Manual Gearboxes: Symptoms & diagnostic

There are 3 main symptoms to look out for when diagnosing a manual gearbox:

–   Jumping out of Gear
–   Grating into Gear
–   Noisy

 

Jumping out of gear – When a vehicle jumps out of gear, 99% of the time this is due to the gear being worn. In the diagram below you will see that the original gear tooth pattern meshes together with a perfect point of contact. Unfortunately over time this contact pattern wears down to the red line on the gear tooth 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. We will discuss the different options for repair in the next section.

 

Grating into gear – This happens when the synchronizers or synchro rings are worn and they do not lock onto the gear. Let us have a look at a syncro and where exactly the wear occurs…

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 gears while the vehicle is moving, this is usually caused by the syncromesh inside the gearbox.

 

Noise – 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 – Once again, 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 have which in turn changes the contact pattern.
  • 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 of absolute importance to test the noise in various states before coming to a conclusive diagnosis.

Normally, when a customer brings a vehicle in for a gearbox problem they will tell you what their concern is… Grating, Jumping or Noisy.

 

Steps to diagnosing – jumping out of gear

  1. Drive the vehicle and in all gears
  2. While driving in ALL gears you must do the following and a separate record should be made for each gear:
    • 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 – grating into gear

  1. With the car stationary, try to select 1st gear and reverse while listening and feeling for a grating sound when engaging. Record your findings.
    (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)
  3. Make sure to record which gears are grating into gear

 

Steps to diagnosing – noise

  1. Before driving the vehicle, make sure that the customer has given precise details of when he/she can hear the noise.
  2. Begin driving the vehicle to replicate the noise and take note of when it happens.
  3. 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)
  4. 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)
  5. Record the outcome

 

Most of the time a noise cannot be diagnosed easily and the recommended way to diagnose noise is to open the gearbox to inspect.

NOTE: The recorded information from the steps above is extremely important to establish what to look for when the gearbox is open for inspection.