Question about liftmaster Chamberlain Garage Door Opener Transmitter 300MC

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Opener travels 2 ft.stops and reverses in the other direction. I have checked the sensors and adjusted the travel, both up and down, nothing does anything.

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Usually when the door acts like this , it is because the eye at the bottom of the openning is not lined up correctly, or these is something in the path of the beam from the eye. Also check to see if the track is binding or the door is hitting some obstruction on the way down.

Posted on May 04, 2010


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4 Answers

Door opens itselv after closing

most likely the down stop is set to low, the door hits ground and reverses. so either change the down stop position or increase the down force so it will not reverse upon contact.

Dec 06, 2015 | Building Materials

1 Answer

My l/m door goes down but come right back up help

This can be one of two problems ! First problem can be that their is something blocking the infrared sensors that would cause the door to go back to the up position but in most cases if their is something blocking one of the sensors the door should not even move and just make a click sound and you will see the lights flashing on the garage door opener , it can also be a mis aligned sensor not pointing directly at the recieving sensor or vice versa! Ok if that was,nt the issue then you can make shure that the door isn,t traveling further then where it needs to stop to be closed, sometimes the chain or belt depending on your opener loosens up and can add an extra inch of travel to the door causing it to return to the up position, to fix this you have 2 adjustments on your opener ...1 will be a pressure adjustment can turn the adjustment to midway #6 to add some pressure so that the opener does not sense the xtra travel of the door , You can also backoff on the downtravel adjustment a turn or two and that will make the door not come down as far ...This adjustment is used to set how far the door goes up and how far you want the door to come down, you will see a picture of the door on both of these adjustments with an arrow letting you know which way to turn the screw ..remember each full turn of the screw equals 1/4" inch of travel to the door just follow the direction of the arrow or turn the screw counter clockwise for less travel !,,,Hope this helps you Sincerely Glenn

Jun 13, 2014 | liftmaster Garage Door 1/2 Hp

1 Answer

I have Sears Craftman 1/2 HP garage door open that want stay shut when you push the remote the sensor wire broke fix that. Now the door goes down and come back up. All sensor are led light are green...

Check the sensors again. Our unit has one amber and one green when working and if something (even a weed) breaks the optical beam it reverses and the lights blink. We've also had bright sunlight interrupt the beam. (The test for this issue is to stand such that you cast a shadow onto the sensor without being in the beam and then attempt to close the door with the remote). Next check if the door is binding during travel. If it is, check the tracks for plumb with a level and make sure the door rollers and tracks are undamaged. If a rail is crimped anywhere, it could be cause the problem. You can find some garage door sales places that sell the track sections and rollers to replace damaged parts. Check your local phone book and call around. In some cases, the door jamb may be damaged (lowering the header and shrinking the entire door area). This may require calling a professional to fix.

If the door itself is moving easily, then it sounds like the door might be hitting the floor too lightly or trying to go past the floor and triggering a safety reverse. On most of the openers, there are several adjustments you can work on to reset this. First try adjusting the down limit to above the floor and then bring it back to the floor. (However, usually these do not make the opener light blink.)

Alternatively, adjust the downward force to a higher value.

Without knowing the model of your Craftsman opener, I'll just use the directions for mine. All of these are adjusted with a flat blade screwdriver. Always test each adjustment with a complete up and down cycle.

The height limit control is on the side. (It's the forward one of two screws inside the cover - you should be able to reach them thtrough holes in the cover. Unfortunately, sometimes it's easier to get to them after removing the cover. The cover is held by 4 screws - 2 on each side.) To decrease the travel, turn the Down screw clockwise. One full turn of the screw is 2" of door travel.

The force controls are on the back with the down adjustment on the right. Turn this clockwise in 10 degree increments and test each time.

It is possible to overheat the opener with too many open and close cycles. In that case stop adjusting the unit and wait 15 minutes for it to cool.

I hope this helps.

Cindy Wells

Nov 27, 2010 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

No matter when i adjust the force the door goes down, stops briefely, them returns to open position. can't get force adjust to let it stay down.

If the door is hitting the floor and then reversing it is traveling too far in the down direction. Locate the 'down limit' on the side of the motor unit and turn it in the opposite direction of the arrow printed on the cover. Some tweaking of this control will be necessary to get the travel just right. The force adjustment affects the safety of your opener. Try to put it back where it was.

Sep 30, 2010 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

Door opens about 1 foot too far, the use of the travel up screw either appears to do nothing or it does too much when adjusted. How do you know how many inches the screw is affecting in a turn (or partial...

Are you turning travel screw, or force screw? If travel, One is for how far up and other is for how far down. Insert screwdriver into UP adjustement and turn it opposite direction of arrow. Try 2-3 turns to start. Yu may have to turn back CW. If while adjusting door just stops, go have a cup of coffee. You overheated motor. Give it 15 minutes to cool down.

Dec 17, 2009 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

Craftsman - chain drive. now open won't travel more than 1 ft.

Sounds like you have a bad RPM sensor board. This is a small printed circuit board just in front of the motor.

Sep 06, 2009 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

Craftsman garage door opener won't close completely

look on the side of the openeer usually lekt looking at it from the rear theese are your travel adjusters one turn = 2" travel normally

Jan 07, 2009 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

Sears Craftsman 3/4

Open up the caseand look at the travel assy. Sometimes the limit (up or down) do fall off the assy. Just snap it back on.

Jan 06, 2009 | Sears Craftsman LiftMaster Chamberlain...

1 Answer

Opener will not reverse


Unplug power
Release the door from the opener close the door manually. take the chain loose and slide the traveler down to reengage with the trolley. Plug in power, run the unit with chain off untill it completes a clockwise cycle and stops. reconnect the chain and set tension.

Adjust the chain tension.

If it still moves 6-8 inches and flashes 5 times. the problem is in the rpm sensor.

Full diagnostics are in the owners manual - FREE at

Oct 06, 2008 | Sears Craftsman LiftMaster Chamberlain...

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