Question about liftmaster Chamberlain Garage Door Opener Transmitter 300MC

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The door goes down to almost the bottom (about 10") and then it reverses itself and the light blinks. One sensor is green and the other is yellow. Any suggestions?

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Hello. On the side of the motor unit you will find two limits screws. One for up and one for down. Make tiny adjustments to the up limit until it works like it used to. Joe

4 thumbs up please

Posted on Oct 10, 2010

  • Alvaro Jinete Oct 10, 2010

    I tried it but it did not work. Help!!!

  • Joe Hogan Oct 10, 2010

    Ok do this. Make wee adjustments to those two small electric eyes near the floor until both light up green. Someone may have accidently hit one and taken it out of alignment. J.

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DOOR CLOSES MOST OF THE WAY AND THEN OPENS. LIGHT THEN BLINKS TEN TIMES. I NOTICED THAT ONE SAFETY SENSOR IS GREEN AND ONE IS YELLOW. PLEASE ADVISE.


Hi. The sensor with the yellow light is telling you that it is miss-aligned, Usually it only takes getting bumped to knock them out of alignment.
I would slowly turn or adjust the positioning of the sensor.
The one with the yellow light should turn green when it is all the way in line so that the beam is completing it's path.

It's out of adjustment, so the sensor is looking at it like there is an object blocking the door from closing.
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If it moves 6-8 inches and then stops it is most likely the rpm sensor.

Yellow and green are what the lights should be.

Does the led on the wall control blink?? If is does press the lock button.
If it doesn't you will need to reprogram the remote.

Watch the eyes as the door closes, does one of them go off? If so the track may be flexing causing misalignment. (Widen the track near the bottom)

If all of the above don't help check the sensitivity adjustment. Turn it about 1/8 turn clockwise and try the door again - continue this until the door goes full cycle without reversing. If you have to set the sensitivity past 3/4 the door needs to be adjusted.

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If the main light flashes 10 times when the door reverses it is a sensor problem. Check them very closely. If they are just a little off one of them will be flickering but it is difficult to see. This can also cause the problem to be intermittent. Tweak them just a little.

It is normal for that light to blink randomly.

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If the door reverses when it hits the floor it is traveling too far. Locate the 'down limit' control on the side of the motor unit and turn this in the opposite direction of the arrow printed on the cover. Some tweaking will most likely be necessary. You can always recognize a sensor problem because the main light will flash 10 times. If the main light isn't flashing it isn't a sensor problem.

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Anytime the main light flashes 10 times and the door reverses to the open position it is a door sensor problem or the wiring to them. Nothing else causes this symptom.

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The only thing that will cause the main light to flash 10 times and the door to reverse to the open position is a door sensor problem. Check to make sure one of the sensors is not blinking ever so slightly. It can be hard to see. Also make sure direct sunlight is not hitting the lens. Try shading them.

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The orange light is a constant, the green sensor needs to be adjusted until you get a solid green light.

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