What are the signs of a failing manual gearbox?

The main signs of a failing manual gearbox are grinding or crunching when you select a gear, a whine or rumble that changes with road speed, the car jumping out of gear, a gear lever that feels notchy or stiff, and oil marks under the car.

Gearboxes rarely fail without warning, but the hints are easy to talk yourself out of. This guide explains what is usually behind each one and says plainly when to stop driving.

Why does my gearbox grind when I change gear?

A graunch on one particular gear usually points to the synchromesh, the part that matches the speed of the gear to the speed of the shaft so the change goes in smoothly. When a synchro ring wears, the gear teeth clash instead of meshing. It often starts on second or third, the gears used most in town driving.

Grinding on every gear is different. That points more towards a clutch that is not fully releasing, which is a clutch or hydraulic matter rather than a gearbox one.

• Grinding on one gear only: often synchro wear
• Grinding on every gear: often clutch release or hydraulics
• Grinding only when cold: may be oil condition or wear

What does a whining or rumbling gearbox mean?

Listen to whether the noise follows engine speed or road speed. A whine or hum that rises and falls as you change gear and varies with the speed of the car often comes from worn bearings or worn gear surfaces inside the box. Low or tired oil makes it worse, because the bearings and gears depend on that oil film.

A noise that disappears when you press the clutch pedal in, and returns when you let it up, can have other sources, including the clutch release bearing.

Why is my car jumping out of gear or hard to shift?

Jumping out of gear, most often on the overrun when you lift off the throttle, suggests wear in the parts that hold a gear engaged, such as selector forks, detents or the gear teeth themselves. Worn mounts or a stretched linkage can also let a gear pop out.

A lever that has become stiff, vague or reluctant to go into gear can come from linkage or cable wear, low or wrong oil, a clutch that is not releasing properly, or worn synchros. Again, the symptom is only a starting point. The diagnosis comes from inspection.

What can I safely check myself?

You can look without taking anything apart. Look under the car for fresh oil or damp patches around the gearbox, and note whether the symptom appears when cold or warm.

Check your handbook for any gearbox oil guidance, as intervals and oil types differ between vehicles.

• Look for oil on the ground or on the gearbox casing
• Write down which gear, which speed and hot or cold
• Listen with the clutch pedal up and down
• Check your handbook for oil and service guidance

When should I stop driving and what will a garage do?

Stop driving if the car will not stay in gear, if you cannot select a gear at all, if there is a burning smell, or if you notice a sudden loss of drive. A gearbox that drops out of gear at the wrong moment, such as when overtaking or pulling out of a junction, is a safety risk for you and others.

Depending on what is found, the answer may be a simple adjustment, an oil service, a linkage repair, or work on the gearbox itself. Cost depends on the vehicle, parts and labour access.