1. Automatic Transmissions
Chapter 18
Automatic Transmissions

2. Objectives (1 of 2)
Identify the components of a simple planetary gearset.
Explain the operating principles of a planetary gear train.
Define a compound planetary gearset and explain how it operates.
Describe a multiple-disc clutch and explain its role in the operation of an automatic transmission.

3. Objectives (2 of 2)
Outline torque path powerflow through a typical four- and five-speed automatic transmission.
Describe the hydraulic circuits and flows used to control automatic transmission operation.
List the two types of hydraulic retarders used on Allison automatic transmissions and explain their differences.

4. Planetary Gear Set (1 of 3)
A simple planetary gear set can provide:
Overdrive
Reverse
Forward reduction
Neutral
Direct drive ratios
There are three main components in a simple planetary gearset.
A sun gear
A carrier with multiple planetary pinion gears mounted to it
An internally toothed ring gear or annulus
PLANETARY GEARSET
COMPONENTS
A simple planetary or epicyclic gearset can provide overdrive, reverse, forward reduction, neutral, and direct drive ratios. It can also supply fast and slow speeds for each operating range, with the exception of neutral and direct drive. There are three main components in a simple planetary gearset: a sun gear, a carrier with planetary pinion gears mounted to it, and an internally toothed ring gear or annulus. The sun gear is located in the center of the assembly (Figure 18–2). It is the smallest gear in the assembly and is located at the center of the axis. The sun gear can be either a spur or helical gear design. It meshes with the teeth of the planetary pinion gears. Planetary pinion gears are small gears fitted into a framework called the planetary carrier; each turns on its own axis. The planetary carrier can be made of cast iron, aluminum, or steel plate and is designed with a shaft for each of the planetary pinion gears. For simplicity, we will use the term planetary pinions in place of planetary pinion gears. Planetary pinions rotate on needle bearings positioned between the planetary carrier shaft and the planetary pinions. The number of planetary pinions in a carrier depends on the load the gear set is required to carry. Passenger vehicle automatic transmissions might use only three planetary pinions while heavy-duty automatics could use as many as five planetary pinions in a planetary carrier. The carrier and its pinions are considered one unit—the mid-size gear member. The planetary pinions rotate in mesh with the sun gear and are surrounded by the ring gear. The ring gear is the largest component of a simple planetary gearset. In some ways, the ring gear acts like a band to hold the entire gearset together. The ring gear is located the greatest distance from the axis of the sun gear and is therefore capable of exerting the most leverage. To help remember the design of a simple planetary gearset, use the solar system as an example. The sun is the center of the solar system with the planets rotating around it, hence the name planetary gearset.

5. Planetary Gear Set (2 of 3)
The sun gear is located in the center of the assembly.
Planetary pinion gears are small gears fitted into a planetary carrier.
The planetary carrier can be made of cast iron, aluminum, or steel plate.
Planetary pinions rotate on needle bearings.
PLANETARY GEARSET COMPONENTS
A simple planetary or epicyclic gearset can provide overdrive, reverse, forward reduction, neutral, and direct drive ratios. It can also supply fast and slow speeds for each operating range, with the exception of neutral and direct drive. There are three main components in a simple planetary gearset: a sun gear, a carrier with planetary pinion gears mounted to it, and an internally toothed ring gear or annulus. The sun gear is located in the center of the assembly (Figure 18–2). It is the smallest gear in the assembly and is located at the center of the axis. The sun gear can be either a spur or helical gear design. It meshes with the teeth of the planetary pinion gears. Planetary pinion gears are small gears fitted into a framework called the planetary carrier; each turns on its own axis. The planetary carrier can be made of cast iron, aluminum, or steel plate and is designed with a shaft for each of the planetary pinion gears. For simplicity, we will use the term planetary pinions in place of planetary pinion gears. Planetary pinions rotate on needle bearings positioned between the planetary carrier shaft and the planetary pinions. The number of planetary pinions in a carrier depends on the load the gear set is required to carry. Passenger vehicle automatic transmissions might use only three planetary pinions while heavy-duty automatics could use as many as five planetary pinions in a planetary carrier. The carrier and its pinions are considered one unit—the mid-size gear member. The planetary pinions rotate in mesh with the sun gear and are surrounded by the ring gear. The ring gear is the largest component of a simple planetary gearset. In some ways, the ring gear acts like a band to hold the entire gearset together. The ring gear is located the greatest distance from the axis of the sun gear and is therefore capable of exerting the most leverage. To help remember the design of a simple planetary gearset, use the solar system as an example. The sun is the center of the solar system with the planets rotating around it, hence the name planetary gearset.

6. Planetary Gear Set (3 of 3)
The carrier and its pinions are considered one unit—the mid-size gear member.
The ring gear is the largest component of a simple planetary gearset.
The ring gear is located the greatest distance from the axis of the sun gear and is therefore capable of exerting the most leverage.
PLANETARY GEARSET COMPONENTS
A simple planetary or epicyclic gearset can provide overdrive, reverse, forward reduction, neutral, and direct drive ratios. It can also supply fast and slow speeds for each operating range, with the exception of neutral and direct drive. There are three main components in a simple planetary gearset: a sun gear, a carrier with planetary pinion gears mounted to it, and an internally toothed ring gear or annulus. The sun gear is located in the center of the assembly (Figure 18–2). It is the smallest gear in the assembly and is located at the center of the axis. The sun gear can be either a spur or helical gear design. It meshes with the teeth of the planetary pinion gears. Planetary pinion gears are small gears fitted into a framework called the planetary carrier; each turns on its own axis. The planetary carrier can be made of cast iron, aluminum, or steel plate and is designed with a shaft for each of the planetary pinion gears. For simplicity, we will use the term planetary pinions in place of planetary pinion gears. Planetary pinions rotate on needle bearings positioned between the planetary carrier shaft and the planetary pinions. The number of planetary pinions in a carrier depends on the load the gear set is required to carry. Passenger vehicle automatic transmissions might use only three planetary pinions while heavy-duty automatics could use as many as five planetary pinions in a planetary carrier. The carrier and its pinions are considered one unit—the mid-size gear member. The planetary pinions rotate in mesh with the sun gear and are surrounded by the ring gear. The ring gear is the largest component of a simple planetary gearset. In some ways, the ring gear acts like a band to hold the entire gearset together. The ring gear is located the greatest distance from the axis of the sun gear and is therefore capable of exerting the most leverage. To help remember the design of a simple planetary gearset, use the solar system as an example. The sun is the center of the solar system with the planets rotating around it, hence the name planetary gearset.

7. How Planetary Gears Work (1 of 4)
Power transmission is possible only when one of the three members is held stationary, or if two of the members are locked together.
Any one of the three members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.
At the same time, another member might be kept from rotating and thus becomes the held or stationary member.
The third member then becomes the driven or output member.
HOW PLANETARY GEARS WORK
Each member of a planetary gearset—the sun gear, pinion gear carrier, and ring gear—can revolve or be held at rest. Power transmission through a planetary gearset is possible only when one of the three component members is held stationary, or if two of the members are locked together. Any one of the three members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member. At the same time, another member might be kept from rotating and thus becomes the held or stationary member. The third member then becomes the driven or output member. Depending on which member is the driver, which is held, and which is driven, either a torque increase or a speed increase is produced by the planetary gearset. Output direction can also be reversed through various combinations. You can take a look at the ratio and directional capabilities of a single planetary gearset by studying Table 18–1. Table 18–1 summarizes the input and output rules of a simple planetary gearset. It shows relative output speed, torque, and direction of the various combinations available. It is also helpful to remember the following two points with regard to direction of rotation: 1. When an external-to-external gear tooth set is in mesh, there will be a change in the direction of rotation at the output (Figure 18–3A). 2. When an external gear tooth is in mesh with an internal gear, the output rotation for both gears will be the same (Figure 18–3B).

8. How Planetary Gears Work (2 of 4)
Depending on which member is the driver, which is held, and which is driven, either a torque increase or a speed increase is produced by the planetary gearset.
Output direction can also be reversed through various combinations.

9. How Planetary Gears Work (3 of 4)
Laws of simple planetary gear operation
See Table 18-1 on page 531 of the textbook.

10. How Planetary Gears Work (4 of 4)
When an external-to-external gear tooth set is in mesh, there will be a change in the direction of rotation at the output.
When an external gear tooth is in mesh with an internal gear, the output rotation for both gears will be the same.

11. Maximum Forward Reduction
With the ring gear stationary and the sun gear turning clockwise, the external sun gear teeth rotate the planetary pinions counterclockwise.
Each planetary pinion pushes against its shaft, moving the planetary carrier clockwise.
The small sun gear must rotate several times to turn the larger planetary carrier through one complete revolution.
This combination represents the most gear reduction that can be achieved in one planetary gearset.
Maximum Forward Reduction
With the ring gear stationary and the sun gear turning clockwise, the external sun gear teeth rotate the planetary pinions counterclockwise. This option is shown as combination 1 in Table 18–1 and in Figure 18–4. The inside diameter of each planetary pinion pushes against its shaft, moving the planetary carrier clockwise. The small sun gear (driving) will rotate several times, to turn the larger planetary carrier through one complete revolution, resulting in an underdrive. This combination represents the most gear reduction or the maximum torque multiplication that can be achieved in one planetary gearset. Input speed is high, but output speed will be low.

12. Minimum Forward Reduction
The ring gear drives the planetary pinions clockwise and walks around the stationary sun gear.
The planetary pinions drive the planetary carrier in the same direction as the ring gear—forward.
More than one turn of the input is needed for one complete revolution of the output.
But because a large gear is driving a small gear, the amount of reduction is not as great.
Minimum Forward Reduction
In this combination, the sun gear is stationary and the ring gear rotates clockwise. This option is shown as combination 2 in Table 18–1 and in Figure 18–5. The ring gear drives the planetary pinions clockwise and walks around the stationary sun gear. The planetary pinions drive the planetary carrier in the same direction as the ring gear—forward. This results in more than one turn of the input as compared to one complete revolution of the output. The result is torque multiplication. But because a large gear is driving a small gear, the amount of reduction is not as great as in combination 1. The planetary gearset is operating in a forward reduction with the large ring gear driving
the small planetary carrier. Therefore, the combination produces minimum forward reduction.

13. Maximum Overdrive
The ring gear is stationary and the planetary carrier rotates clockwise.
The planetary pinion shafts push against planetary pinions.
The pinions are forced to walk around the inside of the ring gear, driving the sun gear clockwise.
The carrier is rotating less than one turn input compared to one turn output.
Maximum Overdrive
In this combination, the ring gear is stationary and the planetary carrier rotates clockwise. This option is shown as combination 3 in Table 18–1 and in Figure 18–6. The three planetary pinion shafts push against the inside diameter of the planetary pinions. The pinions are forced to walk around the inside of the ring gear, driving the sun gear clockwise. The carrier is rotating less than one turn input compared to one turn output, resulting in an overdrive condition. In this combination, the middle size planetary carrier is rotating less than one turn and driving the smaller sun gear at a speed greater than the input speed. The result is a fast overdrive with maximum speed increase.

14. Slow Overdrive
In this combination, the sun gear is stationary and the carrier rotates clockwise.
As the carrier rotates, the pinion shafts push against the pinions and they are forced to walk around the held sun gear.
This drives the ring gear faster.
The carrier turning less than one turn causes the pinions to drive the ring gear one complete revolution in the same direction as the planetary carrier.
Slow Overdrive
In this combination, the sun gear is stationary and the carrier rotates clockwise. This option is shown as combination 4 in Table 18–1 and in Figure 18–7. As the carrier rotates, the pinion shafts push against the inside diameter of the pinions and they are forced to walk around the held sun gear. This drives the ring gear faster and the speed increases. The carrier turning less than one turn causes the pinions to drive the ring gear one complete revolution in the same direction as the planetary carrier. As in combination 3, an overdrive condition exists, but the middle size carrier is now driving the larger size ring gear so only slow overdrive occurs.

15. Slow Reverse
Here, the sun gear is driving the ring gear while the planetary carrier is held.
The planetary pinions, rotate counterclockwise on their shafts.
While the sun gear is driving, the planetary pinions are used as idler gears to drive the ring gear counterclockwise.
Because the driving sun gear is small and the driven ring gear is large, the result is slow reverse.
Slow Reverse
Here, the sun gear is driving the ring gear while the planetary carrier is held stationary. This option is shown as combination 5 in Table 18–1 and in Figure 18–8. The planetary pinions, driven by the external sun gear, rotate counterclockwise on their shafts. The external teeth of the planetary pinions drive the internal teeth of the ring gear in the same direction. While the sun gear is driving, the planetary pinions are used as idler gears to drive the ring gear counterclockwise. This means the input and output shafts are operating in opposite directions to provide a reverse power flow. Because the driving sun gear is small and the driven ring gear is large, the result is slow reverse.

16. Fast Reverse
For fast reverse, the carrier is still held as in slow reverse, but the sun gear and ring gear reverse roles, with the ring gear now being the driving member and the sun gear driven.
In this combination, the input ring gear uses the planetary pinions as idlers to drive the output sun gear in reverse.
The sun gear rotates in reverse to the input ring gear.
Fast Reverse
For fast reverse, the carrier is still held as in slow reverse, but the sun gear and ring gear reverse roles, with the ring gear now being the driving member and the sun gear driven. This option is shown as combination 6 in Table 18–1 and in Figure 18–9. As the ring gear rotates counterclockwise, the pinions rotate counterclockwise as well, while the sun gear turns clockwise. In this combination, the input ring gear uses the planetary pinions to drive the output sun gear. The sun gear rotates in reverse to the input ring gear. In this operational combination, the large ring gear rotating counterclockwise drives the small sun gear clockwise, providing fast reverse.

17. Direct Drive Both the ring gear and the sun gear are input members.
Opposing forces lock the planetary pinions against rotation so that the entire planetary gearset rotates as one complete unit.
For direct drive, both input members must rotate at the same speed.
Direct Drive
In the direct drive combination, both the ring gear and the sun gear are input members. This option is shown as combination 7 in Table 18–1 and in Figure 18–10. They turn clockwise at the same speed. The internal teeth of the clockwise turning ring gear will try to rotate the planetary pinions clockwise as well. But the sun gear, an external gear rotating clockwise, will try to drive the planetary pinions counterclockwise. These opposing forces lock the planetary pinions against rotation so that the entire planetary gearset rotates as one complete unit. This ties together the input and output members and provides direct drive. For direct drive, both input members must rotate at the same speed.

18. Neutral
When no member is held stationary or locked, there will be input into the gearset, but no output.
The result is a neutral condition.
Neutral Operation
Combinations 1 through 7 (Table 18–1) all produce an output drive of various speeds, torques, and direction. In each case, one member of the planetary gear set is held or two members are locked for the output to take place. When no member is held stationary or locked, there will be input into the gearset, but no output. The result is a neutral condition (Figure 18–11).

19. Summary of Simple Planetary Gearset Operation
When the planetary carrier is the input, the gearset produces an overdrive. Speed increases and torque decreases.
When the planetary carrier is the driven (output) member, the gearset produces a forward reduction. Speed decreases and torque increases.
When the planetary carrier is held, the gearset will produce a reverse. To determine if the speed produced is fast or slow, remember the rules regarding large and small gears.
A large gear driving a small gear increases speed and reduces torque of the driven gear.
A small gear driving a large gear decreases speed and increases torque of the driven gear.
Summary of Simple Planetary
Gearset Operation
When the planetary carrier is the drive (input) member, the gearset produces an overdrive condition. Speed increases and torque decreases.
When the planetary carrier is the driven (output) member, the gearset produces a forward reduction. Speed decreases and torque increases.
When the planetary carrier is held, the gearset will produce a reverse. To determine if the speed produced is fast or slow, remember the rules regarding large and small gears.
A large gear driving a small gear increases speed and reduces torque of the driven gear.
A small gear driving a large gear decreases speed and increases torque of the driven gear.

20. Planetary Gear Controls
Multiple-disc clutches serve as both braking and power transfer devices.
A multiple disc clutch uses a series of circular friction discs to transmit torque or apply braking force.
The discs have internal teeth that are sized and shaped to mesh with splines on the clutch assembly hub.
This hub is connected to a planetary geartrain component so that gearset members receive the desired braking or transfer force when the clutch is applied or released.
Planetary Gear Controls
As we have seen, for a planetary gearset to produce an output, there must be some method of holding one planetary gear member stationary and applying drive torque to it. In most truck transmissions, this job is performed by multiple-disc clutches, which serve as both braking and power transfer devices. A multiple disc clutch uses a series of hollow friction discs to transmit torque or apply braking force. The discs have internal teeth that are sized and shaped to mesh with splines on the clutch assembly hub. In turn, this hub is connected to a planetary geartrain component so that gearset members receive the desired braking or transfer force when the clutch is applied or released.

21. Multiple Disc Clutch Design
The drum holds all other clutch components: the cylinder, hub, piston, piston return springs, seals, pressure plate, clutch pack (including friction plates), and snap rings.
The cylinder in a multiple-disc clutch is relatively shallow. The cylinder bore acts as a guide for piston travel.
The piston is made of cast aluminum or steel with a seal ring groove around the outside diameter.
A seal ring seats in the groove. This rubber seal retains fluid pressure required to stroke the piston and engage the clutch pack.
The piston return springs overcome the residual fluid pressure in the cylinder and move the piston to the disengaged position when clutch action (holding or transfer) is no longer needed.
Design. Multiple-disc clutches have a large drum shaped housing that can be either a separate casting or part of the transmission housing (Figure 18–12). This drum housing holds all other clutch components: the cylinder, hub, piston, piston return springs, seals, pressure plate, clutch pack (including friction plates), and snap rings. The cylinder in a multiple-disc clutch is relatively shallow. The cylinder bore acts as a guide for piston travel. The piston is made of cast aluminum or steel with a seal ring groove around the outside diameter. A seal ring seats in the groove. This rubber seal retains fluid pressure required to stroke the piston and engage the clutch pack. The piston return springs overcome the residual fluid pressure in the cylinder and move the piston to the disengaged position when clutch action (holding or transfer) is no longer needed. The clutch pack consists of clutch plates, friction discs, and one thick plate known as the pressure plate. The pressure plate has tabs around the outside diameter to mate with grooves in the clutch drum. It is held in place by a large snapring. An actuated piston forces the engaging clutch pack against the fixed pressure plate. Because the pressure plate cannot move or deflect, it provides reaction to the engaging clutch pack. The number of clutch plates and friction discs contained in the pack depends on the transmission size and application. The clutch plates are made from carbon steel and have several tabs in their outside diameter. These tabs are located in grooves machined into the inside diameter of the clutch drum housing. Clutch plates must be perfectly flat. Although the surface of the plate might appear smooth, it is machined to produce a friction surface to transmit torque. The friction discs are sandwiched between the clutch plates and the pressure plate. Friction discs are designed with a steel core plate center with friction material bonded to either side. Cellular paper fibers, graphites, and ceramics are used as friction facings. Cooling. During clutch engagement, friction takes place and develops heat between the clutch plates and friction discs. The transmission fluid absorbed by the paper-based friction material transfers heat from the discs to the plates. The plates then transfer the heat to the drum housing, where it can be cooled by the surrounding transmission fluid. Operating temperatures can reach 1,100° F, so clutch disc cooling is an important design consideration in transmission manufacturing. To assist cooling and provide other performance advantages, friction disc surfaces are normally grooved. Grooving is generally thought to provide the following advantages: • It allows the disc to store fluid to lubricate, cool, and quiet clutch engagement. • When the clutch is engaging, fluid between the disc and plates can be squeezed out. Fluid remaining between the friction elements lowers the coefficient of friction between them. Grooves in the friction disc surface provide channels through which the fluid can escape and drain quickly. • When a clutch is disengaging, any fluid on the disc and plate surfaces can create a suction effect, making the two surfaces more difficult to separate. Grooved surfaces help eliminate this problem and allow for clean disengagement at high speeds.

22. Multiple Disc Clutch Cooling
Operating temperatures can reach 1,100° F, so clutch disc cooling is an important design consideration in transmission manufacturing.
Friction disc surfaces are grooved to improve cooling.
Grooving provides the following advantages:
It allows the disc to store fluid to lubricate, cool, and quiet clutch engagement.
Grooves in the friction disc surface provide channels through which the fluid can escape and drain quickly.
When a clutch is disengaging, any fluid on the disc and plate surfaces can create a suction effect, making the two surfaces more difficult to separate.
Design. Multiple-disc clutches have a large drum shaped housing that can be either a separate casting or part of the transmission housing (Figure 18–12). This drum housing holds all other clutch components: the cylinder, hub, piston, piston return springs, seals, pressure plate, clutch pack (including friction plates), and snap rings. The cylinder in a multiple-disc clutch is relatively shallow. The cylinder bore acts as a guide for piston travel. The piston is made of cast aluminum or steel with a seal ring groove around the outside diameter. A seal ring seats in the groove. This rubber seal retains fluid pressure required to stroke the piston and engage the clutch pack. The piston return springs overcome the residual fluid pressure in the cylinder and move the piston to the disengaged position when clutch action (holding or transfer) is no longer needed. The clutch pack consists of clutch plates, friction discs, and one thick plate known as the pressure plate. The pressure plate has tabs around the outside diameter to mate with grooves in the clutch drum. It is held in place by a large snapring. An actuated piston forces the engaging clutch pack against the fixed pressure plate. Because the pressure plate cannot move or deflect, it provides reaction to the engaging clutch pack. The number of clutch plates and friction discs contained in the pack depends on the transmission size and application. The clutch plates are made from carbon steel and have several tabs in their outside diameter. These tabs are located in grooves machined into the inside diameter of the clutch drum housing. Clutch plates must be perfectly flat. Although the surface of the plate might appear smooth, it is machined to produce a friction surface to transmit torque. The friction discs are sandwiched between the clutch plates and the pressure plate. Friction discs are designed with a steel core plate center with friction material bonded to either side. Cellular paper fibers, graphites, and ceramics are used as friction facings. Cooling. During clutch engagement, friction takes place and develops heat between the clutch plates and friction discs. The transmission fluid absorbed by the paper-based friction material transfers heat from the discs to the plates. The plates then transfer the heat to the drum housing, where it can be cooled by the surrounding transmission fluid. Operating temperatures can reach 1,100° F, so clutch disc cooling is an important design consideration in transmission manufacturing. To assist cooling and provide other performance advantages, friction disc surfaces are normally grooved. Grooving is generally thought to provide the following advantages: • It allows the disc to store fluid to lubricate, cool, and quiet clutch engagement. • When the clutch is engaging, fluid between the disc and plates can be squeezed out. Fluid remaining between the friction elements lowers the coefficient of friction between them. Grooves in the friction disc surface provide channels through which the fluid can escape and drain quickly. • When a clutch is disengaging, any fluid on the disc and plate surfaces can create a suction effect, making the two surfaces more difficult to separate. Grooved surfaces help eliminate this problem and allow for clean disengagement at high speeds.

23. Compound Planetary Gearsets (1 of 3)
Multiple planetary gearsets are coupled to produce the required gear ratios and direction.
The four-speed heavy-duty transmissions have three simple gearsets that we call front, center, and rear.
Five-speed transmissions can add a low planetary gearset to the three-gear configuration for a total of four gearsets.
Input torque is directed to one of the planetary sun gears through the use of clutches.
COMPOUND PLANETARY GEARSETS
Automatic transmissions for both passenger car and heavy-duty truck applications use more than one simple planetary gearset coupled to generate the required range of output gear ratios and direction. The number of planetary gearsets that can be combined is limited by practical weight, space, and cost considerations. The four-speed heavy- duty transmissions discussed in this chapter have three simple gearsets that we call front, center, and rear. Five-speed transmissions can add a low planetary gearset to the three-gear configuration for a total of four gearsets in this compound arrangement. Any one of the three members of the simple gearset can serve as the input or drive member. And any of the three members could be held stationary. In a working transmission, this is not practical. There is a single path for input torque from the engine to supply drive torque. This input torque is directed to one of the planetary sun gears through the use of clutches, which are then used to option multiple torque path flows through the planetaries. Similarly, having a clutch (brake) to hold each member of each planetary gearset is not practical. A four- peed, three-gearset transmission uses only five clutches, excluding the torque converter lockup clutch. A five-speed, four-gearset transmission uses six clutches. They function as follows: • Forward clutch locks converter turbine to main input drive shaft. • First clutch anchors rear planetary ring gear. • Second clutch anchors front planetary carrier. • Third clutch anchors sun gear shaft. • Fourth clutch locks main input drive shaft to center sun gear shaft. • Low clutch (five speeds only) anchors low planetary carrier. The three planetaries are connected by a sun gear shaft assembly, mainshaft assembly, and a planetary connecting drum. This interconnection of planetary input, reaction, and output components produces the forward and reverse speeds and torque ranges required. Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset. Therefore one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset. The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain. With two simple planetaries working together, torque input is com-

24. Compound Planetary Gearsets (2 of 3)
A four-speed, three-gearset transmission uses only five clutches, excluding the torque converter lockup clutch.
A five-speed, four-gearset transmission uses six clutches. They function as follows:
Forward clutch locks converter turbine to main input drive shaft.
First clutch anchors rear planetary ring gear.
Second clutch anchors front planetary carrier.
Third clutch anchors sun gear shaft.
Fourth clutch locks main input drive shaft to center sun gear shaft.
Low clutch (five speeds only) anchors low planetary carrier.
COMPOUND PLANETARY GEARSETS
Automatic transmissions for both passenger car and heavy-duty truck applications use more than one simple planetary gearset coupled to generate the required range of output gear ratios and direction. The number of planetary gearsets that can be combined is limited by practical weight, space, and cost considerations. The four-speed heavy- duty transmissions discussed in this chapter have three simple gearsets that we call front, center, and rear. Five-speed transmissions can add a low planetary gearset to the three-gear configuration for a total of four gearsets in this compound arrangement. Any one of the three members of the simple gearset can serve as the input or drive member. And any of the three members could be held stationary. In a working transmission, this is not practical. There is a single path for input torque from the engine to supply drive torque. This input torque is directed to one of the planetary sun gears through the use of clutches, which are then used to option multiple torque path flows through the planetaries. Similarly, having a clutch (brake) to hold each member of each planetary gearset is not practical. A four- peed, three-gearset transmission uses only five clutches, excluding the torque converter lockup clutch. A five-speed, four-gearset transmission uses six clutches. They function as follows:
Forward clutch locks converter turbine to main input drive shaft.
First clutch anchors rear planetary ring gear.
Second clutch anchors front planetary carrier.
Third clutch anchors sun gear shaft.
Fourth clutch locks main input drive shaft to center sun gear shaft.
Low clutch (five speeds only) anchors low planetary carrier.
The three planetaries are connected by a sun gear shaft assembly, mainshaft assembly, and a planetary connecting drum. This interconnection of planetary input, reaction, and output components produces the forward and reverse speeds and torque ranges required. Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset. Therefore one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset. The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain. With two simple planetaries working together, torque input is compounded. The gearsets used to produce forward and reverse ranges in typical four-speed and five-speed transmissions are summarized in Table 18–2. Table 18–3 summarizes the clutches applied to produce the required powerflow paths.

25. Compound Planetary Gearsets (3 of 3)
Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset.
Therefore, one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset.
The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain.
With two simple planetaries working together, torque input is compounded.
COMPOUND PLANETARY GEARSETS
Automatic transmissions for both passenger car and heavy-duty truck applications use more than one simple planetary gearset coupled to generate the required range of output gear ratios and direction. The number of planetary gearsets that can be combined is limited by practical weight, space, and cost considerations. The four-speed heavy- duty transmissions discussed in this chapter have three simple gearsets that we call front, center, and rear. Five-speed transmissions can add a low planetary gearset to the three-gear configuration for a total of four gearsets in this compound arrangement. Any one of the three members of the simple gearset can serve as the input or drive member. And any of the three members could be held stationary. In a working transmission, this is not practical. There is a single path for input torque from the engine to supply drive torque. This input torque is directed to one of the planetary sun gears through the use of clutches, which are then used to option multiple torque path flows through the planetaries. Similarly, having a clutch (brake) to hold each member of each planetary gearset is not practical. A four- peed, three-gearset transmission uses only five clutches, excluding the torque converter lockup clutch. A five-speed, four-gearset transmission uses six clutches. They function as follows: • Forward clutch locks converter turbine to main input drive shaft. • First clutch anchors rear planetary ring gear. • Second clutch anchors front planetary carrier. • Third clutch anchors sun gear shaft. • Fourth clutch locks main input drive shaft to center sun gear shaft. • Low clutch (five speeds only) anchors low planetary carrier. The three planetaries are connected by a sun gear shaft assembly, mainshaft assembly, and a planetary connecting drum. This interconnection of planetary input, reaction, and output components produces the forward and reverse speeds and torque ranges required. Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset. Therefore one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset. The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain. With two simple planetaries working together, torque input is com- The three planetaries are connected by a sun gear shaft assembly, mainshaft assembly, and a planetary connecting drum. This interconnection of planetary input, reaction, and output components produces the forward and reverse speeds and torque ranges required. Because the simple planetary gearsets are interconnected by common shafts and drums, the output of one planetary gearset can be used as the input of another planetary gearset. Therefore one gearset can take engine input torque, modify speed, torque, and direction, and pass this onto a second gearset. The second gearset then further modifies speed, torque, and direction before directing the power flow to the output shaft and vehicle drivetrain. With two simple planetaries working together, torque input is com-pounded. The gearsets used to produce forward and reverse ranges in typical four-speed and five-speed transmissions are summarized in Table 18–2. Table 18–3 summarizes the clutches applied to produce the required powerflow paths.

26. Laws of Simple Planetary Gear Operation
See Table 18-1 on page 531 of the textbook.

27. Planetary Gearset Combinations
See Table 18-2 on page 537 of the textbook.

28. Oil Pump An oil pump is used to circulate oil for:
Lubrication
Cooling
Hydraulic clutch application
The pump is located ahead of the gearing and clutches, and it will not be driven when the vehicle is towed or pushed.
Whenever the vehicle must be towed or pushed more than a short distance (one-half mile maximum), the driveline must be disconnected or the driving wheels raised.
Failure to do this will result in premature wear and damage to internal components.
Oil Pump
An oil pump is used to circulate transmission fluid within the transmission for both lubrication and the hydraulic application of clutches. This pump is driven from the torque converter and is located between the torque converter and the transmission gearing and clutches (Figure 18–13). When the torque converter rotates, its rear hub drives the pump drive gear, which is in mesh with the driven gear. As the gears rotate, they draw oil into the pump and move the oil throughout the hydraulic system. Because the pump is located ahead of the transmission gearing and clutches, it will not be driven when the vehicle is towed or pushed. Therefore, any time the vehicle must be towed or pushed more than a short distance (one-half mile maximum), the driveline must be disconnected or the driving wheels raised. Failure to do this will result in premature wear and damage to internal components.

29. Four-speed Clutch Application
See Table 18-4 on page 543 of textbook.

30. Five-speed Clutch Application
See Table 18-5 on page 546 of textbook.

31. Converter Cooler Circuit
Converter-in oil flows to the torque converter. Oil must flow through the converter continuously to keep it filled and to remove heat generated by the converter. The converter pressure regulator valve regulates converter-in pressure by exhausting excessive oil to sump.
Converter-out oil, exiting the torque converter, flows to an external cooler (a vehicle OEM-supplied component). Ram air or engine coolant is used to remove heat from the transmission oil at the cooler heat exchanger.
Lubrication oil is directed from there through the transmission to those components requiring continuous lubrication and cooling.

32. Modulator Pressure Circuit
Modulator pressure is used to modify shift points in response to throttle position.
Modulator pressure is highest when the engine is running at idle and decreases in proportion to accelerator pedal travel.
The modulator valve assists the governor pressure in moving any of the shift signal valves during an upshift.
It can also delay a downshift. A decrease in modulator pressure will cause a downshift if governor pressure is not sufficient to oppose the shift valve spring force.
The trimmer boost and trimmer regulator valve are controlled by modulator pressure.
MODULATOR PRESSURE CIRCUIT
The modulator valve assists the governor pressure in moving any of the shift signal valves during an upshift. It can also delay a downshift. The modulator valve controls a regulated pressure derived from main pressure. The modulator valve receives main pressure and regulates it as a direct result of the accelerator position. The modulator actuator varies modulator pressure as the accelerator pedal moves. It can be controlled in a number of ways. On gasoline engines, vacuum pressure is used. On diesel engines, air pressure or a mechanical linkage can be used to operate the modulator. The mechanical linkage is most common. Modulator pressure is highest when the engine is running at idle and decreases in proportion to accelerator pedal travel. In a mechanical linkage system, the modulator is controlled by a push-or-pull cable connected to the throttle. When there is no throttle pedal pressure, the valve is moved to the right by a spring at the left end of the valve. As the driver depresses the throttle pedal, a wedge-shaped actuator forces the modulator valve to the left, decreasing modulator pressure. When spring force at the left of the valve is in balance with the spring force and modulator pressure at the right of the valve, modulator pressure is regulated. When the accelerator is depressed, the movement of the actuator cam forces the modulator valve to the left. This leftward movement reduces modulator pressure. When the pedal travel is reduced, the downward movement of the actuator cam allows the spring at the left to push the valve to another regulating position. Because the throttle pedal angle varies with load and engine speed, modulator pressure also varies. This varying pressure is directed to the 1–2, 2–3, and 3–4 shift signal valves (four-speed); the 2–3, 3–4, and 4–5 shift signal valves (five-speed); the trimmer regulator valve; and to the modulated lockup valve. Earlier models do not include the modulated lockup valve. At the shift signal valves, modulator pressure assists governor pressure to overcome shift valve spring force. The surface areas and spring tension of each one of the shift valves are so calibrated that the valves will shift in the proper sequence at the right time. A decrease in modulator pressure will cause a downshift if governor pressure is not sufficient to oppose the shift valve spring force. The trimmer boost and trimmer regulator valve are controlled by modulator pressure. At the modulated lockup valve, modulator pressure assists governor pressure in moving (or holding) the valve downward. The variation in modulator pressure will vary the timing of lockup clutch engagement and release. The primary purpose of lockup modulation is to prolong the engagement of the lockup clutch (at closed throttle) in lower ranges to provide greater engine braking. All modulated lockup valves are adjustable, but not all transmissions have modulated lockup. Models with adjustable lockup can be adjusted by rotating the adjusting ring at the bottom of the valve until the desired shift point is attained (see Chapter 19).

33. Trimmer Regulator Valve
The trimmer regulator valve reduces main pressure to a regulated pressure. The regulated pressure is raised or lowered by changes in modulator pressure.
This varies the clutch apply pressure pattern of the trimmer valves.
A higher modulator pressure at zero accelerator pedal travel will reduce trimmer regulator pressure.
A lower modulator pressure (open throttle) results in higher regulator pressure and a higher initial clutch apply pressure.
TRIMMER REGULATOR VALVE
Late generation Allison hydromechanical transmissions are equipped with trimmer regulator valves. The trimmer regulator valve reduces main pressure to a regulated pressure. The regulated pressure is raised or lowered by changes in modulator pressure. Trimmer regulator pressure is directed to the lower sides of the first and second trimmer plugs on some models, or to the lower sides of the first, second, third, and fourth trimmer plugs on models equipped with smooth shift. This varies the clutch apply pressure pattern of the trimmer valves. A higher modulator pressure at zero accelerator pedal travel will reduce trimmer regulator pressure. This results in a lower initial clutch- apply pressure. Conversely, a lower modulator pressure (open throttle) results in higher regulator pressure and a higher initial clutch apply pressure.

34. Lockup Circuit
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.
The purpose of the modulated lockup valve is to prolong lockup clutch engagement while vehicle speed decreases (closed throttle).
This feature provides engine-braking action at speeds lower than the normal lockup disengagement point.
In its downward position, the modulated lockup valve directs main pressure to the top of the lockup valve. Main pressure pushes the lockup valve downward. In its downward position, the valve directs main pressure to the lockup clutch. This engages the clutch.
LOCKUP CIRCUIT
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve. The modulated lockup shift valve is actuated by governor pressure or governor plug modulator pressure. At full throttle, governor pressure will move the valve downward. At part throttle, governor and modulator pressures will move the valve downward. The purpose of the modulated lockup valve is to prolong lockup clutch engagement while vehicle speed decreases (closed throttle). This feature provides engine-braking action at speeds lower than the normal lockup disengagement point. In its downward position, the modulated lockup valve directs main pressure to the top of the lockup valve. Main pressure pushes the lockup valve downward. In its downward position, the valve directs main pressure to the lockup clutch. This engages the clutch. The position of the lockup valve affects the volume of oil flowing to the torque converter. When the valve is upward (lockup clutch released), there is maximum flow to the converter. When the valve is downward (clutch engaged), converter-in flow is restricted by an orifice, and flow is reduced. When the lockup clutch is engaged, a signal pressure is sent to the main-pressure regulator valve. This signal pressure reduces main pressure to a lower pressure schedule. On most models with adjustable lockup, modulator assistance at the modulated lockup valve is not present. By setting the adjusting ring to the shift point specifications, with the assistance of governor pressure, lockup engagement is initiated. Models that do not have modulated lockup do not include the modulated lockup valve. Governor pressure is directed to the top of the lockup valve.

35. Priority Valve Gives the control system priority
The priority valve ensures that the control system upstream from the valve will have sufficient pressure during shifts to perform its automatic functions.
Without the priority valve, the filling of the clutch might require a greater volume of oil (momentarily) than the pump could supply and still maintain pressure for the necessary control functions.
In many models, first clutch pressure can bypass the priority valve when the transmission is in reverse.

36. Inhibitor Valve Inhibits downshifts Prevents overspeeding
In five-speed transmissions, a 2–1 inhibitor valve prevents a downshift from second gear to first gear when road speed is too high.
Prevents overspeeding
It will also protect the engine from overspeeding during downgrade operation in first gear by making a 1–2 upshift if road speed exceeds that which is safe for first gear operation.
INHIBITOR VALVE
In five-speed transmissions, a 2–1 inhibitor valve prevents a downshift from second gear to first gear when road speed is too high. It will also protect the engine from overspeeding during downgrade operation in first gear by making a 1–2 upshift if road speed exceeds that which is safe for first gear operation. When road speed is great enough to produce a governor pressure that forces the 2–1 inhibitor valve downward, D1 pressure to the 1–2 shift valve is blocked. Thus, if a manual shift to drive 1 (first gear) is made, D1 pressure cannot reach the 1–2 shift valve. The transmission will remain in second gear until road speed (governor pressure) decreases to a value that permits the 2–1 inhibitor valve to move upward. In its upward position, D1 pressure can reach the 1–2 shift valve and move it downward to the first gear position. While operating in first gear, the 2–1 inhibitor valve will move downward if road speed (governor pressure) exceeds that which is safe for first-gear operation. This will block D1 pressure and exhaust the cavity above the 1–2 shift valve. The shift valve will move upward to give second gear operation. While operating in first gear, the 2–1 inhibitor valve will move downward if road speed (governor pressure) exceeds that which is safe for first-gear operation. This will block D1 pressure and exhaust the cavity above the 1–2 shift valve. The shift valve will move upward to give second gear operation.
There are five separate clutches in four-speed transmissions
and six clutches in five-speed models. As
covered earlier, various clutch combinations are used
in various drive ranges (see Table 18–3).
Each clutch has its own circuit. The first, second,
third, and fourth clutches are each connected to a relay
valve and to a trimmer valve. The forward clutch is
connected directly to the selector valve and does not
connect to a trimmer valve. Because the vehicle is not
moving, the application of a trimmer valve is not required.
The low clutch is not trimmed because there is
no automatic upshifting from, or downshifting to, this
clutch.
The low clutch circuit (five-speeds) connects the
clutch to the 1–2 shift valve. The 1–2 shift valve also
receives 1–2 (low and first) feed from the 2–3 relay
valve. In neutral, the 1–2 shift valve is held upward by
spring pressure. It cannot move downward unless
drive 1 line is charged. In the upward position, 1–2
(low and first) feed pressure is sent to the first clutch.
The first clutch circuit connects the clutch to the first
trimmer valve and to either the 1–2 relay valve (fourspeeds)
or to the 1–2 shift valve (five-speeds).
In neutral the 1–2 shift valve is held upward by
spring pressure. The 1–2 shift valve cannot move
downward unless the 2–1 signal line is charged. This
will not occur in neutral (vehicle standing) because
there is no governor pressure to shift the 2–1 inhibitor
valve. Only the first clutch is applied, so the transmission
output cannot rotate. A bypass and check ball
are provided between the reverse and first trimmer
passages to ensure a rapid and more positive shift
from first gear to reverse.
The first clutch is also applied in first gear operation
(four-speeds), second gear operation (five-speeds),
and reverse gear operation. Shifting the selector from
neutral to D, D3, D2, or D1 charges the forward clutch
circuit and applies the forward clutch. Shifting from
neutral to reverse (R) charges the fourth clutch, while
the first clutch remains charged. In reverse, fourth
clutch (reverse signal) pressure is also directed to the
bottom of 1–2 relay valve (four-speeds) or 2–3 relay
valve (five-speeds). The pressure at this location prevents
either relay valve from moving downward.
When the circuits are charged and the selector valve
is at D, four-speed transmissions will automatically
shift from first to second, second to third, and third to
fourth. Five-speed models will automatically shift from
second to third, third to fourth, and fourth

37. Clutch Circuits
There are five separate clutches in four-speed transmissions and six clutches in five-speed models.
The first, second, third, and fourth clutches are each connected to a relay valve and to a trimmer valve.
The forward clutch is connected directly to the selector valve and does not connect to a trimmer valve.
Four-speed transmissions will automatically shift from first to second, second to third, and third to fourth.
Five-speed models will automatically shift from second to third, third to fourth, and fourth to fifth. These shifts occur as a result of governor pressure, and (if the throttle is not fully open) modulator pressure.
CLUTCH CIRCUITS
There are five separate clutches in four-speed transmissions and six clutches in five-speed models. As covered earlier, various clutch combinations are used in various drive ranges (see Table 18–3). Each clutch has its own circuit. The first, second, third, and fourth clutches are each connected to a relay valve and to a trimmer valve. The forward clutch is connected directly to the selector valve and does not connect to a trimmer valve. Because the vehicle is not moving, the application of a trimmer valve is not required. The low clutch is not trimmed because there is no automatic upshifting from, or downshifting to, this clutch. The low clutch circuit (five-speeds) connects the clutch to the 1–2 shift valve. The 1–2 shift valve also receives 1–2 (low and first) feed from the 2–3 relay valve. In neutral, the 1–2 shift valve is held upward by spring pressure. It cannot move downward unless drive 1 line is charged. In the upward position, 1–2 (low and first) feed pressure is sent to the first clutch. The first clutch circuit connects the clutch to the first trimmer valve and to either the 1–2 relay valve (four-speeds) or to the 1–2 shift valve (five-speeds). In neutral the 1– shift valve is held upward by spring pressure. The 1–2 shift valve cannot move downward unless the 2–1 signal line is charged. This will not occur in neutral (vehicle standing) because there is no governor pressure to shift the 2–1 inhibitor valve. Only the first clutch is applied, so the transmission output cannot rotate. A bypass and check ball are provided between the reverse and first trimmer passages to ensure a rapid and more positive shift from first gear to reverse. The first clutch is also applied in first gear operation (four-speeds), second gear operation (five-speeds), and reverse gear operation. Shifting the selector from neutral to D, D3, D2, or D1 charges the forward clutch circuit and applies the forward clutch. Shifting from neutral to reverse (R) charges the fourth clutch, while the first clutch remains charged. In reverse, fourth clutch (reverse signal) pressure is also directed to the bottom of 1–2 relay valve (four-speeds) or 2–3 relay valve (five-speeds). The pressure at this location prevents either relay valve from moving downward. When the circuits are charged and the selector valve is at D, four-speed transmissions will automatically shift from first to second, second to third, and third to fourth. Five-speed models will automatically shift from second to third, third to fourth, and fourth to fifth. These shifts occur as a result of governor pressure, and (if the throttle is not fully open) modulator pressure. The position of the selector valve determines the highest gear that can be reached automatically while the vehicle is moving. Four-speed models that do not incorporate a second gear start will automatically shift 1–2, 2–3, and 3–4 when the selector valve is in drive 4 position. In drive 3, automatic 1–2 and 2–3 shifts occur. In drive 2, an automatic 1–2 shift occurs. Drive 1 does not permit an upshift to occur unless an overspeed of the transmission output occurs. The position of the selector valve in a second-gear start is the same but transmission performance will be altered. The transmission always starts in second gear when any forward range other than drive 1 is selected. When drive 4 is selected, the transmission starts in second gear and automatically shifts 2–3 and through to 3–4. In drive 3, an automatic 2–3 shift occurs. Drive 2 does not permit an upshift to occur unless an overspeed of the transmission output occurs. Drive 1 is the only position in which first gear is available and upshifting is not permitted unless an overspeed of the transmission output occurs. In five-speed transmissions drive (D) automatic 2–3, 3–4, and 4–5 shifts can occur. In drive 3 (D3), automatic 2–3 and 3–4 shifts can occur. With some early transmissions an automatic 2–3 shift can occur in drive 2 (D2). In recent models, no shift from 2 can occur unless overspeed occurs. In drive 1 (D1), no shift from 1 can occur unless overspeed occurs. The various drive ranges limit the highest gear attainable by introducing a pressure that prevents governor pressure from upshifting the signal valves (unless governor pressure is well above that normally attained). This pressure is a regulated, reduced pressure
sourced from the main pressure circuit at the hold regulator valve. Main pressure is directed to the hold regulator valve through the hold feed line when the selector lever is in drive 1, drive 2, or drive 3 position. The pressure produced in the hold regulator valve is directed to the 3–4 shift valve (four-speeds) or the 4–5 shift valve (five-speeds) when the selector is in drive 3. The hold pressure is directed to the 2–3 and 3–4 (four-speeds) or the 3–4 and 4–5 (five-speeds) when the selector is in drive 2 position. In some later generation five-speed models, hold pressure is directed to the 2–3, 3–4, and 4–5 shift valves. In other four- and five-speeds, pressure can be directed to all shift signal valves when the selector is in drive 1. Hold regulator pressure at each shift signal valve will push the modulator valve upward and exert downward force on the shift valve. So, when hold pressure is present, upshifts are inhibited except in extreme overspeed conditions.

38. Automatic Upshifts
A combination of governor pressure and modulator pressure, or governor pressure alone, upshifts the transmission to second or third gear, respectively.
At closed or part throttle, modulator pressure exists and will assist governor pressure. When the throttle is closed, shifts occur at lower wheel speeds.
At full throttle, there is no modulator pressure, meaning that upshifts occur at higher road speeds.
The greater the vehicle speed, the greater the governor pressure.
When governor pressure is sufficient, the first upshift will occur. With a further increase in governor pressure, the second upshift will occur, etc.
In any automatic upshift, the shift signal valve acts first. This directs a shift pressure to the relay valve. The relay valve shifts, exhausting the applied clutch and applying a clutch for a higher gear.
AUTOMATIC UPSHIFTS
When the transmission is operating in first gear (four-speed) or second gear (five-speed), with the selector valve at drive (D), a combination of governor pressure and modulator pressure, or governor pressure alone, upshifts the transmission to second or third gear, respectively. At closed or part throttle, modulator pressure exists and will assist governor pressure. At full throttle, there is no modulator pressure, meaning that upshifts occur at higher road speeds. When the throttle is closed, shifts occur at lower wheel speeds. Governor pressure is dependent upon the rotational speed of the transmission output. The greater the output speed (vehicle speed), the greater the governor pressure. When governor pressure is sufficient, the first upshift will occur. With a further increase in governor pressure (and vehicle speed), the second upshift will occur. A still further increase will cause a third upshift. Modulator pressure, when present, will assist governing pressure in overcoming shift valve spring force. In any automatic upshift, the shift signal valve acts first. This directs a shift pressure to the relay valve. The relay valve shifts, exhausting the applied clutch and applying a clutch for a higher gear.

39. Automatic Downshifts
Automatic downshifts, like upshifts, are controlled by governor and modulator pressures.
Downshifts occur in sequence as governor pressure and/or modulator pressure decrease.
Low modulator pressure (full accelerator pedal travel) will hasten the downshift; high modulator pressure (zero accelerator pedal travel) will delay downshifts.
In any automatic downshift, the shift signal valve acts first. This exhausts the shift signal holding the relay valve downward. The relay valve then moves upward, exhausting the applied clutch and applying the clutch for the new lower gear.
AUTOMATIC DOWNSHIFTS
Automatic downshifts, like upshifts, are controlled by governor and modulator pressures. Downshifts occur in sequence as governor pressure and/or modulator pressure decrease. Low modulator pressure (full accelerator pedal travel) will hasten the downshift; high modulator pressure (zero accelerator pedal travel) will delay downshifts. In any automatic downshift, the shift signal valve acts first. This exhausts the shift signal holding the relay valve downward. The relay valve then moves upward, exhausting the applied clutch and applying the clutch for the new lower gear.

40. Downshift and Reverse Inhibiting
The system is designed, to prevent downshifts at too rapid a rate or to prevent a shift to reverse while moving forward.
The progressive downshift occurs because the pressure and the valve sectional areas are calibrated to only shift the signal valves downward against governor pressure when governor pressure decreases to a safe downshift speed.
So if speed is too great, governor pressure is sufficient to hold the shift signal valve upward.
As governor pressure decreases, all shift signal valves move downward in sequence.
Downshift and Reverse Inhibiting
The system is designed, as a result of valve sectional areas and spring forces, to prevent downshifts at too rapid a rate or to prevent a shift to reverse while moving forward. For example, if the vehicle is traveling at a high speed in fourth gear (4) and the selector valve is inadvertently moved to D1, the transmission will not immediately shift to first gear. Instead it will shift 4–3—2–1 as speed decreases. (It will remain in fourth gear if speed is not decreased sufficiently to require an automatic downshift.) The progressive downshift occurs because the hold regulator pressure is calibrated, along with the valve sectional areas, to shift the signal valves downward against governor pressure only when governor pressure decreases to a value corresponding to a safe downshift speed. So if speed is too great, governor pressure is sufficient to hold the shift signal valve upward against drive 3, drive 2, or drive 1 pressure, all of which are the regulated holding pressures originating in the hold regulator valve. As governor pressure decreases, all shift signal valves move downward in sequence.

41. Hydraulic Retarders
Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks.
Some retarder systems are designed as an integral component of the automatic transmission.
They provide some vehicle braking on severe downgrades and help maximize service brake life by providing braking action at the driveline.
Integral retarders are usually available in two automatic transmission configurations:
The input retarder
The output retarder

42. Input Retarders
The input retarder is located between the torque converter housing and the main housing. It is designed primarily for highway applications.
The unit uses a paddle wheel design with a vaned rotor mounted between stator vanes in the retarder housing.
Retardation or brake mode occurs when transmission oil is directed into the retarder housing.
The oil causes resistance to the rotation of a vaned rotor.
Retarding capacity can be enhanced by downshifting.
Variable control is achieved by moving a hand lever or activating a separate foot pedal.
Heat is dissipated by circulating the oil through a high-capacity transmission oil cooler.

43. Output Retarders
Most output retarders are mounted on the rear of the transmission without adding additional length to it.
The output retarder uses a two-stage principle.
The first stage consists of a rotor/stator hydraulic design. The second stage can be a powerful, oil-cooled, friction clutch pack.
On activation of the output retarder, quick initial response is provided by a momentary application of the friction clutch pack while the hydraulic section is being charged with oil.
The hydraulic section provides better retardation at higher speeds. As the vehicle slows, the second stage friction clutch pack phases in, providing the low-speed retardation power capable of slowing the vehicle to a virtual stop.
OUTPUT RETARDERS
Most output retarders are mounted on the rear of the transmission without adding additional length to it. They apply retarding force directly to the driveline. The output retarder uses a two-stage principle. The first stage consists of a rotor/stator hydraulic design. The second stage can be a powerful, oil-cooled, friction clutch pack. On activation of the output retarder, quick initial response is provided by a momentary application of the friction clutch pack while the hydraulic section is being charged with oil. The hydraulic section provides better retardation at higher speeds. As the vehicle slows, the second stage friction clutch pack phases in, providing the low-speed retardation power capable of slowing the vehicle to a virtual stop. Because it is located at the rear of the transmission and transmits retarding force directly to the driveline for greater effectiveness, the output retarder functions independently of engine speed or transmission range. Heat generated by retardation is dissipated through the transmission cooling system. Activation of the output retarder can be by a hand lever or separate foot pedal for highway downhill speed control. Retarder control can also be incorporated into the service brake pedal for stop-and-go traffic operations. This method provides a smooth transition from retarder to service brakes, depending on braking effort needed. For flexibility in varied driving conditions both systems can be used together.

44. Summary (1 of 6)
Automatic transmissions may be hydromechanical or electronic. All automatic transmissions upshift and downshift with no direct assistance from the driver.
Factors such as road speed, throttle position, and governed engine speed control shifting between gears.
Automatic transmissions use compounded planetary gearsets.
The simple planetary gearset consists of three main components: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus.
Automatic transmissions may be hydromechanical or electronic. All automatic transmissions upshift and downshift with no direct assistance from the driver.
Factors such as road speed, throttle position, and governed engine speed control shifting between gears.
Automatic transmissions use compounded planetary gearsets. The simple planetary gearset consists of three main components: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus.
Advantages of the planetary gearset are as follows:
constantly meshed gears
gear torque loads are divided equally
compact and versatile
additional ratio and direction variations can be added by compounding
In a planetary gearset, any one of the three main members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.
Depending on which member of the planetary gearset is the driver, which is held, and which is driven, torque or speed increases are produced.
To produce an output, one of the planetary gear members must be held stationary. In heavy-duty truck transmissions this is achieved with multiple- disc clutches, which can serve as both braking and power transfer devices.
Compound planetary combinations are several planetary gearsets coupled to produce the required gear ratios and direction. Four-speed, heavy-duty transmissions have three simple planetary gearsets—front, center, and rear. Five speed transmissions add an additional low planetary gearset for a total of four gearsets.
In all automatic transmissions, power flows from the torque converter through the transmission planetary gearing and out to the output shaft.
Transmission oil is drawn from the sump through a filter by the input-driven oil pump.
Oil pressurized by the pump flows into the bore of the main pressure regulator valve.
The converter/cooler/lubrication circuit originates at the main pressure regulator valve.
The selector valve/forward regulator circuit is manually shifted to select the operating range desired.
Main pressure is directed to the governor valve. The speed of the transmission output shaft controls the position of the governor valve, which determines the amount of pressure in the governor circuit.
The modulator actuator varies modulator pressure as the accelerator pedal moves.
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.
There are five separate clutches in four-speed transmissions and six clutches in five-speed models, each clutch having its own circuit.
Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks; they are applied by the driver as needed.

45. Summary (2 of 6) Advantages of the planetary gearset are as follows:
Constantly meshed gears
Gear torque loads divided equally
Compact and versatile
Additional ratio and direction variations can be added by compounding
In a planetary gearset, any one of the three main members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.
Depending on which member of the planetary gearset is the driver, which is held, and which is driven, torque or speed increases are produced.
Automatic transmissions may be hydromechanical or electronic. All automatic transmissions upshift and downshift with no direct assistance from the driver.
Factors such as road speed, throttle position, and governed engine speed control shifting between gears.
Automatic transmissions use compounded planetary gearsets. The simple planetary gearset consists of three main components: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus.
Advantages of the planetary gearset are as follows:
constantly meshed gears
gear torque loads are divided equally
compact and versatile
additional ratio and direction variations can be added by compounding
In a planetary gearset, any one of the three main members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.
Depending on which member of the planetary gearset is the driver, which is held, and which is driven, torque or speed increases are produced.
To produce an output, one of the planetary gear members must be held stationary. In heavy-duty truck transmissions this is achieved with multiple- disc clutches, which can serve as both braking and power transfer devices.
Compound planetary combinations are several planetary gearsets coupled to produce the required gear ratios and direction. Four-speed, heavy-duty transmissions have three simple planetary gearsets—front, center, and rear. Five speed transmissions add an additional low planetary gearset for a total of four gearsets.
In all automatic transmissions, power flows from the torque converter through the transmission planetary gearing and out to the output shaft.
Transmission oil is drawn from the sump through a filter by the input-driven oil pump.
Oil pressurized by the pump flows into the bore of the main pressure regulator valve.
The converter/cooler/lubrication circuit originates at the main pressure regulator valve.
The selector valve/forward regulator circuit is manually shifted to select the operating range desired.
Main pressure is directed to the governor valve. The speed of the transmission output shaft controls the position of the governor valve, which determines the amount of pressure in the governor circuit.
The modulator actuator varies modulator pressure as the accelerator pedal moves.
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.
There are five separate clutches in four-speed transmissions and six clutches in five-speed models, each clutch having its own circuit.
Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks; they are applied by the driver as needed.

46. Summary (3 of 6)
To produce an output, one of the planetary gear members must be held stationary.
In heavy-duty truck transmissions this is achieved with multiple-disc clutches that can serve as both braking and power transfer devices.
Compound planetary combinations are several planetary gearsets coupled to produce the required gear ratios and direction.
Four-speed, heavy-duty transmissions have three simple planetary gearsets—front, center, and rear.
Five-speed transmissions add an additional low planetary gearset for a total of four gearsets.
Automatic transmissions may be hydromechanical or electronic. All automatic transmissions upshift and downshift with no direct assistance from the driver.
Factors such as road speed, throttle position, and governed engine speed control shifting between gears.
Automatic transmissions use compounded planetary gearsets. The simple planetary gearset consists of three main components: a sun gear, a carrier with planetary pinions mounted to it, and an internally toothed ring gear or annulus.
Advantages of the planetary gearset are as follows:
constantly meshed gears
gear torque loads are divided equally
compact and versatile
additional ratio and direction variations can be added by compounding
In a planetary gearset, any one of the three main members—sun gear, pinion gear carrier, or ring gear—can be used as the driving or input member.
Depending on which member of the planetary gearset is the driver, which is held, and which is driven, torque or speed increases are produced.
To produce an output, one of the planetary gear members must be held stationary. In heavy-duty truck transmissions this is achieved with multiple- disc clutches, which can serve as both braking and power transfer devices.
Compound planetary combinations are several planetary gearsets coupled to produce the required gear ratios and direction. Four-speed, heavy-duty transmissions have three simple planetary gearsets—front, center, and rear. Five speed transmissions add an additional low planetary gearset for a total of four gearsets.
In all automatic transmissions, power flows from the torque converter through the transmission planetary gearing and out to the output shaft.
Transmission oil is drawn from the sump through a filter by the input-driven oil pump.
Oil pressurized by the pump flows into the bore of the main pressure regulator valve.
The converter/cooler/lubrication circuit originates at the main pressure regulator valve.
The selector valve/forward regulator circuit is manually shifted to select the operating range desired.
Main pressure is directed to the governor valve. The speed of the transmission output shaft controls the position of the governor valve, which determines the amount of pressure in the governor circuit.
The modulator actuator varies modulator pressure as the accelerator pedal moves.
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.
There are five separate clutches in four-speed transmissions and six clutches in five-speed models, each clutch having its own circuit.
Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks; they are applied by the driver as needed.

47. Summary (4 of 6)
In all automatic transmissions, power flows from the torque converter through the transmission planetary gearing and out to the output shaft.
Transmission oil is drawn from the sump through a filter by the input-driven oil pump.
Oil pressurized by the pump flows into the bore of the main pressure regulator valve.

48. Summary (5 of 6)
The converter/cooler/lubrication circuit originates at the main pressure regulator valve.
The selector valve/forward regulator circuit is manually shifted to select the operating range desired.
Main pressure is directed to the governor valve.
The speed of the transmission output shaft controls the position of the governor valve, which determines the amount of pressure in the governor circuit.

49. Summary (6 of 6)
The modulator actuator varies modulator pressure as the accelerator pedal moves.
Lockup clutch engagement and release are controlled by the modulated lockup valve and the lockup relay valve.
There are five separate clutches in four-speed transmissions and six clutches in five-speed models, each clutch having its own circuit.
Hydraulic retarders are one of several types of auxiliary braking systems used on heavy-duty trucks; they are applied by the driver as needed.