Door Keeps on Closing

Garage door continues to close after limit switch has turned the motor off.  Door slowly moves down until it hits the floor. Ratcheting sound heard while moving.
What could be at fault?
Thanks!

What make/model of operator do you have? Is the door balanced? Is the door hard to lift manually?

GDZone wrote:What make/model of operator do you have? Is the door balanced? Is the door hard to lift manually?

Thanks for your reply Brian!.  Sorry I could not reply earlier.
It is a Craftsman 315Mhz, 1/2 HP, Model 139.53920D, about ten years old.  I do not know if the door is balanced, but it was running ok.  How do I check for balance?
Door is somewhat hard to lift, but requires same effort as before.  I have not noticed any increase in effort.
Your help is appreciated!

To check the balance on your door you pull the red emergency cord on the opener (when the door is closed). Then lift the garage door until it is about 1/2 way open. Let go of the door (and make sure your hands & feet are out of the way ). If the door stays in the 1/2 way open position then the springs on the door are fine & the door is balanced. If the door slams to the ground then the door is not balanced & the springs most likely need replaced (or adjusted). The operator should only be lifting around 30 - 40 lbs. of weight while the door springs should be lifting the rest of the door weight.

You could also take the cover off of the operator & look at the gears. If the operator has been lifting too much weight over a period of time then the white plastic drive gear inside the operator will wear out or break.

GDZone wrote:To check the balance on your door you pull the red emergency cord on the opener (when the door is closed). Then lift the garage door until it is about 1/2 way open. Let go of the door (and make sure your hands & feet are out of the way ). If the door stays in the 1/2 way open position then the springs on the door are fine & the door is balanced. If the door slams to the ground then the door is not balanced & the springs most likely need replaced (or adjusted). The operator should only be lifting around 30 - 40 lbs. of weight while the door springs should be lifting the rest of the door weight.

You could also take the cover off of the operator & look at the gears. If the operator has been lifting too much weight over a period of time then the white plastic drive gear inside the operator will wear out or break.

Door stays in the halfway open position (manually), but withing a foot or two up it just slams to the ground.