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My GDO motor sounds like it is running but the door is not moving. The chain is overly tight on one side and drooping about 4 inches on the other side. Would a chain problem cause the door not to open?
 

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My first guess would be that the chain came off of the drive gear. You will need a step ladder, in order to look at the top of the unit, and confirm this. If you do not have one, go to Liftmaster's website, and download a manual, so that you can then properly reinstall and adjust your chain.
 

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First thing to do is trip the emergency release mechanism and see if the door works OK manually. Can you easily lift the door up, does it stay open, and does it seem to work smoothly up and down. If so then proceed to repairing the opener.

Very likely the main drive gear in your opener has failed, the gear is plastic and a very common failure. You can confirm this,

Unplug the opener first!

Then you can remove the cover, you will likely see lots of white shavings inside, if so the shredded gear is the culprit. The plastic gear kit can be purchased for around $10-20 depending on how much else is wrong. Sometimes the shaft and upper sprocket bushing are worn too and can be purchased all in a kit, this could also be part of your problem.

If you need to hire someone to do the repair; it will likely be cheaper to just replace the opener. Either way it is not a difficult job if you have the mechanical ability and some basic tools.

The gear set can last many years on a properly working door, or it can be destroyed in just a few weeks on a door that catches, binds or is improperly adjusted. They often do fail for a reason.

Many other brands are identical, Liftmaster, Chamberlain, Sears, Master Mechanic, Do it best brand openers all use this very same design.

Here is just one of many videos on Youtube detailing the procedure:

http://www.youtube.com/watch?v=OMPFJcdyJk0
 

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doordr
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If the motor runs and the chain does not move its definitely a gear. Take the cover off with no power and tell us what you see... Unless its self evident. The gear is a common fix. Make sure and lube the new one real good and check door balance.
 

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doordr
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If the motor runs and the chain does not move its definitely a gear. Take the cover off with no power and tell us what you see... Unless its self evident. The gear is a common fix. Make sure and lube the new one real good and check door balance. It also sounds like the gear went out because it was hitting the stopper bolt that's put through the rail to prevent over travel. The trolley should not be hitting that bolt if you need to, adjust the length of the bar attaching it to the garage door. Then you will need to adjust the limit screws that should be located on the side of the Motörhead NOT the back(side facing away from the door)the adjustment screws should have a picture of a door closing and a door opening. The direction of the arrow means travel further and opposite means not so far. I'm assuming the close setting may be out of adjustment and that caused the chain to droop. I hope that helps.
 
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