LuK Clutch Diagnostics: Main Causes of Clutch Failure

Noise can resonate from many areas of a vehicle’s driveline. There are several types of noise associated with the clutch assembly.

The release bearing is most often blamed for being the cause of noise, when in many cases it is not the release bearing at all. For example, technicians often describe a squeaking noise emanating from the bell housing. This noise occurs while the vehicle is idling in neutral, and goes away when slight pressure to the clutch pedal is applied. Technicians frequently mistake the release bearing as the source of this squeaking. The true cause is low release system preload. A defective or worn release bearing will make more noise when the clutch pedal is depressed.

Modern hydraulic and self-adjusting cable clutch release systems require a preload on the release bearing. The bearing requires a preload of approximately 28 lbs. This preload causes the release bearing to run constantly on the clutch diaphragm fingers when the engine is running. Insufficient preload allows the release system components, such as the clutch release fork, to be loose on the pivot points. The fork will move around more than when the preload is correct, often causing a squeaking or clunking noise.

In hydraulic systems, the preload is provided by a spring in the slave cylinder. To prevent or eliminate noise associated with low preload, install a new slave cylinder.

In cable systems, the preload is provided by the ratchet and pawl. To prevent or eliminate noise associated with low preload, replace the cable with a new cable. Replacing the cable will allow the ratchet and pawl mechanism to maintain the proper tension and provide the correct preload to eliminate noise. Additionally, be sure to inspect the ratchet and pawl for excessive wear.

Important Tip: A faulty release bearing will make MORE noise when a load is applied (clutch pedal depressed) NOT LESS.