Clutch adjustments are made to compensate for wear of the clutch disc lining and linkage between the clutch pedal and the clutch release lever. This involves setting the correct amount of free play in the release mechanism. Too much free play causes the clutch to drag during clutch disengagement. Too little free play causes clutch slippage. It is important for you to know how to adjust the three types of clutch release mechanisms.
Mechanical clutch linkage is adjusted at the release rod going to the release fork (Figure 10-2). One end of the release rod is threaded. The effective length of the rod can be increased to raise the clutch pedal (decrease free travel). It can also be shortened to lower the clutch pedal (increase free travel). To change the clutch adjustment, loosen the release rod nuts. Turn the release rod nuts on the threaded rod until you have reached the desired free pedal travel.
When a new pressure plate is installed, do not forget to check the plate for proper adjustments. These adjustments will ensure proper operation of the pressure plate. The first adjustment ensures proper movement of the pressure plate in relation to the cover. With the use of a straightedge and a scale as shown in Figure 9-9, begin turning the adjusting screws until you obtain the proper clearance between the straight-edge and the plate as shown. For exact measurements, refer to the manufacturer’s service manual.
Figure 9-9 — Pressure plate adjustment
The second adjustment positions the release levers and allows the release bearing to contact the levers simultaneously while maintaining adequate clearance of the levers and disc or pressure plate cover. This adjustment is known as finger height. To adjust the pressure plate, place the assembly on a flat surface and measure the height of the levers, as shown in Figure 9-10. Adjust it by loosening the locknut and turning. After the proper height has been set, make sure the locknuts are locked and staked with a punch to keep them from coming loose during operations. Exact release lever height can be found in the manufacturer’s service manual.
Figure 9-10 — Pressure plate release lever adjustment
Like the mechanical linkage, a clutch cable adjustment may be required to maintain the correct pedal height and free travel. Typically the clutch cable will have an adjusting nut. When the nut is turned, the length of the cable housing increases or decreases. To increase clutch pedal free travel, turn the clutch cable housing nut to shorten the housing, and, to decrease clutch pedal free travel, turn the nut to lengthen the housing.
The hydraulically operated clutch is adjusted by changing the length of the slave cylinder pushrod. To adjust a hydraulic clutch, simply turn the nut or nuts on the pushrod as needed.
When a clutch adjustment is made, refer to the manufacturer's service manual for the correct method of adjustment and clearance. If no manuals are available, an adjustment that allows 1 1/2 inches of clutch pedal free travel will allow adequate clutch operation until the vehicle reaches the shop and manuals are available.
An automotive clutch normally provides dependable service for thousands of miles. However, stop and go traffic will wear out a clutch quicker than highway driving. Every time a clutch is engaged, the clutch disc and other components are subjected to considerable heat, friction, and wear.
Operator abuse commonly causes premature clutch troubles. For instance, "riding the clutch," resting your foot on the clutch pedal while driving, and other driving errors can cause early clutch failure.
When a vehicle enters the shop for clutch troubles, you should test drive the vehicle. While the vehicle is being test driven, you should check the action of the clutch pedal, listen for unusual noises, and feel for clutch pedal vibrations.
Gather as much information as you can on the operation of the clutch. Use this information, your knowledge of clutch principles, and a service manual troubleshooting chart to determine which components are faulty.
There are five types of clutch problems—slipping, grabbing, dragging, abnormal noises, and vibration. It is important to know the symptoms produced by these problems and the parts that might be causing them.
Slipping occurs when the driven disc fails to rotate at the same speed as the driving members when the clutch is fully engaged. This condition results whenever the clutch pressure plate fails to hold the disc tight against the face of the flywheel. If clutch slippage is severe, the engine speed will rise rapidly on acceleration while the vehicle gradually increases in speed. Slight but continuous slippage may go unnoticed until the clutch facings are ruined by excessive temperature caused by friction.
Normal wear of the clutch lining causes the free travel of the clutch linkage to decrease, creating the need for adjustment. Improper clutch adjustment can cause slippage by keeping the release bearing in contact with the pressure plate in the released position. Even with your foot off the pedal, the release mechanism will act on the clutch fork and release bearing.
Some clutch linkages are designed to allow only enough adjustment to compensate for the lining to wear close to the rivet heads. This prevents damage to the flywheel and pressure plate by the rivets wearing grooves in their smooth surfaces.
Other linkages will allow for adjustment after the disc is worn out. When in doubt whether the disc is worn excessively, remove the inspection cover on the clutch housing and visually inspect the disc. Binding linkage prevents the pressure plate from exerting its full pressure against the disc, allowing it to slip. Inspect the release mechanism for rusted, bent, misaligned, sticking, or damaged components. Wiggle the release fork to check for free play. These problems result in slippage.
A broken motor mount (engine mount) can cause clutch slippage by allowing the engine to move, binding the clutch linkage. Under load, the engine can lift up in the engine compartment, shifting the clutch linkage and pushing on the release fork.
Grease and oil on the disc will also cause slippage. When this occurs, locate and stop any leakage, thoroughly clean the clutch components, and replace the clutch disc. This is the only remedy.
If clutch slippage is NOT caused by a problem with the clutch release mechanism, then the trouble is normally inside the clutch. You have to remove the transmission and clutch components for further inspection. Internal clutch problems, such as weak springs and bent or improperly adjusted release levers, will prevent the pressure plate from applying even pressure. This condition allows the disc to slip.
To test the clutch for slippage, set the emergency brake and start the engine. Place the transmission or transaxle in high gear. Then try to drive the vehicle forward by slowly releasing the clutch pedal. A clutch in good condition should lock up and immediately kill the engine. A badly slipping clutch may allow the engine to run, even with the clutch pedal fully released. Partial clutch slippage could let the engine run momentarily before stalling.
Never let a clutch slip for more than a second or two. The extreme heat generated by slippage will damage the flywheel and pressure plate faces
A grabbing or chattering clutch will produce a very severe vibration or jerking motion when the vehicle is accelerated from a standstill. Even when the operator slowly releases the clutch pedal, it will seem like the clutch pedal is being pumped rapidly up and down. A loud bang or chattering may be heard as the vehicle body vibrates.
Clutch grabbing and chatter is caused by problems with components inside the clutch housing (friction disc, flywheel, or pressure plate). Other reasons for a grabbing clutch could be oil or grease on the disc facings, glazing, or loose disc facings. Broken parts in the clutch, such as broken disc facings, broken facing springs, or a broken pressure plate, will also cause grabbing.
There are several things outside of the clutch that will cause a clutch to grab or chatter when it is being engaged. Loose spring shackles or U-bolts, loose transmission mounts, and worn engine mounts are among the items to be checked. If the clutch linkage binds, it may release suddenly to throw the clutch into quick engagement, resulting in a heavy jerk. However, if all these items are checked and found to be in good condition, the trouble is inside the clutch itself and will have to be removed for repair.
A dragging clutch will make the transmission or transaxle grind when trying to engage or shift gears. This condition results when the clutch disc does not completely disengage from the flywheel or pressure plate when the clutch pedal is depressed. As a result, the clutch disc tends to continue turning with the engine and attempts to drive the transmission.
The most common cause of a dragging clutch is too much clutch pedal free travel. With excessive free travel, the pressure plate will not fully release when the clutch pedal is pushed to the floor. Always check the clutch adjustments first. If adjustment of the linkage does not correct the trouble, the problem is in the clutch, which must be removed for repair.
On the inside of the clutch housing, you will generally find a warped disc or pressure plate, oil or grease on the friction surface, rusted or damaged transmission input shaft, or improper adjustment of the pressure plate release levers causing the problem.
Faulty clutch parts can make various noises. When an operator reports that a clutch is making noise, find out when the noise is heard. Does the sound occur when the pedal is moved, when in neutral, when in gear, or when the pedal is held to the floor? This will assist you in determining which parts are producing these noises.
An operator reports hearing a scraping, clunking, or squeaking sound when the clutch pedal is moved up or down. This is a good sign of a worn or unlubricated clutch release mechanism. With the engine off, pump the pedal and listen for the sound. Once you locate the source of the sound, you should clean, lubricate, or replace the parts as required.
Sounds produced from the clutch when the clutch is initially engaged are generally due to friction disc problems, such as a worn clutch disc facing, which causes a metal-to- metal grinding sound. A rattling or a knocking sound may be produced by weak or broken clutch disc torsion springs. These sounds indicate problems that require the removal of the transmission and clutch assembly for repair.
If clutch noises are noticeable when the clutch is disengaged, the trouble is most likely the clutch release bearing. The bearing is probably either worn or binding, or, in some cases, is losing its lubricant. Most clutch release bearings are factory lubricated; however, on some larger trucks and construction equipment, the bearing requires periodic lubrication. A worn pilot bearing may also produce noises when the clutch is disengaged. The worn pilot bearing can let the transmission input shaft and clutch disc vibrate up and down, causing an unusual noise.
Sounds heard in neutral, which disappear when the clutch pedal is pushed, are caused by problems inside the transmission. Many of these sounds are due to worn bearings. However, always refer to the troubleshooting chart in the manufacturer's manual.
A pulsating clutch pedal is caused by the runout (wobble or vibration) of one of the rotating members of the clutch assembly. A series of slight movements can be felt on the clutch pedal. These pulsations are noticeable when light foot pressure is applied. This is an indication of trouble that could result in serious damage if not corrected immediately. There are several conditions that can cause these pulsations. One possible cause is misalignment of the transmission and engine.
If the transmission and engine are not in line, detach the transmission and remove the clutch assembly. Check the clutch housing alignment with the engine and crankshaft. At the same time, check the flywheel for runout, since a bent flywheel or crankshaft flange will produce clutch pedal pulsation. If the flywheel does not seat on the crankshaft flange, remove the flywheel. After cleaning the crankshaft flange and flywheel, replace the flywheel, making sure a positive seat is obtained between the flywheel and the flange. If the flange is bent, the crankshaft must be replaced.
Other causes of clutch pedal pulsation include bent or maladjusted pressure plate release levers, a warped pressure plate, or a warped clutch disc. If either the clutch disc or pressure plate is warped, they must be replaced.
When adjustment or repair of the linkage fails to remedy problems with the clutch, you must remove the clutch for inspection. Discard any faulty parts and replace them with new or rebuilt components. If replacement parts are not readily available, a decision to use the old components should be based on the manufacturer’s and the maintenance supervisor’s recommendations.
Transmission or transaxle removal is required to service the clutch. Always follow the detailed directions in the service manual. To remove the clutch in a rear-wheel drive vehicle, remove the drive shaft, the clutch fork, the clutch release mechanism, and the transmission. With a front-wheel drive vehicle, the axle shafts (drive axles), the transaxle, and, in some cases, the engine must be removed for clutch repairs.
When the transmission or transaxle is removed, support the weight of the engine. Never let the engine, the transmission, or the transaxle be unsupported. The transmission input shaft, clutch fork, engine mounts, and other associated parts could be damaged.
After removal of the transmission or transaxle bolts, remove the clutch housing from the rear of the engine. Support the housing as you remove the last bolt. Be careful not to drop the clutch housing as you pull it away from the dowel pins.
Using a hammer and a center punch, mark the pressure plate and flywheel. You will need these marks when reinstalling the same pressure plate to assure correct balancing of the clutch.
With the clutch removed, clean and inspect all components for wear and damage. After cleaning, inspect the flywheel and pressure plate for signs of unusual wear, such as scoring or cracks. Use a straightedge to check for warpage of the pressure plate. Using a dial indicator, measure the runout of the flywheel. The pressure plate release levers should show very limited or no signs of wear from contact with the release bearing. If you note excessive wear, cracks, or warping on the flywheel and/or pressure plate, you should replace the assembly. This is also a good time to inspect the ring gear teeth on the flywheel. If they are worn or chipped, install a new ring gear.
A clutch disc contains asbestos—a cancer-causing substance. Be careful how you clean the parts of the clutch. Avoid using compressed air to blow clutch dust from the parts.
While inspecting the flywheel, you should check the pilot bearing in the end of the crankshaft. A worn pilot bearing will allow the transmission input shaft and clutch disc to wobble up and down. Using a telescoping gauge and a micrometer, measure the amount of wear in the bushing. For wear measurements of the pilot bearing, refer to the service manual. If a roller bearing is used, rotate them. They should turn freely and show no signs of rough movement. If replacement of the pilot bearing is required, the use of a slide hammer puller will drive the bearing out of the crankshaft end. Before installing a new pilot bearing, check the fit by sliding it over the input shaft of the transmission. Then drive the new bearing into the end of the crankshaft.
Inspect the disc for wear; inspect the depth of the rivet holes, and check for loose rivets and worn or broken torsion springs. Check the splines in the clutch disc hub for a "like new" condition. Inspect the clutch shaft splines by placing the disc on the clutch shaft and sliding it over the splines. The disc should move relatively free back and forth without any unusual tightness or binding. Normally, the clutch disc is replaced anytime the clutch is torn down for repairs.
Another area to inspect is the release bearing. The release bearing and sleeve are usually sealed and factory packed (lubricated). A bad release bearing will produce a grinding noise whenever the clutch pedal is pushed down. To check the action of the release bearing, insert your fingers into the bearing; then turn the bearing while pushing on it. Try to detect any roughness; it should rotate smoothly. Also, inspect the spring clip on the release bearing or fork. If bent, worn, or fatigued, the bearing or fork must be replaced.
The last area to check before reassembly is the clutch fork. If it is bent or worn, the fork can prevent the clutch from releasing properly. Inspect both ends of the fork closely.
Also, inspect the clutch fork pivot point in the clutch housing; the pivot ball or bracket should be undamaged and tight.
When you install a new pressure plate, do not forget to check the plate for proper adjustments. These adjustments were covered in a previous section.
Reassemble the clutch in the reverse order of disassembly. Mount the clutch disc and pressure plate on the flywheel. Make sure the disc is facing in the right direction.
Usually, the disc's offset center (hub and torsion springs) fit into the pressure plate.
If reinstalling, line up the old pressure plate using the alignment marks made before disassembly. Start all of the pressure plates bolts by hand. Never replace a clutch pressure plate bolt with a weaker bolt. Always install the special case-hardened bolt recommended by the manufacturer.
Use a clutch alignment tool to center the clutch disc on the flywheel. If an alignment tool is unavailable, you can use an old clutch shaft from the same type of vehicle. Tighten each pressure plate bolt a little at a time in a crisscross pattern. This will apply equal pressure on each bolt as the pressure plate spring(s) are compressed. When the bolts are snugly in place, torque them to the manufacturer’s specifications found in the service manual. Once the pressure plates bolts are torqued to specification, slide out the alignment tool. Without the clutch disc being centered, it is almost impossible to install the transmission or transaxle.
Next, install the clutch fork and release bearing in the clutch housing. Fit the clutch housing over the rear of the engine. Dowels are provided to align the housing on the engine. Install and tighten the bolts in a crisscross manner.
Install the transmission and drive shaft or the transaxle and axle shafts. Reconnect the linkages, the cables, any wiring, the battery, and any other parts required for disassembly. After all parts have been installed, adjust the clutch pedal free travel as prescribed by the manufacturer, and test drive the vehicle for proper operation.