Manual Gearbox Problems: Symptoms, Noises and Causes

Crunching when you change gear

A crunch or graunch as you select a gear normally means the synchromesh is struggling. Synchromesh rings are small brass or steel collars that match the speed of the gear you want to the speed of the shaft before the teeth engage. When they wear, you hear the teeth clashing instead.

Before blaming the gearbox, rule out the clutch. If the clutch is not fully releasing, the gearbox input shaft keeps spinning and every gear can crunch, including reverse. If the crunch is limited to one gear, particularly second or third on a car that has done a lot of town driving, the synchro for that gear is the likely culprit.

Jumping out of gear and stiff shifts

If the lever pops back to neutral on the overrun or under load, the gear is not being held in properly. Causes include worn selector forks, tired detent mechanisms, worn gear teeth or, less often, a problem with the engine and gearbox mountings letting the whole assembly move. This is a safety matter as much as an annoyance, because losing drive unexpectedly on a roundabout or when pulling out of a junction is not something to live with.

A stiff or vague shift is often outside the gearbox altogether. Worn linkage bushes, a stretched or sticking gear cable and old, contaminated oil can all make the lever feel heavy or sloppy. It is worth finding that out first, because it changes the repair completely.

• Crunch in one gear only: likely synchromesh wear
• Crunch in every gear: check the clutch first
• Pops out of gear: selectors, detents or worn teeth
• Vague or sloppy lever: linkage, bushes or cable
• Heavy lever when cold: old or wrong oil, worn synchros

Whining, humming and rumbling

Noise that changes with road speed rather than engine speed is the classic sign of bearing or gear wear inside the box. A whine in a particular gear that fades when you press the clutch in suggests worn gear teeth. A constant rumble that is there in every gear, and gets louder as you speed up, often points to a failing bearing.

It can be tricky to separate this from wheel bearings, tyres or the differential, which is why a road test and proper listening matter. Tyres with uneven wear can mimic a gearbox hum surprisingly well.

Oil level and oil condition

Manual gearboxes hold a fairly small amount of oil, and some are sealed from new with no dipstick or obvious filler. Low oil, often from a leaking output seal or a weeping casing joint, speeds up wear in everything described above. Gearbox oil that is dark, gritty or smells burnt tells its own story.

Different gearboxes call for different oil specifications, and the wrong one can make shifting worse, not better. Follow the manufacturer's guidance for your particular vehicle rather than topping up with whatever is to hand, and ask a garage to check the level if you are unsure how.

When to get it diagnosed

Do not wait for the box to lock up. Crunching, jumping out of gear, grinding that gets worse week by week, metal glitter on the drain plug or a sudden loss of a gear are all reasons to book transmission diagnostics sooner rather than later. A small repair, such as a seal, linkage or oil change, is a very different job from a full gearbox rebuild.

At Banks of the Tyne in Hebburn we can look at manual gearbox repairs, gearbox replacement and the related driveshafts and differentials, and we will explain what we find before any decision is made. What you pay depends on what is actually worn, how accessible the box is on your vehicle and the parts needed, which is why we do not guess before inspection.

Frequently asked questions