All about faulty Gearboxes

Causes of a faulty gearbox:

Gears jumping out of gear

General wear and tear over a long period of time causes the contact patterns of different gears to wear down and jump out of gear. This is usually seen in cars with high mileage.

Gears grating

Gears grating happens when the synchronizers/ synchro-rings (a kind of clutch that allows gear components turning at different speeds to synchronize their speed by using friction) are worn down and they do not lock onto the gear. 

Gearbox Noise

Gearbox noise is the most difficult problem to diagnose and can be caused by a variety of reasons including: 

Worn down or noisy bearings 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.

As a result of wear and tear on the contact patterns of gears, gear meshing creates excessive noise. It can also be as a result of a bearing wearing away giving the gear more play than it should.

The lack of oil, incorrect lubrication, under filling, overfilling and incorrect compatibility result in gearboxes overheating and burning and because of this, gear teeth lose their shape and start making noise. Even though the difference in shape is not visible to the naked eye, it is easily identified as the gear will turn blue in colour. 

Water contamination disables the lubricating properties of oil and causes foaming of
the oil. Water in oil can result from condensation or failing seals allowing water ingress.

Further, particulate contamination can happen from putting dirty oil into the gearbox, leaving a filling plug open or from wear or failure particles within the gearbox. Not only can this be an indicator of problems within the gearbox itself but the particles can also accelerate wear and component breakdown.

Improper handling and installation can cause component failure. This applies in particular to bearings where the cages are particularly prone to careless handling, installation or removal, but can equally apply to misaligned shafts which can then lead to bearing or gear failure.


Steps in diagnosing a faulty gearbox:

When diagnosing a noise in a gearbox, it is important to test the noise in various states before coming to a conclusive diagnosis.

Gears jumping out of gear

  1. Drive the vehicle and in all gears
  2. Release and compress the clutch quickly and slowly
  3. With the clutch released, play with the throttle… full, release, full, release
  4. Make sure to record which gears are jumping out and when it occurred

Gears grating

  1. 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)
  2. Test each gear change for grating (Upshift and downshift)

Noisy gears

  1. Begin driving the vehicle to replicate the noise.
  2. 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)
  3. 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)

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Posted on Sep 21, 2020 by Auto Repair Directory

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