There are 3 main symptoms to look out for when diagnosing a manual gearbox:
– Jumps out of gear
– Gears grating
Jumping out of gear
When a vehicle jumps out of gear, 99% of the time this is due to a gear being worn. Unfortunately over time the contact pattern of the gears wear down and the 2 gears no longer mesh correctly. The most common cause of this symptom is high mileage and simple wear & tear.
This is the most basic symptom to diagnose because when you drive the vehicle, the gearbox will jump out of gear.
This happens when the synchronizers (is part of a synchromesh manual transmission that allows the smooth engagement of gears) or synchro-rings are worn and they do not lock onto the gear.
When trying to diagnose this fault, be aware that the clutch can also cause a grating into gear. We determine the difference by which gears grate when engaging. If the grating occurs when engaging 1st gear or reverse while the vehicle is stationary, the grating in usually caused by the clutch. If the grating occurs in any other gear while the vehicle is moving, this is usually caused by the syncromesh inside the gearbox.
Noise is by far the most difficult problem to diagnose. The main causes of noise can be: worn/noisy bearings; gear meshing (wear & tear); lack of oil (Burnt Gears) or Excessive play on the gears/shafts.
- Bearing Noise – Over time bearings tend to wear down which create an excess of play inside the gearbox allowing the gears and shafts to move around more than they should. This results in a whining noise.
- Gear Meshing – Overtime the contact pattern on the gears slowly wears down causing the gears to mesh incorrectly resulting in noise. This can also be the result of a bearing wearing away giving the gear more play than it should.
- Lack of oil – If a manual gearbox runs without oil the internals of the gearbox overheat and burn. Not visible to the naked eye, the gear itself and the teeth on the gear lose their shape which will create a noise from that particular gear. Even though the difference in shape is not visible, we are able to tell this very easily as the gear will turn blue in colour.
When diagnosing a noise in a gearbox, it is important to test the noise in various states before coming to a conclusive diagnosis.
Steps to diagnosing – jumping out of gear
- Drive the vehicle and in all gears
- Release and compress the clutch quickly and slowly
- With the clutch released, play with the throttle… full, release, full, release
- Make sure to record which gears are jumping out and when it occurred
Steps to diagnosing – gears grating
- With the car stationary, try to select 1st and reverse gear.
(Should there be grating into 1st or Reverse, it is most likely the clutch. However, the other gears still need to be tested with the car in motion)
- Test each gear change for grating (Upshift and downshift)
Steps to diagnosing – noise
- Begin driving the vehicle to replicate the noise.
- Test for the noise in all gears, especially in 4th
(If the noise is apparent in all gears but quiet in 4th gear, the fault is on the cluster and spigot shafts and they both need to be replaced. 4th gear is a direct drive @ a 1:1 ratio and does not run through the cluster gears)
- Replicate the noise at a constant speed and compress the clutch to see if the noise goes away or stays there. (If the noise goes away when the clutch is pressed in, the problem is most likely in the gearbox. If the noise does not go away with the clutch compressed, this can mean a bearing in the gearbox is noisy or that the noise is coming from the differential or another part of the vehicle. Eg. wheel bearings)