1 Feb 14, 2009

    I have the very same problem.

    Did you try this? Did it work?

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I HAD the same problem. I just solved it.

Here's what's wrong...

There is a black plastic "notched" wheel that attaches to the very end of the motor shaft and operates the RPM sensor. I found that when I reassembled my unit I did not get this wheel far enough onto the shaft and therefore it wasn't fitting far enough into the RPM sensor.

I had to take the motor back off, loosen the Allen screws on the shaft collar and push the collar further onto the shaft. There should be very little back and forth "play" in the shaft. This makes the RPM wheel fit further onto the shaft where it should and makes the motor work like it should.

Posted on Feb 15, 2009

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You have to sync the limit switch with the trolly. Take the small white piece off the bottom of the gear and sprocket assembly. Rotate the center screw on the limit switch until it is all the way up. Put the chain back on the sprocket with the trolley in the up position. Put the small white piece on the bottom of the gear and sprocket assembly. Operate the opener with the trolley dissconnected from the door. You will have to adjust the limit screws on the side of the opener accordingly.

Posted on Feb 14, 2009

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