My garage door won't (chain drive opener) open except for 1-3 inches, and then it stops. There are two wires that connect from the bottom of the door to a rotary gear above the door on both sides where the rollers are. What are these for, and can they stop the door from opening? Any ideas why the door starts to open but stops? Its a Sears 1/2 HP motor, probably 15 years old.