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

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Sears Garage Door Opener Problem

HELP! I've changed the worm dirve and sprocket and now I can't get the door to go more than 2 feet in either direction. I've disconnected the limits switches to troubleshoot and I've also removed the chain. The motor runs one way a couple seconds and then reverses.

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  • lftmstr1245r Mar 12, 2008

    I have the same problem. I messed with it enough so that the garage will open all the way. However, it will only go down for 2-3 seconds and will go back up. I don't think the sensors are the problem because the light doesn't blink when the garage goes back up.

  • bentonmj Apr 16, 2008

    Man, am I glad to find this post! I just swapped out the gear set on my Craftsman opener, plugged it in, and had exactly the same thing happen. Even without the chain on, the motor runs one way for a few seconds, then reverses for a few seconds, then stops. I can't find anything miswired, misassembled, etc, although that doesn't mean it isn't. Has the posted solution worked for anyone?

  • Nokhodi Apr 19, 2008

    Craftsman garage door opener was working well and all of the sudden started to malfunction; goes up (or down) a few inches to a couple of feet and then reverses itself. I uninstalled it to reduce/remove stress for testing, but it does the same thing off the ceiling. Lubrication made no difference.

  • poppy2four Apr 27, 2008

    To all who read this... The fix is exactly as posted. You have to make sure the cap over the spring washers is FULLY compressed. To compress it, I ran a channel locks down the shaft and compressed the cap over the spring washers down ALL THE WAY and then clamped the channel locks to the shaft to hold the cap in place while tightenting the screws The Key is that the black cap (RPM Sensor) cannot move in or out more than 1/16 inch or the unit believes it's got a problem.

  • benv Aug 29, 2008

    Same problem. I checked that rotary flags that runs between the U shaped sensor. The door goes up and down few inches: That's all.

    I unplugged the sensor (A connector with 4 prongs); same thing happens.

    It seems like the sensor is bad...

    Any suggestions? Thanks: Ben.

  • bentonmj May 11, 2010

    lftmstr1245r's solution worked like a charm. The piece he's talking about is the collar that holds the bearing and washers on the motor shaft. It's held on the shaft by 2 set screws. Loosen them, push the collar so the spring washer is fully compressed (this is apparently what I failed to do), and retighten. I'm not sure how or why this worked, but it did. Thank you, lftmstr1245r!



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The problem lies in the RPM sensor, most likely due to the interrupter cup assembly. The interrupter cup assembly is the black cap that runs through the RPM sensor. You may have forgotten to re-attach this piece or it is probably moving too far out when the garage sets in motion. To fix this, remove the cup assembly, unscrew the two screws on the top metal piece with a 1/8 allen wrench, push the metal piece as far in as possible and put the screws back on. This will give more room for the cup assembly to spin and be read by the RPM sensor. Hope this helps!

Posted on Mar 12, 2008

  • Phat_Vinny Jul 09, 2011

    When I make the worm gear replacement with Sears' kits, the setscrews provided stuck out a bit from the collar. The door worked great but I didn't notice the setscrews rubbed on the RPM sensor board. Everything was fine for about 6 months then it would open 4" and then reverse. I have 3 door openers so I resorted to the rob Peter to pay Paul troubleshooting. I found the RPM sensor board was the problem. The emitter diode had been rubbed until the solder joints broke. I cleaned and resoldered the connections. Before reassembly, I used my dremel to grind the setscrews down even with the collar. It works fine and quiet. I made the same mod to the other 2 openers.


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OK...thanks to
lftmstr1245r...he got me on my way. However....mine was an older model...vintage 1990 + or -. I ordered the repair kit..(screw drive, gear assembly, etc). When reassembling the screw drive, my original set up ONLY had the black cap. There was no top metal piece with two allen screws. So I included the metal ring with two screws, put the black cap on...and ran into the sameproblem..up one foot, down one foot. Worked on it for about an hour, came in, and read lftmstr1245r's post. However, seeing as mine didn't have the metal ring to begin with....I removed it and put the black cap back on. It is working like a charm. One other thing...if you can remove the cover to the encasement, watch the adjusters underneath. It made it a lot easier...when the door goes down, it touches the connection on one side to stop it...when it goes touches the connection on the other side and stops. That's all there is to it. Easier to adjust it this way and see how far the metal bar goes before it touches either sensor. Good Luck.

Posted on Apr 26, 2008


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