Why a gear pops out on its own
When you select a gear, a sleeve (the sliding hub) locks the gear to the shaft via engagement teeth known as dog teeth. A selector fork holds the sliding hub in place while the torque is transmitted. If the dog teeth are worn to a bevel, or if the fork has developed play, the engine load pushes the hub out of engagement: the gear jumps out and the lever returns to neutral.
Likely causes
- Worn dog teeth or synchromesh on the affected gear. The gear always jumps out on the same ratio, either under acceleration or on the overrun. This is the most common cause, often preceded by crunching noises.
- Worn or bent selector fork. It can no longer hold the sliding hub in place. Also an internal fault.
- Broken engine or gearbox mounts. The whole assembly moves under load and pulls on the gear linkage: several gears jump out, especially on rough roads.
- Gear linkage out of adjustment (cables, ball joint, rods).
| What you notice | Most likely cause |
|---|---|
| Always the same gear, under load | Dog teeth, synchromesh or selector fork on that gear |
| Several gears, on rough roads or under load | Engine or gearbox mounts, gear linkage |
Is it safe to keep driving?
Only with caution. A gear jumping out during an overtake or on a hill leaves you without drive at the worst possible moment. Avoid that gear and have the gearbox checked promptly: the dog teeth wear a little more with every jump.
Repair or replace the gearbox?
Replacing the dog teeth, synchromesh or selector fork means removing and stripping the gearbox. If the mounts are not to blame, a reconditioned exchange gearbox replaces all the wear parts in one go, with a warranty of up to 24 months.
