Question about liftmaster Chamberlain Garage Door Opener Transmitter 300MC

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Door wont sense when shuts.

Will not reverse open only cloes.

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  • eric_hughes Jan 12, 2009


  • Joel Wiedeman May 11, 2010

    Does the motor try to keep running when the door is closed?


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  • 16 Answers

Sounds like a limit switch internally is out of adjustment. Check the owners manual. What make and model is it? There may be a manual online.

Posted on Jan 13, 2009

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Door closes and reverse itself

The most likely cause of this problem is that the door is unexpectedly hitting the ground before the shut off of the motor. It is possible that the door is also jamming which makes the unit think there is an obstruction. If there is no mechanical problem with the door itself, you can make adjustments to the up/down force and the up/down limits. These adjustments are at your lifter motor unit and you should check or download your particular manual. I have linked a video showing the basics for this adjustment.

May 06, 2015 | liftmaster Chamberlain Garage Door Opener...


How to safety check your garage door opener

Place a 2x4 (or other object that is solid and about 2" thick)flat on the ground in the middle of the door. Activate the opener and lower the door until it makes contact with the 2x4. It should hit the 2x4 and then reverse in the upward direction. It can put pressure on the 2x4 just not stay there. If it reverses then it has passed. If it stays down then find the down force adjustment on the opener and turn it down. This adjustment can be found in your manual and will differ in location from brand to brand of garage door opener. Check the belt/chain for slack when it is running(there is no need to check this on a worm drive opener). If there is more than 3/4" slack then tighten it up with the tension adjuster on the trolley. Check the force adjustments per the manual as well. Do not pay attention to this slack when the door is shut as the trolley arm presses on the opener arm to gain the best seal at the bottom of the door. If you have a screw drive you can lubricate the screw with liquid graphite. All other types need none unless you want to lubricate the nylon gear inside the unit with rail grease, but be careful here and unplug the unit first. If your garage door opener has safety eyes start the door down and then cross the beam. The opener should reverse upward. If it does it passes, if not then call a reputable repair firm. Thanks BB

on Sep 03, 2010 | liftmaster Chamberlain Garage Door Opener...

1 Answer

We have a liftmaster pro9000 and the garage door will not open very far. How do I adjust the sensor?

If the sensor sees something under the door, it will stop and blink the lights.

So I think you need to check out the motor, there's dial to increase the lifting strength.

Jun 03, 2014 | liftmaster Chamberlain Garage Door Opener...

1 Answer

Installation instructions for a Cooper Wiring Devices OS310U-W-K Motion-Activated Occupancy Sensor Wall Switch, White

LED bulbs with leaking motion sensors tend to flicker as well, more so than CFL. I switched to Cooper style - but now my Cooper OS310u will sense motion - and keep sensing motion almost all the time. If it's in test mode - it will shut off but otherwise it keeps sensing it. This is in a garage - and door open or closed does not seem to affect it. I have the same unit working in my basement just fine.

suggestion - don't try LEDs to solve the problem and yes - it will burn out your CFLs real fast.

Jan 12, 2012 | Light Switches & Dimmers

2 Answers

Door closes then goes back up

it needs adjustment, there are usually an adjustment knobs on the unit it self, (check from the owners manual)...

Jul 10, 2011 | liftmaster Chamberlain Garage Door Opener...

2 Answers

I bought a second hand Digidoor 1 with garage door. The door opens and closes perfectly when the digidoor is off. When I power up the digidoor the motor will not open the garage door when the door is...

You are receiving interference from the digi door. You will have to return this unit and go with a different one on a different frequency. Chamberlain makes a clicker that would work well with that unit. Thanks BB

Sep 21, 2010 | liftmaster Chamberlain Garage Door Opener...

1 Answer

My liftmaster garage door shuts then just automatically re opens there is nothing in the sensors path

If it hits the floor and then reverses then you will have to first turn up the down force a little bit and try. If it still does then turn down the down travel to stop sooner. If it reverses before it hits the ground then turn up the down force. If this does not do it then more than likely the problem is in the door itself. Thanks BB

Jul 18, 2010 | liftmaster Chamberlain Garage Door Opener...

1 Answer

Garage door opens about 8 inches and then revereses

Formula what? they have over forty different types. though sounds something like either limit switches may be faulty. Is this door new? Are there any odd noises from the motor?

Mar 21, 2009 | liftmaster Chamberlain Garage Door Opener...

2 Answers

Grage door reverces and goes back up after touching the ground

try adjusting the close shut off switch.(limiting switch) It sounds like when the door shuts, it has too much pressure. The sensor picks this up and reverses the door.This is a saftey feature. It just needs adjusting

Mar 06, 2009 | liftmaster Chamberlain Garage Door Opener...

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