my craftsman garage door opener just hums for a second then stops

Diagnosis

This specific behavior in the Craftsman 57915—a brief hum followed by a complete stop—indicates that the motor is attempting to engage but is encountering an immediate resistance that triggers the internal thermal overload protection or the force-limit sensor. Because this is a belt-drive system, the humming sound is the 1/2 HP motor attempting to turn the drive sprocket, but the lack of movement suggests either a mechanical jam or a failure in the capacitor to provide the necessary starting torque. The root cause is typically one of three things: a broken torsion spring causing the door to be too heavy for the motor to lift, a stripped drive gear/belt that is slipping under tension, or a failing start capacitor. To confirm, observe the belt during the "hum" phase. If the motor hums but the belt does not move at all, the issue is likely electrical (capacitor) or a jammed sprocket. If the belt twitches or moves slightly before stopping, the motor is sensing too much resistance from the door itself.

How to Fix It

Follow these steps in order to isolate and resolve the resistance issue:

1. Manual Operation Test: Pull the red emergency release cord to disconnect the carriage from the belt. Attempt to lift the garage door manually. If the door feels significantly heavier than usual or will not stay open halfway, your torsion springs are broken. Replace the springs before attempting to run the motor again, as forcing the motor to lift a dead-weight door will burn out the windings.
2. Inspect the Drive Belt: Examine the belt for any fraying or slack. If the belt has stretched or a tooth has stripped, the motor will hum without transferring power. Tighten the tensioning bolt located at the motor head if the belt is sagging.
3. Adjust the Force Settings: Locate the "UP Force" and "DOWN Force" adjustment screws on the side or back of the motor housing. Using a small flat-head screwdriver, turn the "UP Force" screw clockwise by 1/4 turn increments. This increases the amount of torque the motor will apply before the safety shut-off triggers.
4. Clear the Tracks: Inspect the side tracks for debris, pebbles, or bent sections. Use a silicone-based lubricant on the rollers to reduce friction, which often triggers the overload sensor on older 1/2 HP units.
5. Reset the Logic Board: Unplug the unit from the 120V outlet for 60 seconds, then plug it back in to clear any temporary electronic glitches in the force-sensing memory.

If That Didn't Work

• Replace the Start Capacitor: If the motor hums but never moves the belt, the start capacitor has likely failed. This component provides the initial electrical "push" to get the motor spinning. Locate the capacitor (usually a black or silver cylinder) inside the motor housing, discharge it safely, and replace it with a matching microfarad (uF) rated capacitor.
• Inspect the Drive Gear: While the 57915 is belt-driven, the internal drive gear (which turns the pulley) can wear down. Open the motor housing and check for plastic "snow" or shavings. If the gear is stripped, the motor will spin/hum without engaging the pulley. Replace the gear and sprocket kit.
• Check the Photo-Eye Alignment: Ensure the safety sensors at the bottom of the tracks are perfectly aligned. While a sensor issue usually results in a flashing light or reversal, a partial short in the wiring can sometimes cause the motor to hesitate and shut down immediately.
• Test the Motor Windings: Use a multimeter to check for continuity across the motor windings. If you find an open circuit or a short to ground, the 1/2 HP motor has burnt out and the entire power head assembly must be replaced.

Heads Up

• Never adjust force settings to "Maximum" to compensate for a broken spring; this can lead to the door crashing down or the motor overheating and causing a fire hazard.
• Lubrication Warning: Only use lithium-based or silicone lubricants on the tracks and rollers. Never use WD-40 or grease, as these attract dust and grime which increase friction and trigger the overload sensor.
• Spring Danger: Torsion springs are under extreme tension. If you identify a broken spring during the manual test, do not attempt to tighten or replace it yourself unless you have the proper winding bars and safety equipment.
• Belt Maintenance: Check the belt tension every six months. A loose belt causes "slippage" that the motor interprets as an obstruction, leading to the humming/stopping cycle.