My Chamberlain garage door opener only opens 8 inches.

If the closer has adjustable limits and are not set correctly, then it behaves normal. Time to set limit to correct door position.

The chain should be 5/8 inch above the rail when measured at the center of the rail.

Here is some information that may really help you.

http://www.supersneaky.com/index.html

I just had that problem this morning and the cause was that the springs that help hold the weight of the garage door were broken. This caused the garage door opener to raise the garage door a fe inches and then drop again. We called in a professional and $200 later we had new springs and everything works fine now.

Remove the cover and check the drive gear. It is white nylon and is just in front of the motor. It may be stripped. Take a close look at those door springs. They can sometime appear to be okay when one is broken.

Try this link
http://www.garagedooropeners.us/helpgifs/chamberlain_troubleshooting.html

Make a slight adjustment to the Upward force on the side of the machine. Turn the screw a 1/4 turn each time and test, see if that helps. Also, make sure there are no obstructions or friction when the door opens and closes. Try it manually first.

is bulb on motor flashing 10 times. then you have electric eye problem. otherwise, sounds like you need to increase down force a hair. adjustement is on logic board, right knob.

I would try jumping a wire across the white & red terminal to see if your safety sensors are working Properly! If your door opens you have a loose connection or faulty sensors.

try adjusted the sensor controls on the sode of the unit other wise replace the logic board
http://www.chamberlain-garage-door-openers.com/circuitboardcompatibilitychart.htm