The clutch assembly is located between the engine and the transmission/transaxle. The purpose and function of a clutch include the following:
To disconnect the engine from the transmission/transaxle to permit the engine to remain running when the vehicle is stopped and to permit the transmission/transaxle to be shifted
Connect and transmit engine torque to the transmission/ transaxle
To dampen and absorb engine impulses and drivetrain vibration
To provide a smooth engagement and disengagement between the engine and the transmission/transaxle
NOTE: The term transmission refers to rear-wheel-drive vehicles and the term transaxle usually refers to front-wheel-drive vehicles that have a differential built into the unit. A separate differential is used with a transmission.
A clutch assembly consists of a clutch disc that is splined to the input shaft of the transmission/transaxle. When the driver depresses the clutch pedal, a throwout bearing ( release bearing ) is forced against the fingers of the pressure plate , which is bolted to and rotates with the flywheel. When force is exerted on the center of the pressure plate, the pressure is released from the clutch disc that has been forced against the engine flywheel. With the pressure removed from the clutch disc, the engine can be operated without transferring torque to the transmission/ transaxle. See Figure 94–1.
COMPONENT PARTS AND OPERATION OF A CLUTCH ASSEMBLY Continued
Figure 94–1 Typical automotive clutch assembly showing all related parts. (Courtesy of LUK) Continued
When the driver reduces force on the clutch pedal, the pedal return spring and the pressure plate spring combine to return the clutch pedal to its at-rest position (clutch-engaged position). When the clutch pedal moves up, the pressure on the throwout bearing is released and the force against the pressure plate spring(s) is released allowing the force of the pressure plate to clamp the clutch tightly between the flywheel and the pressure plate. See Figure 94–2.
Continued To summarize:
When the clutch pedal is up, the clutch is engaged.
When the clutch pedal is down, the clutch is disengaged.
Figure 94–2 (a) When the clutch is in the released position (clutch pedal depressed), the clutch fork is applying a force to the throwout (release) bearing, which pushes on the diaphragm spring, releasing the pressure on the friction disc. Continued
Figure 94–2 (b) When the clutch is in the engaged position (clutch pedal up), the diaphragm spring exerts force on the clutch disc, holding it between the flywheel and the pressure plate. Continued
Additional related parts include the pilot bearing (or bushing ) that supports the front of the transmission input shaft.
Figure 94–3 The transmission has just been removed. Note that this type of transmission incorporates the bell housing, which was therefore removed at the same time as the transmission. The clutch fork and throwout (release) bearing also came off together. All that remained attached to the engine was the flywheel, clutch disc, and pressure plate. The throwout bearing is often supported and rides on the transmission/transaxle front bearing retainer (called the quill ). Continued NOTE: Most front-wheel-drive transaxles do not use a pilot bearing.
Clutch Pedal Linkage
Figure 94–4 A typical cable- operated clutch. Continued
Levers and rods Through a series of levers and rods, the release fork is forced against the throwout bearing. This method was commonly used on many older vehicles.
Cable operation A cable is used similar to a brake cable used on a bicycle.
Clutch Pedal Linkage
Figure 94–6 A typical clutch master cylinder and reservoir mounted on the bulkhead on the driver’s side of the vehicle. Brake fluid is used in the hydraulic system to operate the slave cylinder located on the bell housing. Continued
Hydraulic A small master cylinder and a slave cylinder located near the throwout bearing is a very common method of connecting the clutch pedal to the release fork on vehicles equipped with manual transmission.
See also Figure 94 –5 following.
Figure 94–5 A hydraulic clutch linkage uses a master cylinder and a slave cylinder. Continued
Clutch Disc The clutch disc is round with a splined center hole that slips over the splines of the input shaft of the transmission/ transaxle. Friction material is riveted to both sides of the clutch disc—one side touches the flywheel of the engine and the other side touches the friction surface of the pressure plate. Friction material is either woven or molded from a mixture of other materials. Woven materials are softer and help cushion clutch engagement, but they may not last as long as moldedmaterials. Friction materials operate in high heat and pressure. For many years, asbestos was the most common material used in both clutch and brake linings. Today, clutch friction material may contain paper, cotton, and bits of copper or brass wire with resin holding the mixture together.
High-Performance Clutch Disc Another type of friction material is a ceramic and metallic mixture. This creates a hard, long-lasting lining, but is more expensive and does not cushion clutch engagement as much as a softer lining. Instead of a full circle of softer friction material, the disc may have only a few segments or buttons of this ceramic-metallic material. Clutches that use these discs, which are sometimes called “button clutches,” are found in racing applications where strength and durability are a greater concern than smooth engagement. See Figure 94–7.
CAUTION: Always take precautions when working around clutch lining material and assume that it contains asbestos. These precautions should include wetting the friction disc before removal and preventing any dust from the lining from becoming airborne.
Figure 94–7 A racing or high-performance clutch disc lacks the features of a stock clutch disc that help provide smooth engagement. Continued
Stock Clutch Disc Around the center hub of the clutch disc are torsional dampers that absorb the initial shock of engagement and help dampen engine-firing in pulses being transmitted into and through the transmission/transaxle. See Figure 94–8. The torsional dampers are either coil springs or made of rubber. In the space between the friction surfaces is a wavy spring steel material called a cushioning spring or marcel spring . The marcel spring also helps to absorb the initial shock of engagement and allows for a smooth engagement of the clutch. See Figure 94–9.
Figure 94–8 A typical stock clutch friction disc that uses coil spring torsional dampers. Figure 94–9 A marcel is a wavy spring that is placed between the two friction surfaces to cushion the clutch engagement. Continued
HINT: The larger center hub section of the disc must be installed with the thicker portion facing the pressure plate Figure 94–10 Cutaway of the center section of a clutch plate showing the various layers of steel plates used in the construction.
Pressure Plates The purpose of the pressure plate is to exert a force on the clutch disc so that engine torque can be transmitted from the engine to the transmission/transaxle. A strong spring(s) is required to provide adequate clamping force on the clutch disc. However, a strong pressure plate spring force must be released by the force of the driver’s foot to disengage the clutch. The pressure plate, like the flywheel, is usually made of nodular cast iron. A smooth, machined area on one side forms the friction disc contact surface. When the clutch engages, spring force pushes the pressure plate toward the flywheel so the friction disc is clamped between the flywheel and the pressure plate. Engine torque flows through the clutch to the transmission input shaft.
Coil spring style This style of pressure plate uses coil springs and three release arms. A coil-spring-style pressure plate is also called a lever style because it uses levers to compress the springs.
Figure 94–11 A coil spring (lever style) clutch pressure plate. Continued
Diaphragm spring style This style is the most commonly used pressure plate design. One large, round, spring-steel spring is used to apply even force on the clutch disc. These tend to be smaller assemblies, weigh less, and have fewer parts than coil spring assemblies. The one-piece diaphragm spring does the job of all the release levers and coil springs in a coil spring clutch. See Figure 94–12.
Figure 94–12 Typical diaphragm-style pressure plate that uses a Belleville spring. Continued
The driver must push harder on the clutch pedal to disengage a coil spring clutch than a comparable diaphragm spring design. The pedal effort for a coil spring clutch increases the farther down the pedal is pushed. The pedal effort for a diaphragm spring clutch decreases during the second half of pedal travel. In many cases, aftermarket clutch parts manufacturers supply a diaphragm spring pressure plate assembly when an original coil spring assembly must be replaced. If so, carefully compare the new pressure plate to the old one to make sure it is the correct replacement. Physical characteristics, such as dimensions and bolt-hole patterns, must be identical for the assembled clutch to operate properly. Check that the inner edges of the diaphragm spring fingers are the same height as the release levers on the original unit.
Flywheels The engine flywheel serves four basic purposes:
Smoothes out or dampens engine power pulses.
Absorbs some of the heat created by clutch operation.
Provides the connection point for the starter motor to crank the engine.
Provides application surface for the clutch friction disc.
A flywheel is heavy, or has a large mass, which creates inertia. The inertia provided by the flywheel mass tends to keep crankshaft speed more constant. The flywheel absorbs some of the heat created by clutch operation by acting as a heat sink for the clutch friction disc. An external ring gear is pressed or welded onto the flywheel along its outer circumference. When the starter motor is engaged, the starter-drive gear meshes with the flywheel ring gear. Through gear reduction, the flywheel transfers starter motor rotation to the crankshaft to crank the engine.
The face on the transmission side of the flywheel has a smooth, machined area that creates the application surface for the clutch friction disc. This surface must be properly finished to allow adequate slippage as the clutch engages and disengages, and to prevent slippage when the clutch is engaged.
Figure 94–13 A flywheel after it has been machined (ground) to provide the correct surface finish for the replacement clutch disc. Continued
The flywheel is constructed of cast iron and attaches to the end of the engine crankshaft. The carbon content of the cast iron (about 3%) provides a suitable surface for the clutch disc. The carbon, in the form of graphite, acts as a lubricant to provide a smooth engagement of the clutch. The weight of the flywheel helps to absorb and smooth out engine-firing impulses. A starter ring gear is welded or pressed onto the outside diameter of the flywheel.
Often the ring gear can be replaced separately without having to replace the flywheel in the event of a failure, as shown below.
Figure 94–14 The starter motor will spin but the engine will not crank if the ring gear on the flywheel is broken. Continued The pilot bearing is often installed in the center of the flywheel (or in the end of the crankshaft) to support the end of the input shaft of the transmission.
Dual-Mass Flywheels High-performance vehicles and vehicles equipped with diesel engines, use a dual-mass flywheel, to dampen engine vibrations and keep them from being transmitted to the passenger compartment through the transmission and shift linkage. A dual-mass flywheel consists of two separate flywheels attached with damper springs, friction material, and ball bearings to allow some movement between the primary and secondary flywheel. By allowing a very slight amount of movement between the two flywheels, the damper absorbs engine torque peaks and normal vibration to provide smoother drivetrain operation. The damper assembly is completely sealed, because it also contains a fluid or lubricant, typically silicone-based, which also helps absorb vibration and transmit torque.
Figure 94–15 A cutaway of a dual-mass flywheel used on a Ford diesel pickup truck.
The starter ring gear mounts on the primary flywheel.
Continued Power from the starter motor does not have to flow through the damper assembly to reach the engine crankshaft. The ring gear and pilot bearing are usually attached to the primary flywheel. The clutch friction surface is usually the secondary flywheel.
Whenever replacing a clutch, most experts agree that the flywheel should be removed from the engine and resurfaced. When material is removed from the surface of the flywheel, the geometry (relationship) of the clutch parts changes because the pressure plate is now closer to the rear of the engine by the amount removed from the flywheel. Ask your parts supplier for a shim equal in thickness to the amount of material removed during resurfacing. Generally, these round shims are available in 0.020’’ to 0.100” thicknesses. The shim is installed between the crankshaft flange and the flywheel. If a shim is not used, the flywheel may have to be replaced to properly restore proper clutch operation and service life.
Shim It or Replace It
If engine oil or transmission lube gets onto the friction surface of the clutch, the clutch will chatter when engaged. This grabbing and releasing of the clutch is not only harmful to the drivetrain (transmission, driveshaft, etc.) but also is disturbing to the driver when the vehicle vibrates and shakes while driving. To avoid the possibility of a chattering clutch, always repair oil leaks as soon as possible. Rocker (valve) cover gaskets, intake manifold gaskets, oil galley plugs, rear main seals, as well as the input shaft seal on the transmission/transaxle itself can all lead to clutch contamination.
NOTE: If the dual-mass flywheel fails, the symptom is the same as a slipping clutch. The torque-limiting friction material connecting the primary and secondary flywheels can fail. This failure requires the replacement of the flywheel assembly. Repair the Oil Leaks Before Replacing the Clutch - Part 1
Figure 94–16 (a) Before replacing the clutch, the bell housing should be cleaned and the clutch fork pivot lightly lubricated. Figure 94–16 (b) The input shaft seal should also be replaced to prevent the possibility of getting transmission lubricant on the friction surfaces of the clutch. Repair the Oil Leaks Before Replacing the Clutch - Part 2
Pilot Bearing or Bushing The transmission input shaft from the transmission, through the clutch assembly, to the engine. Transmission bearings support the transmission end of the shaft. Depending on the length of the input shaft, it may also need a pilot bearing or bushing to support it at the engine end. A front-wheel-drive vehicle with a stepped flywheel, compact clutch assembly, and a transaxle has a short input shaft that may not reach all the way to the flywheel. This design does not need a pilot bushing or bearing to support the engine end of the input shaft. See Figure 94–17.
Figure 94–17 A transaxle assembly has been removed to replace the clutch. Note the short input shaft. This vehicle did not use a pilot bearing (bushing). Continued
Other designs, such as a rear-wheel-drive vehicle with a flat flywheel, have a much longer transmission input shaft. At the engine, the transmission input shaft rests inside a small bore in the flywheel or crankshaft flange. The pilot bearing or bushing, which is pressed into this bore, supports the engine end of the input shaft and provides a low-friction surface for the shaft to ride on. This keeps the shaft and friction disc perfectly aligned with the flywheel and pressure plate. The pilot bearing or bushing rotates with the crankshaft when the engine is running. At times, the input shaft does not rotate when the engine is running. The pilot bearing or bushing lowers the friction between these two moving parts. Pilot bushings are usually a sintered metal sleeve pressed into the bore in the crankshaft flange.
The driver of a Mazda pickup complained it was difficult to shift the manual transmission into first gear and reverse. The hydraulic clutch system was checked and bled and seemed to be operating as designed. A defective clutch was not suspected because the clutch did not slip. The problem worsened. When the driver had trouble shifting into all gears, the tech decided that the transmission had to be removed and the clutch inspected. The pilot bearing was inspected. The needle bearings were broken. The input shaft of the transmission is supported by the pilot bearing located in the center of the flywheel. Apparently, what had happened was that the pilot bearing failed and the pieces of the bearing kept the input shaft to the transmission rotating whenever the engine was running whether or not the clutch was depressed or released. The tech replaced the pilot bearing and the other clutch components at the request of the owner and everything worked perfectly.
The Mazda Pickup Truck Story
Always follow the manufacturer’s lubrication instructions. Excess lubricant leaks out onto the clutch friction disc, which causes clutch slippage and early failure. Too little lubricant leads to noisy operation and input shaft damage. Replacement pilot bearings may need to be packed before installation. Sometimes, the bottom of the bore behind the bearing is also packed with grease. Other pilot bearings are permanently lubricated at the factory. Prelubricated bearings have an oil seal to hold the lubricant. If only one side is sealed, that side faces the transmission when installed. Some pilot bushings need to be soaked in oil before installation. Others have a permanent lubricant built into the bushing material. With this type, adding another lubricant actually increases friction, by preventing the permanent lubricant from doing its job.
Release Bearing The throwout (release) bearing is attached to the clutch fork and rides on the fingers of the pressure plate. The clutch operating system moves the clutch release bearing when the driver presses or releases the clutch pedal. Most systems move the release bearing toward the flywheel to disengage the clutch. See Figure 94–18
Figure 94–18 The clutch pedal linkage moves the clutch fork, which then applies a force against the release bearing, which then releases the clamping force the pressure plate is exerting on the clutch disc. Continued
However, some systems have a pull-type mechanism that moves the bearing away from the flywheel to disengage the clutch. The release bearing presses against the diaphragm spring fingers or coil spring levers. This takes spring force off the pressure plate so that it no longer clamps the friction disc against the flywheel. The diaphragm spring fingers or coil spring levers rotate at crankshaft speed, but the clutch operating system is a part of the vehicle chassis and does not rotate. The release bearing is the point where the fixed, stationary clutch operating system meets the rapidly spinning clutch assembly. See Figure 94–19.
Figure 94–19 The release bearing rubs against the tips of the diaphragm spring. Continued
Release Bearing Construction Most clutch release bearings are ball bearings. The bearing absorbs a thrust load when its outer race presses against the diaphragm spring fingers or coil spring levers. When the outer race contacts the spring fingers or levers, it must rotate with them at engine crankshaft speed. The inner bearing race is pressed onto an iron hub, or sleeve. In some designs, the inner bearing race and sleeve are machined as one piece. The inner bearing race and sleeve are stationary and do not spin when the outer race spins.
Release Bearing Installation The transmission front bearing retainer has a long, hollow tube extending toward the engine. The release bearing sleeve slides on the outer surface of this tube, which is commonly called the quill , quill shaft , or candlestick . In a typical system, the outside of the release bearing sleeve has grooves or raised flat surfaces that fit into the clutch release fork. A snap ring, spring clips, or lock pins secure the release bearing to the release fork. The clutch operating system pivots the release fork back and forth when the driver presses and releases the clutch pedal. The pivoting motion of the fork slides the release bearing away from or toward the engine to engage or disengage the clutch.
Types of Release Bearings If the clutch operating system self-adjusts, then there is no clearance between the release bearing outer race and the diaphragm spring fingers or coil spring levers. The release bearing outer race constantly turns at engine crankshaft speed. This is called a constant-running release bearing. In some self-adjusting systems, a snap ring holds the outer race to the spring fingers. This design is typical of pull-type clutch operating systems that move away from the flywheel to disengage the clutch.
If the clutch operating system does not self-adjust, then there must be some clearance between the release bearing outer race and the spring fingers when the clutch is engaged. The outer race does not contact the spring fingers and so it does not turn. As the driver depresses the clutch pedal, the release bearing moves into contact with the fingers and the outer race begins to rotate with them. This type of release bearing is not designed to rotate constantly. If the clutch is not adjusted properly and there is no clearance, the release bearing spins constantly and wears out quickly.
Release Bearing Lubrication The ball bearing portion of the release bearing is usually permanently lubricated and sealed during manufacture. This part of the bearing should not be lubricated during service. The sleeve, or quill shaft, often needs lubrication during clutch service. Typically, a thin film of high-temperature grease coats the sliding surfaces. Always follow the manufacturer’s recommendations for release bearing lubrication, and avoid overlubricating. As with pilot bearings, too much lubricant here can lead to early clutch failure.
CLUTCH PROBLEM DIAGNOSIS
f the clutch is slipping, it cannot transfer engine torque to the transmission or drive wheels. A common method used to check for a slipping clutch:
Drive the vehicle to a safe location where it can be accelerated safely.
Rapidly accelerate the vehicle in first or second gear and rapidly shift the transmission into a higher gear.
The engine speed should drop as the clutch is released after selecting a higher gear. If the clutch is slipping, the engine speed will either rise or not drop after the clutch pedal is released (clutch is engaged).
Another common problem with a clutch is that it sometimes will not fully disengage. The following symptoms will occur if there is a fault in the clutch or in the linkage or hydraulic system that could prevent the clutch from being fully disengaged:
The transmission will be difficult (or impossible) to shift into reverse.
The transmission will be difficult (or impossible) to shift.
See Figures 94–20 through 94–28 for examples of worn clutch components.
Figure 94–20 Heavy chatter marks on the pressure plate indicate that oil or grease has gotten onto the clutch facing. (Courtsey of LUK) Figure 94–21 Hot spots on the pressure plate indicate that this clutch has been slipping or that oil/grease has gotten onto the clutch facing. (Courtesy of LUK) Continued
Figure 94–22 Friction material (facing) worn down to the rivets. Normal wear can cause this or improper clamping force from a defective pressure plate. (Courtesy of LUK) Figure 94–23 Friction material (facing) only makes contact with the flywheel on the inner and outer edge. The likely cause is that the flywheel was not resurfaced or replaced during a previous repair. (Courtesy of LUK) Continued
Figure 94–24 Deep scoring in the friction material (facing) on the flywheel side indicates that the flywheel was not resurfaced or replaced during a previous repair. (Courtesy of LUK) Figure 94–25 A broken cushion segment caused by movement between the engine and transmission. The most likely cause is a missing or defective pilot bearing. (Courtesy of LUK) Continued
Figure 94–26 A destroyed torsional damper is usually caused by driving at too low an engine speed, which would cause too much strain on the torsional damper. (Courtesy of LUK) Figure 94–27 A worn down or broken diaphragm is often caused by a faulty release bearing or a linkage problem that keeps the clutch from fully releasing. (Courtesy of LUK) Continued
Figure 94–28 Rusting hub splines indicate that the transmission shaft was not lubricated, which can cause the clutch to not disengage correctly. (Courtesy of LUK) Continued
Many service technicians try to determine which part causes noise before the clutch assembly is removed from the vehicle. Start your noise analysis by starting the engine with the manual transmission in neutral and the clutch engaged (foot off the clutch pedal). If you hear a growl or grinding sound, the cause is the input shaft bearing in the transmission. If you are hearing a chirping noise, slowly depress the clutch pedal. If the noise stops, the problem is lack of lubrication at the fork and pivot. If the noise gets louder as you depress the clutch, the throwout (release) bearing is the problem. The pilot bearing is the cause of a squealing noise if the sound changes as the clutch pedal is depressed or released. The sound will be loudest when the difference in speed between the engine and the input shaft is greatest.
What Part Causes What Noise and When?
A beginning tech installed a new clutch in a Chevrolet S-10 pickup truck. The tech started the engine to check the operation of the clutch with the truck still on the lift and off the ground. The drive wheels never turned even though the clutch was released. The transmission was removed and the clutch parts inspected. Everything looked okay until the technician slid the clutch disc over the splines of the transmission input shaft. The diameter of the hole in the clutch disc was a lot larger than the diameter of the shaft. The clutch also simply revolved without even touching the input shaft. The tech received the wrong clutch disc from the parts department. The experienced tech explained that not only should all parts be carefully inspected before installation but that the clutch disc should have been slid over the splines of the input shaft to check for any possible burrs that could prevent the clutch disc from disengaging.
Best to Double-Check Your Parts Before Installation
It is always a safe idea to disconnect the negative ( – ) battery cable before performing major work to the vehicle as a safety precaution.
Disconnect the Battery Before Work Begins
Clutch replacement steps for a typical rear-wheel-drive vehicle:
Hoist the vehicle safely and mark and remove the driveshaft. This step ensures that the driveshaft will be reinstalled correctly and in phase.
Disconnect the shift linkage, speedometer connections, and reverse light switch connection.
Support the transmission with a transmission jack and then remove the rear crossmember and bell housing bolts.
Carefully move the transmission toward the rear. Try to keep the transmission level to avoid causing damage to the pilot bearing or clutch components. A slight wiggling of the transmission is usually necessary to allow the input shaft to slide over the spline of the clutch disc.
After the transmission has cleared the clutch, it can be lowered and inspected before being reinstalled after the clutch assembly has been replaced.
Mark the pressure plate and flywheel if they are to be reused to allow them to be reinstalled in the same location to maintain assembly balance.
Remove the clutch pressure plate retaining bolts, and remove the clutch assembly including the release bearing, pressure plate, and clutch disc.
Clean and inspect the flywheel.
Replace or lightly lubricate the pilot bearing/bushing.
NOTE: Most experts recommend that the flywheel be removed and resurfaced. Always use new flywheel bolts and torque to factory specifications.
Install the clutch disc and pressure plate using an aligning tool to center the clutch disc and then torque the pressure plate bolts to factory specifications.
Figure 94–29 A clutch alignment tool is inserted into the pilot bearing and over the splines of the clutch disc to keep the disc properly centered before installing the pressure plate. After the pressure plate has been installed and torqued to factory specifications, the alignment tool can be removed because the pressure plate spring force holds the friction disc tightly against the flywheel. Continued
Inspect the clutch release lever and replace if necessary.
Attach the replacement release bearing and install the transmission and torque the retaining bolts to factory specifications. Reconnect the clutch linkage and place the transmission into high gear and have an assistant depress the clutch pedal. Grasp the output shaft and turn it with your hand
The transmission output shaft should be easy to rotate if the clutch is fully disengaged with the clutch pedal held down. See Figure 94–30.
If the output shaft cannot be rotated, double-check that the clutch linkage and clutch is correctly installed.
Figure 94–30 To check that the clutch is properly installed before replacing all of the components, try to turn the output shaft with the transmission in gear and the clutch pedal depressed by an assistant. Continued
Reinstall the rear crossmember and torque the retaining bolts to factory specifications.
Reconnect the shift linkage, speedometer, and reverse light connections.
Install the driveshaft and torque the U-joint retaining fastener to factory specifications.
Lower the vehicle and test-drive it to verify correct clutch operation.
CLUTCH PEDAL ADJUSTMENT
As the clutch disc wears, the pressure plate moves closer to the flywheel. This causes the clutch to engage when the clutch pedal is higher and higher. Eventually, the clutch will start to slip if the linkage is not adjusted to compensate for the normal wear. Most hydraulic clutch linkage is self-adjusting and does not require any adjustment in normal service. Some cable-operated clutches are either self-adjusting or are adjusted easily. Some cable-operated clutch linkage is adjusted either at the bulkhead (firewall) where the cable exits the passenger compartment or at the clutch fork. Follow the adjustment procedures specified for the exact vehicle being serviced.
Most adjustment procedures specify a clutch pedal freeplay of 0.5 to 1.0 inch. Freeplay is the distance the clutch pedal can be depressed without encountering any noticeable resistance. This slight amount of freeplay assures that the clutch is fully engaged and not partially engaged as could occur if there is no freeplay in the pedal linkage.
Many cable-operated clutch linkage problems are caused by flexing of the bulkhead (firewall) as the clutch pedal is depressed. This movement of the bulkhead can cause the clutch to slip. To prevent unnecessary clutch repairs, have an assistant observe the bulkhead near the clutch cable while the clutch pedal is depressed. If there is movement of the sheet metal bulkhead, add reinforcing steel plate to the area.
Check for Bulkhead Flex
BLEEDING THE HYDRAULIC CLUTCH
If the hydraulic system has been opened, air has to be bled from the system. Fill the master cylinder with DOT 3 brake fluid and open the bleeder valve at the slave cylinder. Gravity should force the brake fluid to flow downward, expelling any trapped air that may be in the system. When brake fluid starts to drip from the bleeder valve, close the bleeder valve and check the clutch for normal operation. Sometimes. pressing the clutch pedal slowly as an assistant opens the bleeder valve at the slave cylinder will force trapped air out. This is manual bleeding and should be repeated until normal clutch operation returns. See Figures 94–31 and 94–32.
Figure 94–31 The clutch slave cylinder is often corroded because of moisture absorbed by the brake fluid used in the hydraulic clutch. This slave cylinder was disassembled to see if it could be overhauled rather than replaced. Continued
Figure 94–32 (a) The replacement hydraulic clutch for a Saturn includes the master cylinder (shown) with the line and the slave cylinder as an assembly. The assembly is even filled with brake fluid! Do not open the master cylinder cap on this unit because Saturn did not provide any method of bleeding air out of the ignition. Continued
Figure 94–32 (b) The slave cylinder attaches to the bell housing of the transaxle. Continued
CLUTCH SYMPTOM GUIDE
Drive the vehicle to a safe location where it can be accelerated safely.
Rapidly accelerate the vehicle in first or second gear and rapidly shift the transmission into a higher gear.
Clutch disc has oil on the surface.
Clutch linkage is binding.
Pilot bearing is defective or worn.
Release bearing is defective or worn.
PHOTO SEQUENCE Clutch Replacement Step-by-Step
PHOTO SEQUENCE Clutch Replacement
( cont. ) Continued
PHOTO SEQUENCE Clutch Replacement
( cont. )
The clutch not only transmits engine torque from the flywheel to the transmission/transaxle it also absorbs engine-firing impulses to reduce the vibration felt by the driver through the shift linkage.
The clutch has to be depressed (clutch disengaged) to keep the engine running with the transmission in gear and to shift from one gear to another.
The clutch disc has friction material on both sides and is clamped between the engine flywheel and the pressure plate.
The pressure plate bolts to and revolves with the flywheel.
A throwout (release) bearing is used to disengage the clutch by depressing a lever or spring, which removes the clamping force from the clutch disc
The pilot bearing is supported by the front bearing retainer of the transmission.
The linkage between the clutch pedal and the clutch fork can be levers and rods, a cable, or a hydraulic connection.
The flywheel attaches to the engine and not only provides the friction surface for one side of the clutch disc but also dampens engine-firing impulses.
A dual-mass flywheel is used on many import, luxury, and diesel vehicles to help dampen engine and driveline vibrations.
The clutch linkage can be adjusted on some vehicles to provide about 1/2 inch to 1 inch of clutch pedal freeplay.
Continued ( cont. )
It is a normal service procedure to replace the clutch disc, pressure plate, and throwout bearing as well as the pilot bearing every time the clutch is replaced. The flywheel should also be removed from the engine and resurfaced or replaced.