9.6 Manual Transmissions

A manual transmission (Figure 9-11) 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 9-11 — 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:

Transmission Construction

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.

ransmission Case

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.

Extension Housing

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.

Front Bearing Hub

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.

Transmission Shafts

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.

Input Shaft. The input shaft, also known as the clutch shaft, transfers rotation from the clutch disc to the countershaft gears (Figure 10-11). 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.

Countershaft. 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 10-11). 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.

Reverse Idler Shaft. The reverse idler shaft is a short shaft that supports the reverse idle gear (Figure 10- 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.

Main Shaft. The main shaft, also called the output shaft, holds the output gears and synchronizers (Figure 10-11). 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

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.

Synchronizers

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 9-12) has two functions:

  1. Lock the main shaft gear to the main shaft.
  2. Prevent the gear from clashing or grinding during shifting.


Figure 19-12— 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

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.

Shift Linkage and Levers

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.

Transmission Types

Modern manual transmissions are divided into two major categories:

Constant Mesh 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.

Synchromesh Transmission

The synchromesh transmission also has gears in constant mesh (Figure 10-11). 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.

Auxiliary Transmissions

The auxiliary transmission is used to provide additional gear ratios in the power train (Figure 9-13). 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 9-13 — 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.

Transmission Troubleshooting

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.

Transmission Overhaul

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:

  1. Unscrew the transmission drain plug and drain the oil.
  2. Remove the drive shaft and install a plastic cap over the end of the transmission shaft.
  3. Disconnect the transmission linkage at the transmission.
  4. Unbolt and remove the speedometer cable from the extension housing.
  5. Remove all electrical wires leading to switches on the transmission.
  6. Remove any cross members or supports.
  7. Support the transmission and engine with jacks. Operate the jack on the engine to take the weight off the transmission. Be careful not to crush the oil pan.

 

CAUTION

Never let the engine hang suspended by only the front motor mounts.

  1. Depending upon what is recommended by the service manual, either remove the transmission-to-clutch cover bolts or the bolts going into the engine from the clutch cover.
  2. Slide the transmission straight back, holding it in alignment with the engine. You may have to wiggle the transmission slightly to free it from the engine.

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:

  1. Unbolt and remove the rear extension housing. It may be necessary to tap the housing off with a soft face mallet or bronze hammer.
  2. Unbolt and remove the front extension housing and any snap rings.
  3. Carefully pry the input shaft and gear forward far enough to free the main shaft.
  4. Using a brass drift pin, push the reverse idler shaft and countershaft out of the transmission case.
  5. Remove the input shaft and output shaft assemblies. Slide the output shaft and gears out of the back of the transmission as a unit. Be careful not to damage any of the gears.

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.

WARNING

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. Step- by-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:

 

Note

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.

  1. Place the transmission on the transmission jack.
  2. Position the transmission behind the engine. Ensure that the release bearing is in place on the clutch fork.
  3. Carefully align the transmission and engine, ensuring that the input and output shaft lines up perfectly with the center line of the engine crankshaft. If the transmission is slightly tilted, it will not fit into place.
  4. With the transmission in high gear, slowly push the transmission into the clutch housing. You may need to raise or lower the transmission slightly to keep it aligned.
  5. When the transmission is almost in place, wiggle the extension housing in a circular motion while pushing toward the engine. This will help start the input shaft in the pilot bearing. The transmission will then slide into position.
  6. With the transmission bolted to the clutch cover, install the rear support or cross member and transmission mount. Reinstall the clutch linkage, the transmission linkage, and any other parts.
  7. Adjust the clutch. With the transmission installed and the clutch adjusted, test drive the vehicle for proper operation. If the transmission is noisy, extremely loose, or binds, it must be removed and disassembled for further inspection and corrective action.

Transmission Service

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:

  1. Before you drain the oil, clean around the drain and fill plugs thoroughly. Remove both the drain and fill plugs to allow the oil to drain.
  2. Drain the transmission immediately after the vehicle has been operated. The oil will then be warm and will readily drain, taking along the suspended contaminants as it drains.
  3. Check the drained oil for any uncommon foreign matter, such as large metal particles (steel or brass). This is a good sign of internal damage to the gears, bearings, or synchronizers. If large particles are found, notify your shop supervisor for further instructions.
  4. Once the transmission has drained completely and no large metal particles are found, replace the drain plug and refill the transmission with the proper grade of oil until it reaches the bottom of the fill plug. Then replace the fill plug.

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.

Power Flow

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 10- 14). In this example neither first gear nor reverse gear are synchronized.

Reverse Gear

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 9-14). 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.


.

Figure 9-14 — Power flow of a five speed manual transmission.

First Gear

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 10-14). The difference in countershaft gear and first gear results in a gear ratio approximately 3.5:1.

Second Gear

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 10-14). Gear ratio is approximately 2.5:1.

Third Gear

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 10-14). Gear ratio is approximately 1.5:1.

Fourth Gear

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 10-14). This is direct drive and gives you a 1:1 gear ratio.

Fifth Gear

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 10-14).

Gear ratio is approximately .7:1.

Test Your Knowledge

2. What is the maximum number of adjustments on a pressure plate before installation?

A. One
B. Two
C. Three
D. Four

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?

A. Clearance height
B. Relation height
C. Finger height
D. Free height

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?

A. 1
B. 2
C. 1 1/2
D. 4

5. A pilot bearing that is worn or lacks lubricant will produce noise in the clutch when which condition exists?

A. Transmission is in gear.
B. Clutch is disengaged.
C. Vehicle is standing still.
D. Clutch is engaged.

6. Which tool(s) are used to measure the amount of wear of a pilot bearing?

A. Inside caliper
B. Outside caliper
C. Telescoping gauge and micrometer
D. Thickness gauge and sliding scale