Set the Door
When the spring breaks when your garage door is in the down position you will need to raise it up. You can do this with the help of a friend because the door will be heavy. Lift the door up and set it in its full and open position. This is needed to relax the springs so they are not stretched and under tension. To do this you will need to disengage the lock bar from the chain mechanism.
Prepare the Door
Once the door is opened attach a pair of vise grips to the track onto the side with the broken spring. Do this under the last roller on the track. This will keep the door from falling down. Disconnect the springs that are broken from the garage door. Verify that the pulleys are still in good condition. If the pulleys or bearings are worn then they should be replaced.
Obtain New Spring
Measure the width of the broken spring by using a tape measure. You can also measure the width of the other spring on the garage door. Take the measurements to a dealer that has garage doors parts that you can purchase. Make sure that you purchase two springs that are identical in length and diameter.
Install New Spring
Examine the unbroken spring on the garage door to see how the cable is placed on the pulleys. Place the new spring on the garage door. Set the cable in place and then tighten to provide tension. Make sure that this cable is not being stretched more than 1-inch.
Replace Unbroken Spring
Replacing the springs for a garage door will require replacing both the broken spring and the unbroken spring. This is required as you should use new springs that have the same strength. Take your vise grips off of the track and attach them to the other side. Remove the unbroken spring and then set the new spring in place.
Test the Door
Reattach the lock bar to the chain mechanism. Remove the vise grips from the track and then lower the door to the ground. If the garage door does not reach the ground then you need to reset the spring to let the door go further down to the ground.
Raise and lower the garage door to test your work. You will be able to tell if the springs are too loose or are too tight.