Door Won't Open But Motor Runs: Trolley vs Spring vs Gear Translation Tree
Why this matters
"The motor runs but the door doesn't move" is a translation call with three distinct mechanical answers: a disengaged or stripped trolley, a broken counterbalance spring, or a stripped opener drive gear. Each makes the same complaint, and each demands a different response, from re-coupling a trolley to replacing a gear set to a spring job that carries real injury risk. Reading the customer's symptom correctly before you stand under the door tells you which repair you are on and, critically, whether you are about to deal with stored spring energy.
Symptom presentation
- Motor runs the full cycle, no door movement, trolley travels the rail empty: disengaged or stripped trolley.
- Motor runs, gear noise or a grinding whir, trolley does not travel: stripped drive gear in the opener head.
- Motor runs, trolley moves and tries, door rises a few inches and stalls or the opener strains and reverses: the door is too heavy for the opener, the hallmark of a broken spring.
- Door is heavy as lead by hand, will not stay up: broken or failed counterbalance spring, independent of the opener.
Quick checks
A garage door counterbalance spring stores enough energy to cause severe injury or death if it releases uncontrolled. Do not loosen, adjust, or attempt to wind a torsion spring without the correct winding bars and training. If diagnosis points to a broken spring, the door must be handled as a heavy, unbalanced load and the spring work treated as a hazardous task.
- Pull the emergency release and lift the door by hand (only if it is safe to do so and the door is down or controlled). A door that lifts smoothly and stays put has good springs; a door that slams down or takes both arms to lift has a spring failure.
- With the release pulled, run the opener and watch the trolley. Trolley travels the rail but never connects to the door equals a coupling or trolley problem. Trolley does not travel at all with the motor running equals a drive-gear failure.
- Look at the springs directly. A torsion spring shows a visible gap of one to two inches where it snapped. An extension spring hangs slack or is in pieces.
- Listen at the opener head. A spinning motor with a quiet rail and no trolley motion is a stripped gear.
Isolation tree
Branch 1, motor runs, trolley travels rail empty. The trolley is disengaged (emergency release pulled and not re-coupled) or the trolley carriage is stripped from the screw or chain. Re-engage the release; if it will not re-couple or slips, replace the trolley.
Branch 2, motor runs, no trolley travel, grinding from the head. The opener's drive gear (commonly a nylon gear on the output shaft) is stripped. The motor spins but cannot drive the rail. Replace the gear and sprocket kit, or the gear assembly, and inspect for the cause (an overloaded door from a weak spring chews gears).
Branch 3, trolley travels and connects, door rises a few inches and stalls or reverses, door heavy by hand. Broken or weakened counterbalance spring. The opener is healthy and overloaded. This is a spring job; treat the stored energy hazard accordingly and balance the door.
Branch 4, door heavy by hand with the release pulled, opener fine. Spring failure confirmed independent of the opener. Same as branch 3.
Branch 5, intermittent, door opens sometimes. A marginal spring nearing failure or a slipping trolley. Test door balance by hand at several positions.
A note on why a stripped opener gear and a broken spring so often arrive together. A weak or broken spring leaves the opener lifting far more weight than it was designed to move, and the softest element in the drive train, the nylon main gear, strips under that overload. So a stripped gear is frequently the downstream symptom of a counterbalance that was already failing. If you replace a stripped gear without checking and correcting door balance, the new gear inherits the same overload and fails again, sometimes within weeks. Always verify the door balances by hand before and after a gear replacement, so the fresh gear is moving a properly counterbalanced door and not the dead weight of a failed spring.
Confirming diagnosis
The decisive test is the manual balance check with the release pulled. A door that lifts easily and holds at mid-travel has good springs, so a no-move complaint is a trolley or gear fault, separated by whether the trolley travels the rail. A door that is heavy and will not hold is a spring failure, and the opener straining against it is a symptom, not the cause. Confirm balance before condemning the opener, and confirm the trolley travels before condemning a gear.
Remediation
- Disengaged trolley: re-engage the carriage to the trolley.
- Stripped trolley: replace the trolley carriage.
- Stripped drive gear: replace the gear and sprocket kit, verify the door balances so the new gear is not immediately overloaded.
- Broken spring: replace the spring as a hazardous task with correct winding bars and procedure, rebalance the door, then retest the opener force.
Always rebalance and retest opener up/down force after any spring or drive repair.
References
- UL 325, Standard for Door, Drape, Gate, Louver, and Window Operators and Systems, opener safety and force requirements.
- ANSI/DASMA 102, Specifications for Sectional Doors, counterbalance and operational criteria.
- DASMA Technical Data Sheet 153, Garage Door and Operator Safety, counterbalance hazard guidance.