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Clutch Symptoms (760) 741-7861
The pressure plate is normally closed clamping the disc between the pressure plate and the flywheel. When the clutch pedal is depressed the pressure plate is forced open creating a very small space between the disc, flywheel and pressure plate. When the disc moves away from the flywheel, power flow from the engine to the transmission is interrupted. When the clutch is engaged, the disc slips briefly to provide smooth engagement and, then clamps the disc against the flywheel. This causes the input shaft to turn, transmitting power to the transmission.
The disc hub is splined to fit the input shaft, provides smooth engagement and dampens engine vibrations via torsion springs in the damper assembly. It is mounted to the input shaft between the flywheel and the pressure plate. It can slide forward and backward on the input shaft, but cannot rotate without rotating the transmission input shaft.
The pressure plate clamps the disc against the flywheel during engagement. During disengagement it releases pressure on the disc, creating a gap large enough for the disc to move away from the flywheel and enable the driver to shift gears.A typical diaphragm spring clutch consists of a pressure plate, diaphragm spring, pivot ring, drive straps, and a cover. When the release bearing contacts the tips of the diaphragm spring fingers, it moves them toward the flywheel. The outside diameter of the diaphragm spring pivots on the pivot ring inside the cover. This action lifts the pressure plate off the flywheel through the drive straps that connect the cover to the pressure plate.Lever style clutches produce clamp load by pressure from coil springs. As the friction material of the disc wears, the springs expand, reducing their clamping force. At the same time, pedal effort remains high. As a result of these disadvantages, passenger cars and light trucks are now almost exclusively equipped with diaphragm spring clutches.Diaphragm spring clutches maintain higher clamp load than lever style clutches throughout the service life of the clutch. As disc friction material wears, clamp load increases during the first half of clutch life before decreasing gradually to its original level. Diaphragm spring clutches require less pedal effort the further the pedal is actuated, reducing stress on release system components.
Bolted to the end of the crankshaft, the flywheel provides the mounting surface for the clutch. During engagement, the disc is clamped against the flywheel by the pressure plate.In addition to its other functions, the flywheel acts as a heat sink, dissipating heat and moving it away from the clutch pressure plate and disc friction material. The flywheel must provide a smooth, flat surface in order for the clutch to operate properly.
When depressing the clutch pedal the fork movement causes the release bearing to move along the transmission-bearing retainer forcing the diaphragm spring fingers to open the pressure plate. The internal bearings in the release bearing enable it to turn while applying pressure to the diaphragm spring fingers. The bearing retainer must be parallel to the input shaft and provide a smooth surface for the release bearing to function properly.Angular-contact bearings, found in hydraulic release systems and self-adjusting cable systems, are in constant contact with the diaphragm spring fingers. Self- centering bearings are designed to compensate for slight misalignment between the engine and transmission. It is normal for these bearings to be "off center" until they contact the diaphragm spring fingers.Some vehicle designs utilize a concentric slave cylinder. It eliminates the need for a number of release system components, including the release fork, pivot ball and bearing retainer. They are located inside the bell housing and should replaced when replacing the clutch to avoid rework and additional labor cost.
The pilot bearing/bushing supports the end of the transmission input shaft and centers the disc on the flywheel. Types of pilots include conventional ball bearings, needle bearings and sintered bronze bushings.A small and relatively inexpensive component, the pilot bearing or bushing should always be replaced during clutch installation. The variety of problems caused by a worn or defective pilot bearing or bushing are not worth the risk of having to remove the transmission to replace this component later.
Hard-pedal conditions are often caused by wear damage or improper lubrication of the components. Inspect the release system for a damaged fork, pivot ball, release bearing or bearing retainer. In a mechanical release system, inspect all of the linkages for binding.
Proper lubrication during clutch installation can reduce the chances of noise problems down the road. Squeals and growls are usually caused by worn or seized bearings. Chirping noises are usually caused by vibration somewhere in the release system. Determine the source of bearing noise by setting the parking brake, putting the vehicle in neutral and starting the engine. Refer to the chart below to determine the source of the noise.
When the pressure plate does not release the disc completely, the disc continues to turn the transmission input shaft and gears. Causes of incomplete release include excessive free play, air in the hydraulic system, binding or wear in release system components and a bent disc.Movement of the master cylinder will also cause release problems. This is especially noticeable when the master cylinder is mounted to the firewall, and the firewall metal is not rigid enough to hold it in place during actuation. Any movement of the master cylinder reduces the force applied to the hydraulic piston, causing incomplete release.
Normal wear is the most likely cause of slipping if the disc is worn down to the rivets and the clutch has high mileage. If a newly installed clutch is slipping, the most likely causes are oil or grease contamination, incorrect release system adjustment or improper flywheel machining of a stepped or cupped flywheel.
To determine if the clutch slips too much, start the engine with the emergency brake set, put the transmission in high gear and slowly release the clutch pedal. If the engine stalls immediately, the clutch is not slipping. If the engine continues to run when the clutch is fully engaged, the clutch is slipping excessively. If the engine slows but continues to run, the clutch is slipping partially.
If the vehicle is equipped with a dual-mass flywheel, the flywheel may be the cause of the slipping. Carefully examine the old clutch to rule out the flywheel as the source of the slipping. Indications include heat marks on the pressure plate, disintegrated disc friction material and contamination of the friction material from external leaks. If these symptoms are not evident, install a new flywheel.