Clutch Troubleshooting Guide
In this clutch troubleshooting guide, NAP highlight the key components of the clutch system and identify common causes of clutch failure.
Damage of the diaphragm fingers by the gearbox input shaft during installation and overstroked diaphragms are common causes of cover assembly malfunction. Ensure that when re-fitting the gearbox that this is carried out without damaging any of the clutch components. To avoid overstroking of the diaphragm, clutch adjustment should be checked and set according to manufacturers specification. Before leaving the production facility all cover assemblies are checked for clamp loading, pressure plate lift and diaphragm run-out utilising computerised test equipment.
Below: Diagram of a diaphragm clutch
Clutch driven plates can become distorted during transit or more commonly during installation by 'hanging' of the gearbox on the driveshaft. The risk of distortion can be minimised by the use of an alignment tool and ensuring that the gearbox is installed without hanging on the clutch units. All driven plates are individually inspected before leaving the factory and tested for run-out and overall consistency of effective thickness using specially designed spin test machines. In house computerised evaluation equipment is available to measure (in both the drive and overun positions) the torsional damping characteristics of the driven plate.
Although it is not possible to fully check the operation of the release bearing when it is removed, it is false economy to re-use the old bearing as this often results in premature clutch failure. Therefore we recommend that it is always replaced. Refitment of the old component can also give excessive bearing noise and abnormal diaphragm finger/release lever wear. The bearing carrier should slide freely on the bearing quill - apply a recommended high temperature grease sparingly.
It is recommended that clutch cables are replaced when clutch components are renewed. Cables often stretch beyond adjust limits resulting in non-clearance. Ensure that the correct cable is fitted and adjusted in accordance with the manufacturers specification.
The flywheel is often ignored during clutch replacement but should always be checked before refitting new clutch components. Uneven facing wear on the flywheel side of the drive plate can indicate severe flywheel wear, usually observed as a badly grooved mating surface. Hot spots and thermal cracks indicate the clutch has been running at high temperatures. Any marking or grooves on the mating surface should be removed or if excessive the flywheel should be replaced. If the flywheel is to be machined, the original overall dimensions must be maintained. The cover assembly mounting flange must be machined down by the same amount as the contact surface of the flywheel. The opportunity should be taken to check the flywheel ring gear.
The spigot bearing can cause severe damage if worn as it may allow the gearbox to 'float'. This can result in excessive noise and misalignment leading to damage of the clutch components. A worn spigot bearing can also cause unbalance resulting in vibration. It is good practice to replace the spigot bearing/bush during clutch replacement.
Engine/Gearbox Oil Seals
Leaking oils seals are a major cause of clutch failure, allowing contamination of the clutch components. A small amount of contamination will affect the efficient operation of the clutch. Always check for traces of oil and replace any suspect oil seals.
Release Fork/Release Mechanism
A substantial amount of the clutch release stroke can be lost if wear on the release mechanism is not corrected. All parts such as the release fork, cross arm, bushes, and pivot blocks/pins should be checked and replaced if any wear is located. Always ensure the free operation of the linkage.
Release Bearing Quill
Wear on the side of the quill can affect the release bearing travel causing grab or chatter of the clutch and off centre release bearing contact with the diaphragm. Ensure that the quill is centred and concentric with the gearbox input shaft.
Engine & Gearbox Mountings
Worn or damaged parts can cause vibration and judder, and are often mistaken for clutch malfunction. Components showing any signs of wear or damage should be replaced.
Inefficient operation of the clutch and non-disengage problems can be caused by old, insufficient or air impregnated hydraulic fluid. Check operation of hydraulic components such as master and slave cylinders, examine for leaks and replace any substandard parts. Flush the system and bleed after clutch replacement.
Automatic Adjust Mechanisms
Faulty auto adjust systems can cause excessive clutch slip, diaphragm wear and non-engage problems. When these systems become inefficient it may be possible to replace with a manual adjustment. Ensure that cables are de-adjusted when new components are fitted and reset to vehicle manufacturers specifications.
As there are many differing specifications for adjustment, always refer to vehicle manufacturers instructions when setting this. Some vehicles require the release bearing to run in constant light contact with the cover assembly diaphragm, where others demand significant free play. Overall clutch adjustment is critical in obtaining efficient operation of the clutch - incorrect adjustment can lead to clutch slip, overheating, excessive diaphragm wear and release carrier contact with the diaphragm. These faults cause abnormal wear, broken components and premature clutch life.
For more detailed information, please refer to the clutch fault finder guide available here.