Question about liftmaster Chamberlain Garage Door Opener Transmitter 300MC

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My Liftmaster Professional 1/2 hp unit opens to the correct limit, however, after it stops, it starts to slide back a few inches, the motor engages and pulls it back up, it slides, it pulls it back up, etc. This happens about 5 or more times until it locks/brakes into the open position. What's going on? Can I adjust something? Thanks

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I replaced a gear sprocket kit on my lift master..... because the mortor was running and nothing was happening. Now the same thing is happening, what is causing this to happen?

Yhank you

Posted on Nov 12, 2009

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There are internal limit switches that look like little plastic wheels on a screw. if these wheels dont push the buttons in on either side as it travels it will try to keep traveling see if you can locate these plastic wheels and the switches they push in both the full up and full down position and make sure they are actually pushing the levers

Posted on Oct 10, 2009

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1 Answer

Garage Door Opener Will Open But Not Close


Your machine is jammed, meaning the carriage part or trolley that has the swingarm attached to the door has went further then it,s limit! your trolley that has the chain or belt connected to it may be jammed against the stop bolt that is just before the powerhead of your opener. In order to get it to operate again you must loosen the chain and take some tension off of it , you should also remove the stop bolt at this point sine a loose chain can add an extra inch or two to the travel limit when the door is coming up or going down . Once the bolt is removed your garage door opener should work again , be shure to retighten the chain after removing the stop bolt and check your travel limits in the up and down position! Hope this helps you in getting it to work again! Glenn

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My Chamberlain Liftmaster Professional 1/2 HP door goes down all the way but then it comes back up again. I see nothing in the way at all. is there some way to adjust the sentsitivity on it???


If the door is hitting the floor and then reversing you need to adjust the down limit control. This control is on the side of the motor unit. Turn the control a couple turns in the opposite direction of the arrow printed on the cover. This should stop the door short of the floor. Now some tweaking of that control will most likely be necessary to get it just right. Just remember if you run the motor too much it will shut off on the thermal overload and you will have to wait as much as 15 minutes for it to cool.

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Opener has been working fine for four years. Today, when I pressed the remote to get into the garage, the door opened for a few inches, then stopped, and now the motor will not run at all, via the remote...


The problem may not be the garage door opener, To determine this, Pull the emergency release to release the door from the carriage. (The red cord that hangs from the opener) Now you can press your garage door opener to see if your motor works. If it still does not work, try unlpugging the opener from the power source for approx10 seconds. This should reset your motor.

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Dec 02, 2010 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

I have a 1/2 hp liftmaster my door goes almost all the way down to about 3 inches from the floor and then stopps like it has hit something and then it will sometimes go back up other times it just freezes...


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Look at your manuel find the page that shows the limit access switch. (Unplug your opener.) It isn't really a switch, but the three white plastic screw like parts. The center has a brown wire connected to it, which has a metal piece that is as wide as all three white screws. The outside screws are your limit access stop guides, one for the up control stop guide and the other for the down control stop guide. If the center screw with the metal part gets caught behind on of the stop guides the motor will not engage. Gently move metal piece from behind the stop guide.

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Okay...upon further inspection, my left spring has broken in two. Going to call around for some quotes to replace the spring. Thanks for the suggestions.

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