In a vehicle, the mechanism that transmits the power developed by the engine
to the wheels and/or tracks and accessory equipment is called the power train.
In a simple application, a set of gears or a chain and sprocket could perform
this task. However, automotive and construction equipment are not designed for
such simple operating conditions. They are designed to provide pulling power, to
move at high speeds, to travel in reverse as well as forward, and to operate on
rough terrain as well as smooth roads. To meet these varying conditions, vehicle
power trains are equipped with a variety of components. This manual discusses
the basic automotive clutch, transmissions (manual and automatic), and
transaxles (manual and automatic).
When you have completed the work in this manual, you will be able to:
An automotive clutch is used to connect and disconnect the engine and manual transmission or transaxle. The clutch is located between the back of the engine and the front of the transmission.
With a few exceptions, the most common types of clutches are the single, double, and multiple-disc types. The clutch that you will encounter the most is the single-disc type (Figure 1). The double-disc clutch is substantially the same as the single-disc, except that another driven disc and an intermediate driving plate are added. This clutch is used in heavy-duty vehicles and construction equipment. The multiple-disc clutch is used in the automatic transmission and for the steering clutch used in tracked equipment.
Figure 1 — Single disk clutch.
The operating principles, component functions, and maintenance requirements are essentially the same for each of the three clutches mentioned. This being the case, the single-disc clutch will be used to acquaint you with the fundamentals of the clutch.
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The clutch is the first drive train component powered by the engine crankshaft. The clutch lets the driver control power flow between the engine and the transmission or transaxle. Before understanding the operation of a clutch, you must first become familiar with the parts and their functions. This information is very useful when learning to diagnose and repair the clutch assembly.
A clutch release mechanism allows the operator to operate the clutch. Generally, it consists of the clutch pedal assembly, a mechanical linkage, cable, or hydraulic circuit, and the clutch fork. Some manufacturers include the release bearing as part of the clutch release mechanism.
A clutch linkage mechanism uses levers and rods to transfer motion from the clutch pedal to the clutch fork. One configuration is shown in Figure 2. When the pedal is pressed, a pushrod shoves on the bell crank and the bell crank reverses the forward movement of the clutch pedal. The other end of the bell crank is connected to the release rod. The release rod transfers bell crank movement to the clutch fork. It also provides a method of adjustment for the clutch.
Figure 2 — Clutch linkage.
The clutch cable mechanism uses a steel cable inside a flexible housing to transfer pedal movement to the clutch fork. As shown in Figure 3, the cable is usually fastened to the upper end of the clutch pedal, with the other end of the cable connecting to the clutch fork. The cable housing is mounted in a stationary position. This allows the cable to slide inside the housing whenever the clutch pedal is moved. One end of the clutch cable housing has a threaded sleeve for clutch adjustment.
Figure 3 — Clutch cable.
A hydraulic clutch release mechanism uses a simple hydraulic circuit to transfer clutch pedal action to the clutch fork (Figure 4). It has three basic parts—master cylinder, hydraulic lines, and a slave cylinder.
Movement of the clutch pedal creates hydraulic pressure in the master cylinder, which actuates the slave cylinder. The slave cylinder then moves the clutch fork.
The master cylinder is the controlling cylinder that develops the hydraulic pressure. The slave cylinder is the operating cylinder that is actuated by the pressure created by the master cylinder.
The clutch fork, also called a clutch arm or release arm, transfers motion from the release mechanism to the release bearing and pressure plate. The clutch fork sticks through a square hole in the bell housing and mounts on a pivot. When the clutch fork is moved by the release mechanism, it pries on the release bearing to disengage the clutch.
A rubber boot fits over the clutch fork. This boot is designed to keep road dirt, rocks, oil, water, and other debris from entering the clutch housing.
The clutch housing is also called the bell housing. It bolts to the rear of the engine, enclosing the clutch assembly, with the manual transmission bolted to the back of the housing. The lower front of the housing has a metal cover that can be removed for flywheel ring gear inspection or when the engine must be separated from the clutch assembly. A hole is provided in the side of the housing for the clutch fork. It can be made of aluminum, magnesium, or cast iron.
The release bearing, also called the throw-out bearing, is a ball bearing and collar assembly. It reduces friction between the pressure plate levers and the release fork. The release bearing is a sealed unit pack with a lubricant. It slides on a hub sleeve extending out from the front of the manual transmission or transaxle and is moved by either hydraulic or manual pressure.
The hydraulic release bearing eliminates the stock mechanical release bearing linkage and slave cylinder. The release bearing mounts on the transmission face or slips over the input shaft of the transmission. When the clutch pedal is pressed, the bearing face presses against the pressure plate to disengage the clutch.
The release bearing snaps over the end of the clutch fork. Small spring clips hold the bearing on the fork. Then fork movement in either direction slides the release bearing along the transmission hub sleeve.
The pressure plate is a spring-loaded device that can either engage or disengage the clutch disc and the flywheel. It bolts to the flywheel. The clutch disc fits between the flywheel and the pressure plate. There are two types of pressure plates—the coil spring type and the diaphragm type.
The coil spring pressure plate uses small coil springs similar to valve springs (Figure 5). The face of the pressure plate is a large, flat ring that contacts the clutch disc during clutch engagement. The back side of the pressure plate has pockets for the coil springs and brackets for hinging the release levers.
Figure 5 — Coil spring pressure plate.
During clutch action, the pressure plate moves back and forth inside the clutch cover. The release levers are hinged inside the pressure plate to pry on and move the pressure plate face away from the clutch disc and flywheel. Small clip-type springs fit around the release levers to keep them rattling when fully released. The pressure plate cover fits over the springs, the release levers, and the pressure plate face. Its main purpose is to hold the assembly together. Holes around the outer edge of the cover are for bolting the pressure plate to the flywheel.
The diaphragm pressure plate (Figure 6) uses a single diaphragm spring instead of coil springs. The diaphragm spring is a large, round disc of spring steel. The spring is bent or dished and has pie-shaped segments running from the outer edge to the center. The diaphragm spring is mounted in the pressure plate with the outer edge touching the back of the pressure plate face. The outer rim of the diaphragm is secured to the pressure plate and is pivoted on rings approximately 1 inch from the outer edge.
Figure 6 — Diaphragm pressure plate.
Application of pressure at the inner section of the diaphragm will cause the outer rim to move away from the flywheel and draw the pressure plate away from the clutch disc, disengaging the clutch.
A “wet” clutch is immersed in a cooling lubricating fluid, which also keeps the surfaces clean and gives smoother performance and longer life. Wet clutches, however, tend to lose some energy to the liquid. Since the surfaces of a wet clutch can be slippery, stacking multiple clutch discs can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.
Wet clutches are designed to provide a long, service-free life. They often last the entire life of the machine they are installed on. If you must provide service to a wet clutch, refer to the manufacturer’s service manual for specific details.
The clutch disc, also called friction lining, is a “dry” clutch and consists of a splined hub and a round metal plate covered with friction material (lining). The splines in the center of the clutch disc mesh with the splines on the input shaft of the manual transmission. This makes the input shaft and disc turn together. However, the disc is free to slide back and forth on the shaft.
Clutch disc torsion springs, also termed damping springs, absorb some of the vibration and shock produced by clutch engagement. They are small coil springs located between the clutch disc splined hub and the friction disc assembly. When the clutch is engaged, the pressure plate jams the stationary disc against the spinning flywheel. The torsion springs compress and soften as the disc first begins to turn with the flywheel.
Clutch disc facing springs, also called the cushioning springs, are flat metal springs located under the friction lining of the disc. These springs have a slight wave or curve, allowing the lining to flex inward slightly during initial engagement. This also allows for smooth engagement.
The clutch disc friction material, also called disc lining or facing, is made of heat-resistant asbestos, cotton fibers, and copper wires woven or molded together. Grooves are cut into the friction material to aid cooling and release of the clutch disc. Rivets are used to bond the friction material to both sides of the metal body of the disc.
he flywheel is the mounting surface for the clutch (Figure 7). The pressure plate bolts to the flywheel face. The clutch disc is clamped and held against the flywheel by the spring action of the pressure plate. The face of the flywheel is precision machined to a smooth surface. The face of the flywheel that touches the clutch disc is made of iron.
Figure 7 — Flywheel and pilot bearing.
Even if the flywheel were aluminum, the face is iron because it wears well and dissipates heat better.
The pilot bearing or bushing is pressed into the end of the crankshaft to support the end of the transmission input shaft (Figure 7). The pilot bearing is a solid bronze bushing, but it also may be a roller or ball bearing.
The end of the transmission input shaft has a small journal machined on its end. This journal slides inside the pilot bearing. The pilot bearing prevents the transmission shaft and clutch disc from wobbling up and down when the clutch is released. It also assists the input shaft center the disc on the flywheel.
|Test Your Knowledge
1. The clutch fork transfers motion from the release mechanism to what components?
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When the operator presses the clutch pedal, the clutch release mechanism pulls or pushes on the clutch release lever or fork (Figure 8). The fork moves the release bearing into the center of the pressure plate, causing the pressure plate to pull away from the clutch disc releasing the disc from the flywheel. The engine crankshaft can then turn without turning the clutch disc and transmission input shaft. When the operator releases the clutch pedal, spring pressure inside the pressure plate pushes forward on the clutch disc. This action locks the flywheel, the clutch disc, the pressure plate, and the transmission input shaft together. The engine again rotates the transmission input shaft, the transmission gears, the drive train, and the wheels of the vehicle.
Figure 8 — Clutch operation.
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Many of the newer vehicles incorporate a clutch start switch into the starting system. The clutch start switch is mounted on the clutch pedal assembly. The clutch start switch prevents the engine from cranking unless the clutch pedal is depressed fully. This serves as a safety device that keeps the engine from possibly starting while in gear. Unless the switch is closed (clutch pedal depressed), the switch prevents current from reaching the starter solenoid. With the transmission in neutral, the clutch start switch is bypassed so the engine will crank and start.
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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 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, 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 — Pressure plate adjustment
Figure 10 — Pressure plate release lever 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 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.
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.
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A manual transmission (Figure 11) is designed with two purposes in mind. One purpose of the transmission is providing the operator with the option of maneuvering the vehicle in either the forward or reverse direction. This is a basic requirement of all automotive vehicles. Almost all vehicles have multiple forward gear ratios, but in most cases, only one ratio is provided for reverse.
Figure 11 — Manual transmission.
Another purpose of the transmission is to provide the operator with a selection of gear ratios between engine and wheel so that the vehicle can operate at the best efficiency under a variety of operating conditions and loads. If in proper operating condition, a manual transmission should do the following:
Before understanding the operation and power flow through a manual transmission, you first must understand the construction of the transmission so you will be able to diagnose and repair damaged transmissions properly.
The transmission case provides support for the bearings and shafts, as well as an enclosure for lubricating oil. A manual transmission case is cast from either iron or aluminum. Because they are lighter in weight, aluminum cases are preferred. A drain plug and fill plug are provided for servicing. The drain plug is located on the bottom of the case, whereas the fill plug is located on the side.
Also known as the tail shaft, the extension housing bolts to the rear of the transmission case. It encloses and holds the transmission output shaft and rear oil seal. A gasket is used to seal the mating surfaces between the transmission case and the extension housing. On the bottom of the extension housing is a flange that provides a base for the transmission mount.
Sometimes called the front bearing cap, the bearing hub covers the front transmission bearing and acts as a sleeve for the clutch release bearing. It bolts to the transmission case, and a gasket fits between the front hub and the case to prevent oil leakage.
A manual transmission has four steel shafts mounted inside the transmission case. These shafts are the input shaft, the countershaft, the reverse idler shaft, and the main shaft.
The input shaft, also known as the clutch shaft, transfers rotation from the clutch disc to the countershaft gears (Figure 11). The outer end of the shaft is splined, except the hub of the clutch disc. The inner end has a machined gear that meshes with the countershaft. A bearing in the transmission case supports the input shaft in the case. Anytime the clutch disc turns, the input shaft gear and gears on the countershaft turn.
The countershaft, also known as the cluster gear shaft, holds the countershaft gear into mesh with the input shaft gear and other gears in the transmission (Figure 11). It is located slightly below and to one side of the clutch shaft. The countershaft does not turn in the case. It is locked in place by a steel pin, force fit, or locknuts.
The reverse idler shaft is a short shaft that supports the reverse idle gear (Figure 11). It mounts stationary in the transmission case about halfway between the countershaft and output shaft, allowing the reverse idle gear to mesh with both shafts.
The main shaft, also called the output shaft, holds the output gears and synchronizers (Figure 11). The rear of the shaft extends to the rear of the extension housing where it connects to the drive shaft to turn the wheel of the vehicle. Gears on the shaft are free to rotate, but the synchronizers are locked on the shaft by splines. The synchronizers will only turn when the shaft itself turns.
Transmission gears can be classified into four groups—input gear, countershaft gears, main shaft gears, and the reverse idler gear. The input gear turns the countershaft gears, the countershaft gears turns the main shaft gears, and, when engaged, the reverse idler gear.
In low gear, a small gear on the countershaft drives a larger gear on the main shaft, providing for a high gear ratio for accelerating. Then, in high gear, a larger countershaft gear turns a small main shaft gear or a gear of equal size, resulting in a low gear ratio, allowing the vehicle to move faster. When reverse is engaged, power flows from the countershaft gear, to the reverse idler gear, and to the engaged main shaft gear. This action reverses main shaft rotation.
The synchronizer is a drum or sleeve that slides back and forth on the splined main shaft by means of the shifting fork. Generally, it has a bronze cone on each side that engages with a tapered mating cone on the second and high-speed gears. A transmission synchronizer (Figure 12) has two functions:
Figure 12— Synchronizer.
When the synchronizer is moved along the main shaft, the cones act as a clutch. Upon touching the gear that is to be engaged, the main shaft is accelerated or slowed down until the speeds of the main shaft and gear are synchronized. This action occurs during partial movement of the shift lever. Completion of lever movement then slides the sleeve and gear into complete engagement. This action can be readily understood by remembering that the hub of the sleeve slides on the splines of the main shaft to engage the cones; then the sleeve slides on the hub to engage the gears. As the synchronizer is slid against a gear, the gear is locked to the synchronizer and to the main shaft. Power can then be sent out of the transmission to the wheels.
Shift forks fit around the synchronizer sleeves to transfer movement to the sleeves from the shift linkage. The shift fork sits in a groove cut into the synchronizer sleeve. The linkage rod or shifting rail connects the shift fork to the operator’s shift lever. As the lever moves, the linkage or rail moves the shift fork and synchronizer sleeve to engage the correct transmission gear.
There are two types of shift linkages used on manual transmissions. They are the external rod and the internal shift rail. They both perform the same function. They connect the shift lever with the shift fork mechanism.
The transmission shift lever assembly can be moved to cause movement of the shift linkage, shift forks, and synchronizers. The shift lever may be either floor mounted or column mounted, depending upon the manufacturer. Floor-mounted shift levers are generally used with an internal shift rail linkage, whereas column-mounted shift levers are generally used with an external rod linkage.
Types Modern manual transmissions are divided into two major categories:
Transmission The constant mesh transmission has two parallel shafts where all forward gears of the countershaft are in constant mesh with the mainshaft gears, which are free to rotate. Reverse can either be a sliding collar or a constant mesh gear. On some earlier versions, first and reverse gears were sliding gears.
To eliminate the noise developed by the spur-tooth gears used in the sliding gear transmission, automotive manufacturers developed the constant mesh transmission. The constant mesh transmission has parallel shafts with gears in constant mesh. In neutral, the gears are free running but, when shifted, they are locked to their shafts by sliding collars.
When the shift lever is moved to third, the third and fourth shifter fork moves the sliding collar toward the third speed gear. This engages the external teeth of the sliding collar with the internal teeth of the third speed gear. Since the third speed gear is meshed and rotating with the countershaft, the sliding collar must also rotate. The sliding collar is splined to the main shaft, and therefore, the main shaft rotates with the sliding collar. This principle is carried out when the shift lever moves from one speed to the next.
The synchromesh transmission also has gears in constant mesh (Figure 11). However, gears can be selected without clashing or grinding by synchronizing the speeds of the mating part before they engage.
The construction of the synchromesh transmission is the same as that of the constant mesh transmission with the exception that a synchronizer has been added. The addition of synchronizers allows the gears to be constant mesh when the cluster gears and the synchronizing clutch mechanisms lock the gears together.
The synchronizer accelerates or slows down the rotation of the shaft and gear until both are rotating at the same speed and can be locked together without a gear clash. Since the vehicle is normally standing still when it is shifted into reverse gear, a synchronizer is not ordinarily used on the reverse gear.
The auxiliary transmission is used to provide additional gear ratios in the power train (Figure 13). This transmission is installed behind the main transmission, and power flows directly to it from the main transmission when of the integral type, or by a short propeller shaft (jack shaft) and universal joints
Figure 13 — Auxiliary transmission.
Support and alignment are provided by a frame cross member. Rubber-mounting brackets are used to isolate vibration and noise from the chassis. A lever that extends into the operator's compartment accomplishes shifting. Like the main transmission, the auxiliary transmission may have either constant mesh gears or synchronizers to allow for easier shifting.
This transmission, when of the two-speed design, has a low range and direct drive. Three- and four-speed auxiliary transmissions commonly have at least one overdrive gear ratio. The overdrive position causes increased speed of the output shaft in relation to the input shaft. Overdrive is common on heavy-duty trucks used to carry heavy loads and travel at highway speeds.
The auxiliary transmission can provide two-speed ratios. When it is in the direct drive position, power flows directly through the transmission and is controlled only by the main transmission. When the auxiliary transmission is shifted into low range, vehicle speed is reduced and torque is increased. When the low range is used with the lowest speed of the main transmission, the engine drives the wheels very slowly and with less engine horsepower.
In this constant mesh auxiliary transmission, the main gear is part of the input shaft, and it is in constant mesh with the countershaft drive gear. A pilot bearing aligns the main shaft output shaft with the input shaft. The low-speed main shaft gear runs free on the main shaft when direct drive is being used and is in constant mesh with the countershaft low-speed gear. A gear type dog clutch, splined to the main shaft, slides forward or backward when you shaft the auxiliary transmission into high or low gear position.
In high gear, when direct drive from the main transmission is being used, the dog clutch is forward and makes a direct connection between the input shaft and the main shaft. When in low gear, the dog clutch is meshed with the low-speed, main shaft gear and is disengaged from the main drive gear.
Transmissions are designed to last for the life of the vehicle when lubricated and operated properly. The most common cause of failure results from shifting when the vehicle is not completely stopped or without waiting long enough to allow the gears to stop spinning after depressing the clutch pedal. This slight clashing of gears may not seem significant at the time, but each time this occurs, small particles of the gears will be ground off and carried with the lubricant through the transmission. These small metal particles may become embedded in the soft metal used in synchronizers, reducing the frictional quality of the clutch. At the same time, these particles damage the bearings and their races by causing pitting, rough movement, and noise. Soon transmission failure will result. When this happens, you will have to remove the transmission and replace either damaged parts or the transmission unit.
As a mechanic, your first step toward repairing a transmission is the diagnosis of the problem. To begin diagnosis, gather as much information as possible. Determine in which gears the transmission acts up—first, second, third, fourth, or in all forward gears when shifting. Does it happen at specific speeds? This information will assist you in determining which parts are faulty. Refer to a diagnosis chart in the manufacturer’s service manual when a problem is difficult to locate. It will be written for the exact type of transmission.
Many problems that seem to be caused by the transmission are caused by clutch, linkage, or drive line problems. Keep this in mind before removing and disassembling a transmission.
Because of the variations in construction of transmissions, always refer to the manufacturer’s service manual for proper procedures in the removal, disassembly, repair, assembly, and installation. The time to carry out these operations varies from 6 to 8 hours, depending on transmission type and vehicle manufacturer. The basic removal procedures are as follows:
Never let the engine hang suspended by only the front motor mounts.
Once the transmission has been removed from the engine, clean the outside and place it on your workbench. Teardown procedures will vary from one transmission to another. Always consult the service manual for the type of transmission you are working on. If improper disassembly methods are used, major part damage could possibly result.
Before disassembly, remove the inspection cover. This will allow you to observe transmission action. Shift the transmission into each gear, and at the same time rotate the input shaft while inspecting the conditions of the gears and synchronizers.
The basic disassembly procedures are as follows:
After the transmission is disassembled, clean all the parts thoroughly and individually. Clean all the parts of hardened oil, lacquer deposits, and dirt. Pay particular attention to the small holes in the gears and to the shifter ball bores in the shifter shaft housing. Remove all gasket material using a putty knife or other suitable tool. Ensure that the metal surfaces are not gouged or scratched. Also, clean the transmission bearings and blow-dry them using low-pressure compressed air.
Always use protective eyewear when you are blowing the bearing dry with compressed air. Do NOT allow the bearing to spin. Air pressure can make the bearing spin at tremendously high rpm, possibly causing the bearing to explode and fly apart.
After all parts of the transmission have been cleaned, inspect everything closely to determine whether they can be reused or have to be replaced. The wear or damage to some of the parts will be evident to the eye. If brass-colored particles are found, one or more of the synchronizers or thrust washers are damaged. These are normally the only transmission parts made of this material. If iron chips are found, main drive gears are probably damaged. To check for damage or wear on other parts, you may have to use measuring tools and gauges to determine their condition.
Any worn or damaged parts in the transmission must be replaced. This is why your inspection is very critical. If any trouble is NOT corrected, the transmission overhaul may fail. You would have to complete the job a second time, wasting man-hours and materials, as well as causing unnecessary equipment downtime.
Always replace all gaskets and seals in the transmission. Even though the seal or gasket may have not been leaking before disassembly, it may start to leak after assembly.
When replacing a main shaft gear either due to wear or damage, you should also replace the matching gear on the countershaft. If a new gear is meshed with an old gear, transmission gear noise will occur. If new bolts are needed, make sure they are the correct thread type and length. Some transmissions use metric bolts. Remember, mixing threads will cause parts damage.
All parts must be lightly coated with a medium grade lubricating oil immediately after the inspection or repair. Oiling the parts gives them a necessary rust-preventive coating and facilitates the assembly process.
After obtaining new parts to replace the worn or damaged parts, you are ready for transmission assembly. To assemble the transmission, use the reverse order of disassembly. Again refer to the service manual for exact directions, as well as proper clearances and wear limits of the parts. The service manual will have an exploded view of the transmission. It will show how each part is located in relation to the others. Stepby-step directions will accompany the illustrations.
Certain key areas of the transmission should be given extra attention during assembly. One area is the needle bearings. To hold the needle bearings into the countershaft or other shafts, you coat the bearings with heavy grease. The grease will hold the bearing in place as you slide the countershaft into the gears. Also, measure the end play or clearance of the gears and synchronizers and the countershaft and case as directed by the service manual.
Before installing, ensure the transmission shifts properly. This will save you from having to remove the transmission if there are still problems. Also, since the transmission is already out, this is an ideal time to inspect the condition of the clutch.
Before installation, place a small amount of grease in the pilot bearing and on the release bearing inner surface. Now the transmission is ready to be installed. Basic transmission installation is as follows:
Do NOT place any lubricant on the end of the clutch shaft input splines or pressure plate release levers. Grease in these locations can spray onto the clutch disc, causing clutch slippage and failure
Check the manual transmission’s oil level at each PM. Recurrent low oil level indicates that there is leakage around the oil seals.
If you notice foaming in the oil, drain the transmission and refill it with clean oil. Foaming is evidence that water or some other lubricant that will not mix with the recommended transmission oil is present.
When it becomes necessary to change the transmission oil, use the following procedure:
Other than the periodic check required on the transmission fluid, drain and refill are performed as prescribed by the manufacturer. You should check the bolts for tightness and inspect the case for damage each scheduled PM.
Now that you understand the basic parts and construction of a manual transmission, we will cover the flow of power through a five-speed synchromesh transmission (Figure 14). In this example neither first gear nor reverse gear are synchronized.
In passing from neutral to reverse, the reverse idler gear has been moved rearward, and power from the countershaft gear flows into the reverse idler gear. The reverse idler gear directs power to the gear on the outside of the first and second synchronizer. Since the outer sleeve of the first and second gear synchronizer has been moved to the center position, power will not flow through first or second gear. The output shaft and synchronizer remain locked together; rotation is reversed to the countershaft gear and is reversed again on its way through the reverse idler gear. Since the power flow has changed three times, an odd number, direction of transmission spin is opposite of that of the engine (Figure 14). The sole function of this gear is to make the main shaft rotate in the opposite direction to the input shaft; it does not affect gear ratio.
To get the vehicle moving from a standstill, the operator moves the gearshift lever into first. The input shaft’s main drive gear turns the countershaft gear in a reverse direction. The countershaft gear turns the low gear in the same direction as the input shaft. Since the outer sleeve on the first-second gear synchronizer has been moved rearward, the low gear is locked to the output shaft (Figure 14). The difference in countershaft gear and first gear results in a gear ratio approximately 3.5:1.
Figure 14 — Power flow of a five speed manual transmission.
In second gear, the input shaft’s main drive gear turns the countershaft gear in a reverse direction. The countershaft gear turns the second gear on the output shaft to reverse the direction again. This action will result in the rotation of the output shaft to turn in the same direction as the input shaft. Since the outer sleeve on the first-second gear synchronizer has been moved forward, the second gear is locked to the output shaft (Figure 14). Gear ratio is approximately 2.5:1.
In third gear, the input shaft’s main drive gear turns the countershaft gear in a reverse direction. The countershaft gear turns the third gear on the output shaft to reverse the direction again. This action will result in the rotation of the output shaft to turn in the same direction as the input shaft. Since the outer sleeve on the third-fourth gear synchronizer has been moved rearward, the third gear is locked to the output shaft (Figure 14). Gear ratio is approximately 1.5:1.
In fourth gear, the synchronizer outer sleeve moves forward to engage the main drive gear. This will lock the input and output shafts together (Figure 14). This is direct drive and gives you a 1:1 gear ratio.
In fifth gear, the input shaft’s main drive gear turns the countershaft gear in a reverse direction. The fifth gear synchronizer outer sleeve moves forward. This engages the fifth gear with the counter gear. Since fifth gear is already in mesh with a gear on the output shaft, the synchronizer has locked the counter gear to the output shaft (Figure 14). Gear ratio is approximately .7:1.
|Test Your Knowledge
2. What is the maximum number of adjustments on a pressure plate before installation?
3. The pressure plate adjustment that positions the release levers and allows the release bearing to contact the levers simultaneously is known by which term?
4. You need to adjust a hydraulic clutch in the field and no manuals are available. What amount of clutch pedal free travel, in inches, will allow for adequate clutch operation until the vehicle reaches the shop?
5. A pilot bearing that is worn or lacks lubricant will produce noise in the clutch when which condition exists?
6. Which tool(s) are used to measure the amount of wear of a pilot bearing?
- To Table of Contents -
The automatic transmission, like the manual transmission, is designed to match the load requirements of the vehicle to the power and speed range of the engine (Figure 15). The automatic transmission, however, does this automatically depending on throttle position, vehicle speed, and the position of the control lever. Automatic transmissions are built in models that have two, three, or four-forward speeds and in some that are equipped with overdrive. Operator control is limited to the selection of the gear range by moving a control lever.
Figure 15 — Automatic transmission
The automatic transmission is coupled to the engine through a torque converter. The torque converter is used with an automatic transmission because it does not have to be manually disengaged by the operator each time the vehicle is stopped.
Because the automatic transmission shifts without any interruption of engine torque application, the cushioning effect of the fluid coupling within the torque converter is desirable. Because the automatic transmission shifts gear ratios independent of the operator, it must do so without the operator releasing the throttle. The automatic transmission does this by using planetary gearsets whose elements are locked and released in various combinations that produce the required forward and reverse gear ratios. The locking of the planetary gearset elements is done through the use of hydraulically actuated multiple-disc clutches and brake bands. The valve body controls the hydraulic pressure that actuates these locking devices. The valve body can be thought of as a hydraulic computer that receives signals that indicate vehicle speed, throttle position, and gearset lever position. Based on this information, the valve body sends hydraulic pressure to the correct locking devices.
The parts of the automatic transmission are as follows:
The torque converter is a fluid clutch that performs the same basic function as a manual transmission dry friction clutch (Figure 16). It provides a means of uncoupling the engine for stopping the vehicle in gear. It also provides a means of coupling the engine for acceleration.
Figure 16 — Torque converter.
A torque converter has four basic parts:
The primary action of the torque converter results from the action of the impeller passing oil at an angle into the blades of the turbine. The oil pushes against the faces of the turbine vanes, causing the turbine to rotate in the same direction as the impeller (Figure 17). With the engine idling, the impeller spins slowly. Only a small amount of oil is thrown into the stator and turbine. Not enough force is developed inside the torque converter to spin the turbine. The vehicle remains stationary with the transmission in gear.
Figure 17 — Torque converter operation.
During acceleration, the engine crankshaft, the converter housing, and the impeller begin to move faster. More oil is thrown out by centrifugal force, turning the turbine. As a result, the transmission input shaft and vehicle starts to move, but with some slippage.
At cruising speeds, the impeller and turbine spin at almost the same speed with very little slippage. When the impeller is spun fast enough, centrifugal force throws oil out hard enough to almost lock the impeller and turbine. After the oil has imparted its force to the turbine, the oil follows the contour of the turbine shell and blades so that it leaves the center section of the turbine spinning counterclockwise.
Because the turbine has absorbed the force required to reverse the direction of the clockwise spinning of the oil, it now has greater force than is being delivered by the engine. The process of multiplying engine torque has begun.
Torque multiplication refers to the ability of a torque converter to increase the amount of engine torque applied to the transmission input shaft. Torque multiplication occurs when the impeller is spinning faster than the turbine. For example, if the engine is accelerated quickly, the engine and impeller rpm might increase rapidly while the turbine is almost stationary. This is known as stall speed. Stall speed of a torque converter occurs when the impeller is at maximum speed without rotation of the turbine. This condition causes the transmission fluid to be thrown off the stator vanes at tremendous speeds. The greatest torque multiplication occurs at stall speed. When the turbine speed nears impeller speed, torque multiplication drops off. Torque is increased in the converter by sacrificing motion. The turbine spins slower than the impeller during torque multiplication.
If the counterclockwise oil were allowed to continue to the center section of the impeller, the oil would strike the blades of the pump in a direction that would hinder its rotation and cancel any gains in torque. To prevent this, you can add a stator assembly.
The stator is located between the pump and the turbine and is mounted on a one-way clutch that allows it to rotate clockwise but not counterclockwise (Figure 16). The purpose of the stator is to redirect the oil returning from the turbine and change its rotation back to that of the impeller. Stator action is only needed when the impeller and turbine are turning at different speeds. The one-way clutch locks the stator when the
impeller is turning faster than the turbine. This causes the stator to route oil flow over the impeller vanes properly. Then, when turbine speed almost equals impeller speed, the stator can freewheel on its shaft so not to obstruct flow.
Even at normal highway speeds, there is a certain amount of slippage in the torque converter. Another type of torque converter that is common on modern vehicles is the lockup torque converter. The lockup torque converter provides increased fuel economy and increased transmission life through the elimination of heat caused by torque converter slippage. A typical lockup mechanism consists of a hydraulic piston, torsion springs, and clutch friction material.
In lower gears, the converter clutch is released. The torque converter operates normally, allowing slippage and torque multiplication. However, when shifted into high or direct drive, transmission fluid is channeled to the converter piston. The converter piston pushes the friction discs together, locking the turbine and impeller. The crankshaft is able to drive the transmission input shaft directly, without slippage. The torsion springs assist to dampen engine power pulses entering the drive train.
A planetary gearset consists of three members--sun gear, ring gear, and planetary carrier, which hold the planetary gears in proper relation with the sun and ring gear (Figure 18). The planetary gears are free to rotate on their own axis while they "walk" around the sun gear or inside the ring gear.
Figure 18 — Planetary gearset.
By holding or releasing the components of a planetary gearset, it is possible to do the following:
Figure 19 shows the simplest application of planetary gears in a transmission. With the application shown, two forward speeds and neutral are possible. High gear or direct drive is shown. The clutch is holding the planet carrier to the input shaft, causing the carrier and sun gear to rotate as a single unit. With the clutch released, all gears are free to rotate and no power is transmitted to the output shaft. In neutral, the planetary carrier remains stationary while the pinion gears rotate on their axis and turn the ring gear. Should the brake be engaged on the ring gear, the sun gear causes the planetary gears to walk around the inside of the ring gear and forces the planet carrier to rotate in the same direction as the sun gear, but at a slower speed (low gear). To provide additional speed ranges or a reverse, you must add other planetary gearsets to this transmission
Figure 19 — Planetary gearset operation.
A compound planetary gearset combines two planetary units into one housing or ring gear. It may have two sun gears or a long sun gear to operate two sets of planetary gears. A compound planetary gearset is used to provide more forward gear ratios than a simple planetary gearset.
Automatic transmission clutches and bands are friction devices that drive or lock planetary gearsets members. They are used to cause the gearset to transfer power.
The multiple-disc clutch is used to transmit torque by locking elements of the planetary gearsets to rotating members within the transmission. In some cases, the multiple-disc clutch is also used to lock a planetary gearset element to the transmission case so it can act as a reactionary member. The multiple-disc clutch is made up of the following components (Figure 20):
Figure 20 — Multiple-disc clutch.
• Discs and plates—The active components of the multiple-disc clutch are the discs and the plates. The discs are made of steel and are faced with a friction material. They have teeth cut into their inner circumference to key them positively to the clutch hub. The plates are made of steel with no lining. They have teeth cut into their outer circumference to key them positively with the inside of a clutch drum or to the inside of the transmission case. Because the discs and plates are alternately stacked, they are locked together or released by simply squeezing them.
Figure 21 — Multiple-disc clutch operation.
The operation of the multiple-disc clutch is as follows (Figure 21):
An overrunning clutch is used in automatic transmissions to lock a planetary gearset to the transmission case so that it can act as a reactionary member. The overrunning clutch for the planetary gears is similar to the one in a torque converter stator or an electric starting motor drive gear. A planetary gearset overrunning clutch consists of an inner race, a set of springs, rollers, and an outer race.
Operation of the overrunning clutch is very simple to understand. When driven in one direction, rollers lock between ramps on the inner and outer race, allowing both races to turn. This action can be used to stop movement of the planetary member, for example. When turned in the other direction, rollers walk off the ramps, and the races are free to turn independently.
The brake band is used to lock a planetary gearset element to the transmission case so that the element can act as a reactionary member.
Figure 22 — Brake band.
The brake band is made up of the following elements (Figure 22):
Figure 23 — Brake band operation.
The operation of the brake band is as follows (Figure 23):
Released—When the brake band is released, there is no hydraulic pressure applied to the servo, and the drum is free to rotate within the band.
Applied—When the brake band is applied, hydraulic pressure is applied to the servo that in turn tightens the band around the drum. The result is that the drum is locked in a stationary position, causing an output change from the planetary gearset.
In the applied circuit of a clutch or band, an accumulator is used to cushion initial application. It temporarily absorbs some of the hydraulic pressure to cause slower movement of the applied piston.
The hydraulic system of an automatic transmission serves four basic purposes:
The hydraulic system for an automatic transmission typically consists of a pump, pressure regulator, manual valve, vacuum modulator valve, governor valve, shift valves, kick down valve, and a valve body.
The typical hydraulic pump is an internal-external rotor or gear-type pump. Located in the front of the transmission case, it is keyed to the torque converter hub so that it is driven by the engine. As the torque converter spins the oil pump, transmission fluid is drawn into the pump from the transmission pan. The pump compresses the oil and forces it to the pressure regulator. The pump has several basic functions:
The pressure regulator limits the maximum amount of oil pressure developed by the oil pump. It is a spring-loaded valve that routes excess pump pressure out of the hydraulic system, assuring proper transmission operation.
A manual valve located in the valve body is operated by the driver through the shift linkage (Figure 24). This valve allows the operator to select park, neutral, reverse, or different drive ranges. When the shift lever is moved, the shift linkage moves the manual valve. As a result, the valve routes hydraulic fluid throughout the transmission to the correct places.
Figure 24 — Manual valve operation.
When the operator selects overdrive, drive, or second, the transmission takes over, shifting automatically to meet driver conditions. When the selector is placed in low and reverse, the transmission is locked into the selected gear.
The vacuum modulator valve is a diaphragm device that uses engine manifold vacuum to indicate engine load to the shift valve (Figure 25). As engine vacuum (load) rises and falls, it moves the diaphragm inside the modulator. This in turn moves the rod and hydraulic valve to change throttle control pressure in the transmission. In this way, the vacuum modulator can match transmission shift points to engine loads.
Figure 25 — Vacuum modulator valve.
The governor valve senses engine speed (transmission output shaft speed) to help control gear shifting (Figure 26). The vacuum modulator and governor work together to determine the shift points. The governor gear is meshed with a gear on the transmission output shaft. Whenever the vehicle and output shaft are moving, the centrifugal weights rotate. When the output shaft and weights are spinning slowly, the weights are held in by the governor springs, causing low-pressure output, and the transmission remains in low gear. As the engine speeds increases, the weights are thrown out further and governor pressure increases, moving the shift valve and causing the transmission to shift into higher gear.
Figure 26 — Governor valve
The shift valves are simple balance type spool valves that select between low and high gear when the manual valve is in drive. Using control pressure (oil pressure from the regulator, governor, vacuum modulator, and manual valves), they operate the bands, servos, and gearsets. Oil pressure from the other transmission valves acts on each end of the shift valve. In this way, the shift valve is sensitive to engine load (vacuum modulator valve oil pressure), engine speed (governor valve oil pressure), and gearshift position (manual valve oil pressure). These valves move according to the forces and keep the transmission shifted into the correct gear ratio for the driving conditions.
The kickdown valve causes the transmission to shift into a lower gear during fast acceleration. A rod or cable links the carburetor or fuel injection throttle body to a lever on the transmission. When the operator depresses the throttle, the lever moves the kickdown valve. This action causes hydraulic pressure to override normal shift control pressure and the transmission downshifts.
The valve body is a very complicated hydraulic system (Figure 27). It contains hydraulic valves used in an automatic transmission, such as the pressure regulator, shift valves, and manual valves. The valve body bolts to the bottom of the transmission case and is housed in the transmission pan. A filter or screen is attached to the bottom of the valve body. Passages in the valve body route fluid from the pump to the valves and then into the transmission case. Passages in the transmission case carry fluid to other hydraulic components.
Figure 27 — Valve body.
Automatic transmission service can be easily divided into the following areas: preventive maintenance, troubleshooting, and major overhaul. Before you perform maintenance or repair on an automatic transmission, consult the maintenance manual for instructions and proper specifications. As a floor mechanic, however, your area of greatest concern is preventive maintenance. Preventive maintenance includes the following:
The operator is responsible for first echelon (operator’s) maintenance. The operator should be trained not only to know to look for the proper fluid level but also to know how to look for discoloration of the fluid and debris on the dipstick.
Fluid levels in automatic transmissions are almost always checked at operating temperature. This is important to know since the level of the fluid may vary as much as three quarters of an inch between hot and cold.
The fluid should be either reddish or clear. The color varies due to the type of fluid. (For example, construction equipment using OE-10 will be clear). A burnt smell or brown coloration of the fluid is a sign of overheated oil from extra heavy use or slipping bands or clutch packs. The vehicle should be sent to the shop for further inspection.
Not all transmission fluids are the same. Before you add fluid, check the manufacturer’s recommendations first. The use of the wrong fluid will lead to early internal parts failure and costly overhaul.
Overfilling the transmission can result in the fluid foaming and the fluid being driven out through the vent tube. The air that is trapped in the fluid is drawn into the hydraulic system by the pump and distributed to all parts of the transmission. This situation will cause air to be in the transmission in place of fluid and in turn cause slow application and burning of clutch plates and facings. Slippage occurs, heat results, and failure of the transmission follows.
Another possible, but remote, problem is water, indicated by the fluid having a "milky" appearance. A damaged fluid cooling tube in the radiator (automotive) or a damaged oil cooler (construction) could be the problem. The remedy is simple. Pressure-test the suspected components and perform any required repairs. After repairs have been performed, flush and refill the transmission with clean, fresh fluid.
The types of linkages found on an automatic transmission are gearshift selection and throttle kickdown. The system can be a cable or a series of rods and levers. These systems do not normally present a problem, and preventive maintenance usually involves only a visual inspection and lubrication of the pivot points of linkages or the cable. When adjusting these linkages, you should strictly adhere to the manufacturer’s specifications.
If an automatic transmission is being used in severe service, the manufacturer may suggest periodic band adjustment. Always adjust lockup bands to the manufacturer’s specifications. Adjust the bands by loosening the locknut and tightening down the adjusting screw to a specified value. Back off the band adjusting screw with a specified number of turns and tighten down the locking nut.
Not all bands are adjustable. Always check the manufacturer’s service manual before any servicing of the transmission.
Perform fluid replacement according to the manufacturer’s recommendations. These recommendations vary considerably for different makes and models. Before you change automatic transmission fluid, always read the service manual first.
Service intervals depend on the type of use the vehicle receives. In the NCF, because of the operating environment, more than a few of the vehicles are subjected to severe service. Severe service includes the following: hot and dusty conditions, constant stopand-go driving (taxi service), trailer towing, constant heavy hauling, and around-the-clock operations (contingency). Any CESE operating in these conditions should have its automatic transmission fluid and filter changed on a regular schedule, based on the manufacturer's specifications for severe service. Ensure the vehicle is on level ground or a lift, and let the oil drain into a proper catchment device.
The draining of the transmission can be accomplished in one of the following three ways:
Oil drained from automatic transmissions contains heavy metals and is considered hazardous waste and should be disposed of according to local instructions.
Once the oil is drained, remove the pan completely for cleaning by paying close attention to any debris in the bottom of the pan. The presence of a high amount of metal particles may indicate serious internal problems. Clean the pan, and set it aside.
All automatic transmissions have a filter or screen attached to the valve body. The screen is cleanable, whereas the filter is a disposable type and should always be replaced when removed. These are retained in different ways: retaining screws, metal retaining clamps, or O-rings made of neoprene. Clean the screen with solvent and use low-pressure air to blow-dry it. Do not use rags to wipe the screen dry, as they tend to leave lint behind that will be ingested into the hydraulic system of the transmission. If the screen is damaged or is abnormally hard to clean, replace it.
Draining the oil from the pan of the transmission does not remove all of the oil—draining the oil from the torque converter completes the process. To do this, remove the torque converter cover and remove the drain plug, if so equipped. For a torque converter without a drain plug, special draining instructions may be found in the manufacturer’s service manual. Before performing this operation, clear it with your shop supervisor.
Reinstall the transmission oil pan, the oil plug, and the fill tube. Fill the transmission with the fluid prescribed by the manufacturer to the proper level. With the brakes applied, start the engine and let it idle for a couple of minutes. Move the gear selector through all gear ranges several times, allowing the fluid to flow through the entire hydraulic system to release any trapped air. Return the selector lever to park or neutral and recheck the fluid level. Bring the fluid to the proper level. Run the vehicle until operating temperature is reached, checking for leaks. Also, recheck the fluid and adjust the level as necessary.
Overfilling an automatic transmission will cause foaming of the fluid. This condition prevents the internal working parts from being properly lubricated, causing slow actuation of the clutches and bands. Eventually, burning of the clutches and bands results. Do NOT overfill an automatic transmission.
The vehicle speed sensor is a device that is mounted on the output shaft of the transmission or transaxle. This device tells the electronic control module (ECM) how fast the vehicle is moving. It consists of a wheel with teeth around it and a magnetic pickup. The wheel can either be attached to the output shaft or be gear driven off the output shaft. As the wheel is turned, it induces an alternating current (AC) in the magnetic pickup. The ECM uses this information to calculate how fast the vehicle is moving.
Electronic transmissions utilize shift solenoids to control when the transmission will shift from one gear to the next. The solenoid affects hydraulic pressure on one side of a shift valve, causing it to move. In some transmissions this solenoid is connected directly to a check ball that acts as a shift valve. Energizing the shift solenoid causes the check ball to move and either open or close pressure passages leading to the holding members.
The ECM works with the shift solenoids, either receiving or sending input to tell the solenoid to operate or hold. If the speed is appropriate for an upshift, however, the throttle position sensor tells the ECM it is wide open; under a heavy load, for example, the ECM may hold the shift solenoid from operation until the throttle is changed.
|Test Your Knowledge
7. In a torque converter, what component is known as the converter pump?
8. The condition that exists when the impeller of a torque converter is at maximum speed and the turbine is almost stationary is known by what term?
9. What type of torque converter eliminates the heat caused by torque converter slippage, which results in increased fuel economy and transmission life?
10. What gear is the center gear in a planetary gearset?
11. What component of a multiple-disc clutch is used to distribute application pressure equally on the surfaces of the clutch discs and plates?
- To Table of Contents -
A transaxle is a transmission and differential combination in a single assembly. Transaxles are used in front-wheel drive vehicles. A transaxle allows the wheels next to the engine to propel the vehicle. Short drive axles are used to connect the transaxle output to the hubs and drive wheels.
Vehicle manufacturers claim that a transaxle and front-wheel drive vehicle has several advantages over a vehicle with rear-wheel drive. A few of these advantages are the following:
Most transaxles are designed so that the engine can be transverse (sideways) mounted in the engine compartment. The transaxle bolts to the rear of the engine. This produces a very compact unit. Engine torque enters the transaxle transmission. The transmission transfers power to the differential. Then the differential turns the drive axles that rotate the front wheels.
Both manual and automatic transaxles are available. Manual transaxle uses a friction clutch and a standard transmission-type gearbox. An automatic transaxle uses a torque converter and a hydraulic system to control gear engagement.
A manual transaxle uses a standard clutch and transmission (Figure 28). A footoperated clutch engages and disengages the engine and transaxle. A hand-operated shift lever allows the operator to charge gear ratios.
Figure 28 — Manual transaxle.
The basic parts relating to a manual transaxle are the following:
The manual transaxle can be broken up into two separate units—a manual transaxle transmission and a transaxle differential. A manual transaxle transmission provides several (usually four or five) forward gears and reverse. You will find that the names of shafts, gears, and other parts in the transaxle vary, depending on the location and function of the components. For example, the input shaft may also be called the main shaft, and the output shaft is called the pinion shaft because it drives the ring and pinion gear in the differential. The output, or pinion, shaft has a gear or sprocket for driving the differential ring gear.
The clutch used on the manual transaxle transmission is almost identical to the manual transmission clutch for rear-wheel drive vehicles. It uses a friction disc and spring-loaded pressure plate bolted to the flywheel. Some transaxles use a conventional clutch release mechanism (release bearing and fork); others use a long pushrod passing through the input shaft.
The transaxle differential, like a rear axle differential, transfers power to the axles and wheels while allowing one wheel to turn at a different speed than the other. A small pinion gear on the gearbox output shaft or countershaft turns the differential ring gear. The ring gear is fastened to the differential case. The case holds the spider gears (pinion gears and axle side gears) and a pinion shaft. The axle shafts are splined to the differential side gears.
An automatic transaxle is a combination automatic transmission and differential combined into a single assembly (Figure 29).
Figure 29 — Automatic transaxle.
The basic parts of an automatic transaxle are as follows:
Many of the components used in the automatic transaxle are also found in the automatic transmission. Operating principles of these components are the same as those of the automatic transmission. The differential of the automatic transaxle is similar to that used on the manual transaxle.
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In this manual, you were introduced to automotive clutches, transmissions and transaxles. You learned how the manual and automatic transmission operates and provides power to the drive wheels of the vehicle. In addition you learned the differences between a transmission and a transaxle. The power flow to the drive wheels is something a construction mechanic needs to understand to enable them to properly troubleshoot and diagnose equipment. When you have mastered the knowledge of these systems you will become a better mechanic.
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1. What device is designed to disconnect the engine from the transmission?
2. What component provides the operator the means with which to operate the clutch assembly?
3. What clutch component can either engage or disengage the clutch disc and flywheel?
4. What springs in the clutch disc absorb vibration and shock produced by clutch engagement?
5. What term refers to the flat metal springs located under the friction lining of the disc that allow for smooth engagement of the clutch?
6. Which clutch component prevents the transmission shaft from wobbling up and down when the clutch is released?
7. Which safety switch prevents the engine from starting unless the clutch pedal is fully depressed?
8. How is a hydraulically operated clutch adjusted?
9. What is the most common cause of premature clutch troubles?
10. Which condition will result in the clutch slipping?
11. If an operator reports that a vehicle has a severe vibration when accelerated from a standstill, what is the most likely cause of this trouble?
12. An operator reports hearing rattling sounds when the clutch is engaged. This condition is generally due to which problem?
13. An operator reports that a vehicle has "clutch-pedal pulsation." A mechanic should know that this means that _______.
14. Clutch-pedal pulsation CANNOT be caused by which condition?
15. If a clutch release bearing is running roughly, what action should the mechanic take?
16. You are reassembling a clutch assembly and a clutch alignment tool is NOT available. You can center the clutch disc on the flywheel by using which item?
17. What component provides a selection of gear ratios so a vehicle can operate under a variety of operating conditions and loads?
18. In a manual transmission, what shaft is locked in place within the transmission case?
19. What are the four gear groups in a manual transmission?
20. Which function pertains to the synchronizer in a manual transmission?
21. What are the two types of shifting linkages used on manual transmissions?
22. When the gears are shifted, what type of transmission locks the gears to their shafts using sliding collars?
23. What is the function of the synchronizer in a synchromesh transmission?
24. What component of an auxiliary transmission is splined to the main shaft and slides backwards or forwards when shifting into high or low positions?
25. When disassembling a manual transmission, you find brass-colored particles. What components are most likely damaged?
26. When replacing a main shaft gear, you should also replace the matching gear on what shaft?
27. You have completed reassembling a transmission. Which action should you take before reinstalling the transmission?
28. Operator control of an automatic transmission is limited to what action?
29. What action within an automatic transmission allows the transmission to shift gear ratios without operator control?
30. The turbine of a torque converter is connected to what component?
31. The blades inside a torque converter are forced to rotate by _______.
32. In a torque converter, what action causes torque multiplication to occur?
33. What clutch locks the stator of a torque converter when the impeller is turning faster than the turbine?
34. What springs in a lockup torque converter assist in dampening engine pulses entering the drive train?
35. Which gear is NOT a part of the makeup of the planetary gearset?
36. What clutch in an automatic transmission is used to transmit torque by locking elements of the planetary gearsets to rotating members within the transmission?
37. What component of a multiple-disc clutch ensures a rapid release of the clutch when hydraulic pressure to the clutch piston is released?
38. What component of an automatic transmission is designed to lock a planetary gearset element to the transmission case so the element can act as a reactionary member?
39. Which is NOT a basic function of the hydraulic system of an automatic transmission?
40. So it can be driven by the engine, the hydraulic pump of an automatic transmission is keyed to what component?
41. Which is NOT a function of the hydraulic pump of an automatic transmission?
42. What valve in an automatic transmission is operated by the shift mechanism, allowing the operator to select park, neutral, reverse, or different drive ranges?
43. What valve works in conjunction with the vacuum modulator to determine shift points in an automatic transmission?
44. What valve causes the transmission to shift into a lower gear during quick acceleration?
45. In addition to giving off a burnt smell, overheated transmission fluid will turn what color?
46. Using a transmission fluid that is incompatible with the unit you are working on may lead to which problem?
47. Air trapped in the hydraulic system of an automatic transmission can cause which problem?
48. Water mixed with automatic transmission fluid will turn the fluid what color?
49. After a vehicle has been operated in severe service, the transmission will require a band adjustment.
50. Oil drained from an automatic transmission should be disposed of according to what instructions?
51. Which factor is NOT an advantage of a vehicle with a transaxle and front-wheel drive?
52. In a manual transaxle the output shaft transfers torque to which component?
53. The flow of fluid to the pistons and servos of an automatic transaxle is controlled by what component of the transaxle?
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