Opener Runs But Door Does Not Move: Trolley vs Gear vs Spring
Why this matters
When the opener motor runs but the door stays put, the drive train has lost its grip on the door somewhere between the motor and the door arm, and the three usual culprits, a disengaged or broken trolley, a stripped drive gear or sprocket, or a broken counterbalance spring, are dangerous to confuse. A broken spring leaves the full weight of the door unsupported, so reconnecting an opener to that load can strip the new gear instantly or, worse, send a tech reaching into a door that will slam. Each cause has a clean signature: the motor's sound, whether the trolley or chain moves, and whether the door is liftable by hand. Reading those signatures in order keeps you from replacing a gear that was a symptom of a broken spring, and it keeps you from operating an opener against a door it was never meant to lift. This tree isolates the fault to the right part safely.
Symptom presentation
The customer reports pressing the button: the opener hums or whirs, lights may come on, but the door does not budge, or it moves an inch and stops. Ask what the motor sounds like. A motor that runs full speed with a fast whirring or rattling and no door movement points at a stripped gear or a disconnected trolley. A motor that hums, strains, or trips a breaker without moving suggests it is bound against a load it cannot lift, classic broken spring. Ask whether the manual release was pulled (someone may have left the door disconnected). Ask whether the customer heard a loud bang recently, the signature of a torsion spring breaking, often blamed on the opener. Note whether the chain or belt and trolley move while the door does not.
Quick checks
Disconnect the opener with the manual release and lift the door by hand. A door that lifts easily and holds at mid travel has a healthy spring; the fault is in the drive. A door that is extremely heavy or will not stay up has a broken or failed spring, and the opener is innocent. With the opener still connected, watch the trolley and chain/belt during a run cycle: if the chain or belt moves but the trolley does not travel, the trolley is disconnected or its release is tripped; if the motor spins but the chain/belt does not move, the drive gear or sprocket is stripped. Inspect the spring above the door (torsion) or alongside the tracks (extension) for a visible gap or separation.
Isolation tree
Branch 1, motor runs, chain/belt does not move at all: drive gear or sprocket failure. On a chain/belt opener the plastic drive gear commonly strips and the motor freewheels; look for white plastic shavings inside the opener housing. The trolley and door are fine; the opener simply cannot transmit power.
Branch 2, chain/belt moves but trolley stays put, or trolley slides without engaging the door: trolley problem. The manual release was pulled and not re-engaged, the trolley carriage is broken, or the release latch failed. Re-engaging the trolley by running the opener until the carriage relatches, or operating the door to the release point, restores it if the part is intact.
Branch 3, door is heavy or will not hold when lifted by hand, motor strains or trips: broken counterbalance spring. The door's weight is no longer counterbalanced, so the opener is being asked to lift the raw door weight, which it cannot and must not do. A visible gap in a torsion spring or a stretched/separated extension spring confirms it.
Branch 4, motor hums but nothing moves, no whirring, possible breaker trip: a seized motor, failed capacitor, or the door is locked or jammed against an obstruction. Verify the door is unlocked and unobstructed, then suspect the opener's motor/capacitor.
Confirming diagnosis
Confirm with the hand-lift test as the gate. Disconnect and lift: easy and holds means the drive is at fault (gear or trolley); heavy or drops means the spring is at fault and the opener stays disconnected until the spring is repaired. For gear: open the opener housing and inspect the drive gear for stripped teeth and shavings. For trolley: inspect the carriage and release mechanism, and confirm whether the chain/belt drives it. For spring: look for the separation gap on a torsion spring or a hanging, stretched extension spring; never judge spring condition by sound alone.
A broken counterbalance spring is a life-safety condition. Do not run the opener against an unsupported door, and do not attempt to wind, replace, or relieve torsion springs without proper winding bars and training; they store enough energy to cause severe injury or death. If the hand-lift test shows a heavy door that will not hold, stop, secure the door, and treat it as a spring repair, not an opener repair.
Remediation
For a stripped drive gear, replace the gear-and-sprocket kit per the opener model and clear the shavings before reassembly. For a disengaged trolley, re-engage the carriage per the manual; for a broken trolley or release, replace the trolley assembly. For a broken spring, repair or replace the counterbalance spring with the correct wire size, length, and wind using proper tools, then re-balance the door before reconnecting the opener. After any drive repair, confirm the door is balanced (hand-lift holds at mid travel), then reconnect the opener, run a full cycle, set force and limits to the minimum that operates the balanced door, and perform the UL 325 obstruction reverse test.
References
- UL 325, Standard for Safety for Operators and Systems (opener operation, force, and entrapment protection)
- DASMA Technical Data Sheet TDS-167, Counterbalance Springs and Door Balance
- DASMA Technical Data Sheet TDS-353, Garage Door Operator Force Settings and Safety Reversing
- LiftMaster, Chamberlain, and Genie opener service manuals (drive-gear replacement, trolley re-engagement, and balance verification)