Limit Switch

A limit switch is the device inside a garage door opener that tells the motor where to stop. When the door travels to its fully open position, the up-limit switch fires and the motor cuts off. When the door reaches fully closed, the down-limit switch fires and stops the motor again. Without working limit switches, the opener motor would keep running past the door's endpoints and damage the door, the opener, or both.

Older mechanical limit switches are physical contacts mounted on the motor unit or the rail. A threaded rod or cam rotates as the trolley travels, and when it reaches the set point, it trips the contact. Modern openers use electronic sensors or encoder counting instead. The motor tracks how many rotations (or encoder pulses) the drive system has made and stops at the programmed count.

What happens when limits are wrong:

• If the down limit is set too low, the door closes past the floor line. It bends the bottom section and stresses the springs.
• If the down limit is too high, the door stops before fully closing. Cold air, pests, and water enter through the gap at the bottom.
• If the up limit is too low, the door barely clears the vehicle. If it is too high, the door slams into the back-hang stops on the track.

Adjusting limit switches is a standard part of a new opener installation and a common service call after a door replacement changes the door height.

For example, a Chamberlain belt-drive opener has two adjustment screws labeled "Up" and "Down" on the motor head. Each partial turn of the screw shifts the stop point by a small increment of door travel; the exact amount varies by model and is specified in the opener's installation manual.

Limit switches work alongside the force setting to control safe door operation. The force setting stops the door if it meets unusual resistance before reaching the limit position. The limit switch stops it if travel completes normally.