Liftmaster Chamberlain Garage Door Opener Transmitter 300MC: Light keeps on flashing on the motor...

here is a YouTube video on battery replacement.
http://www.youtube.com/watch?v=9GYqN5_DRHY

If the motor is spinning but the chain is not, then there is probably a bad gear before the sprocket.
You need to decide the type of gear kit you need.

https://www.superiordoorparts.com/pm-134-12-liftmaster-chamberlain-garage-door-opener-gear-kit-41a2817.aspx

IF YOU HAVE THREE SCREWS THE WALL BUTTON GOES TO 1 AND 2

THE EYES
THE WHITE WIRES W/BLACK LINE GOES TO 3
THE PLAIN WHITE WIRES GO TO 2

IF YOU HAVE SLIDE IN QUICK CONNECT THE WALL BUTTON GOES WHITE WITH RED TRACE TO RED.
THE SAFETY EYES WHITE WITH BLACK TRACE GOES TO GRAY PLAIN WHITE GOES TO WHITE

HOPE THIS HELPS

IF IT IS ONE OF THE NEWER BELT DRIVE OPERATORS THE BELT ATTACHMENT IS ADJUSTABLE.

IF IT IS ONE OF THE VERY OLD OPERATORS AND YOU IN FACT HAVE THE CORRECT BELT YOU CAN NORMALLY SET THE BELT ON THE SMALL PULLY AND LAY IT AGAINST THE LARGE PULLY BY TURNING THE LARGE PULLY YOIU CAN ROLL IT ON.

INSIDE LOOKING OUT
STANDING IN THE GARAGE LOOKING OUT THE DOOR IS THE LEFT AND RIGHT THEY REFER TO.

99% SURE IT IS THE SAFETY EYES - ALIGNMENT, WIRING, OR BAD EYES

HOPE THIS HELPS

You will need to match the 10 dip switches inside the transmitter to the 10 on the motor head. That is is. Thanks BB

sounds like you need a gear replacement, chamberlain sells the kits.

Here is the information you need to set up the code and keep your door working well for many years. Enjoy.

http://www.liftmaster.com/consumerweb

You should have two buttons on the unit in the garage, press the first one and the light should go off, if it's like my Chaimberlain the light signals that the remote is locked out.