OBJECTIVES After studying Chapter 6, the reader should be able to:

OBJECTIVES After studying Chapter 6, the reader should be able to:
Discuss hydraulic principles. Describe how a hydraulic system operates. Identify the parts of a transmission hydraulic system and explain their purpose. Explain the requirements for a transmission hydraulic system. Identify the requirements for automatic transmission fluid and the differences between fluids.

INTRODUCTION The automatic transmission’s hydraulic system has several important functions . It must be able to: Apply the clutches and bands and therefore control the transmission power flow, Transmit sufficient force and motion to completely apply the control units to prevent slippage, Maintain fluid flow through the torque converter for its proper operation, Maintain fluid flow to lubricate and cool the moving parts of the gear train.

INTRODUCTION FIGURE 6-1 A hydraulic diagram for a four-speed transmission with electronic controls. Diagrams are used to determine the relationship of the components. They are often color-coded to help locate the circuits. (Courtesy of Chrysler Corporation)

HYDRAULIC PRINCIPLES Hydraulics, often called fluid power, is a method of transmitting motion and/or force. Hydraulics is based on the fact that liquids can flow easily through complicated paths, but they cannot be compressed. All the components in a hydraulic system are connected so that fluid pressure can be transmitted and allowed to work.

HYDRAULIC PRINCIPLES FIGURE 6-2 Fluid pressure is transmitted undiminished in all directions. Note that the pressure is equal throughout the system.

HYDRAULIC PRINCIPLES FIGURE 6-3 Fluids flow freely and will assume the shape of their container (a), yet they are virtually noncompressible (b).

HYDRAULIC PRINCIPLES FIGURE 6-4 A 100-lb force applied on an input piston that has an area of 1 in. will produce a fluid pressure of 100 psi. Pressure is defined as the amount of force applied to a given area.

HYDRAULIC PRINCIPLES FIGURE 6-5 System pressure can be determined by dividing the input force (50 lb) by the area of the input piston (0.5 in2). Output force can be determined by multiplying the area of the output piston (2 in2) by the fluid pressure (100 psi).

HYDRAULIC PRINCIPLES When discussing hydraulic pistons and computing fluid pressures and forces, it is important to use the area of the piston and not the diameter. The area of a piston or any circle can be determined using this formula: πr2 or π (0.785d2)

HYDRAULIC PRINCIPLES FIGURE 6-6 A simple memory triangle will help you remember the commonly used hydraulic formulas.

SIMPLE HYDRAULIC SYSTEMS
Many hydraulic systems use an engine- or motor-driven pump to produce fluid movement. These systems normally consist of the pump, a fluid intake system usually equipped with a filter, a fluid supply (sump), control valves, and the actuators that provide the system output. FIGURE 6-7 The basic components of a simple hydraulic system.

BASIC AUTOMATIC TRANSMISSION HYDRAULICS
In most automatic transmissions, the pump is built into the front or engine end of the transmission and is driven by the back of the torque converter. FIGURE 6-8 In many transmissions, the oil pump is at the front of the transmission and is driven by the torque converter hub. (Courtesy of Chrysler Corporation)

FIGURE 6-9 This pump assembly (20) is attached to the valve body (51) and channel plate (280). The pump is driven by the oil pump drive shaft (340). (Courtesy of Slauson Transmission Parts,

FIGURE 6-10 A pressure regulator valve. When fluid pressure acting on the right end of the valve exceeds spring tension, the valve will move to the left and open the passage back to the pump inlet. (Courtesy of Chryster Corporation).

FIGURE 6-11 This valve body has two sections that contain 11 sets of valves.

FIGURE 6-11 (CONTINUED) This valve body has two sections that contain 11 sets of valves.

FIGURE 6-12 The manual valve is connected to the gear shift lever so movement of the lever will slide the valve along its bore. (Courtesy of Nissan North America, Inc.)

FIGURE 6-13 A typical shift valve has a spring to move the valve to a downshift position; throttle pressure works with this spring. When governor pressure gets high enough, the valve will move to an upshift position.

FIGURE 6-14 Operation of the valve controls fluid flow to the actuator. It can block operation (a), cause apply (b), or cause release (c).

PRODUCING FLUID FLOW AND PRESSURE
Three common types of rotary pumps are used to produce the fluid flow and resulting pressure in an automatic transmission. They are the internal-external gear with crescent (crescent or gear) pump, the gerotor (rotor) pump, and the vane pump. FIGURE 6-15 An internal-external gear pump (a), gerotor pump (b), and vane pump (c).

FIGURE 6-15 (CONTINUED) An internal-external gear pump (a), gerotor pump (b), and vane pump (c).

FIGURE 6-16 As a pump rotates, a low pressure/vacuum is created as the pumping members move apart in one area, and atmospheric pressure will force fluid into this area. Pressure is created where the pumping members move together.

FIGURE 6-17 A variable displacement vane pump in maximum and minimum output positions. The slide is moved to the high-output position by the spring moving the slide. Decreased pressure comes from the pressure regulator valve. (Reprinted with permission of General Motors)

FIGURE 6-18 This transmission uses a dual-stage, external gear pump. Both stages are used at low engine speeds to produce enough fluid for the transmission’s needs. At higher engine speeds, the secondary stage is vented. (Courtesy of Chrysler Corporation)

PROVIDING CLEAN FLUID A filter is located at the pump inlet to trap dirt, metal, and any other foreign particles that might cause wear in the pump, bearings, bushings, and gear train or cause sticking of the various valves. Three types of filters are used: Surface Depth Paper

PROVIDING CLEAN FLUID FIGURE 6-19 Two filters: a surface/screen filter (a) and a depth/felt filter (b and c).

PROVIDING CLEAN FLUID FIGURE 6-20 A surface filter traps particles that are too big to pass through the openings in the screen. (Courtesy of SPX Filtran)

PROVIDING CLEAN FLUID FIGURE 6-21 The surface area of a surface filter is reduced somewhat by the material that makes up the screen. The size of the screen openings determines how small of a particle can be filtered.

PROVIDING CLEAN FLUID FIGURE 6-22 A depth filter is a group of woven fibers of a certain thickness. Foreign particles are trapped at different levels as they try to flow through. (Courtesy of SPX Filtran)

PROVIDING CLEAN FLUID FIGURE 6-23 Comparison of the filtering ability of four types of filters. (Courtesy of SPX Filtran)

PROVIDING CLEAN FLUID FIGURE 6-24 General Motors transaxles have two fluid reservoirs. One is the lower pan, and one is the valve body cover. The thermostatic element closes when the fluid heats up to raise the fluid level in the upper pan. (Reprinted with permission of General Motors)

CONTROLLING FLUID FLOW
The fluid flow from the pressure regulator valve to the manual valve and into the control circuit is called mainline, line, or control pressure. Flow to and from a transmission hydraulic actuator is controlled by one or more valves. Spool valves sliding in a round bore are used for this purpose. A spool valve gets its name from its resemblance to a spool used for thread

FIGURE 6-25 A spool valve resembles a spool for thread (top). FIGURE 6-26 A spool valve and its bore. Note the names of the various parts. (Courtesy of Chrysler Corporation)

FIGURE 6-27 The sleeve allows the larger primary regulator valve to enter its part of the bore and also provides the bore for the smaller No. 1 and No.2 plungers. (Courtesy of Toyota Motor Sales USA, Inc.)

FIGURE 6-28 When a valve moves in its bore, the side passages are opened or closed to control fluid flow. (Courtesy of Chrysler Corporation) FIGURE 6-29 Fluid pressure acting on the surface area of the valve face can move the valve along the bore.

FIGURE 6-31 Ideally, governor pressure increases with speed so the pressure in psi matches the speed in mph. There should be about 40 psi at 40 mph. FIGURE 6-30 The governor valve produces a fluid pressure that is proportional to the speed of the vehicle. (Courtesy of Nissan North America, Inc.)

FIGURE 6-32 A vacuum-operated throttle valve uses a vacuum modulator to produce an engine-load-sensitive signal at the transmission (a). A mechanical-controlled throttle valve (in this case a cable) transfers an engineload- sensitive signal through mechanical linkage (b). (Reprinted with permission of General Motors)

FIGURE 6-33 This kickdown valve is controlled by the electric solenoid and is closed when there is no electrical signal (a). The valve opens when the solenoid is energized (b). (Courtesy of Nissan North America, Inc.)

FIGURE 6-35 When fluid flows through this shuttle valve from port B to port C, the check ball moves over to close port A (left). Fluid flow from port A will close port B (right). FIGURE 6-34 A check valve is opened by fluid flow in one direction (left) and closes when the fluid tries to flow in the other direction.

FIGURE 6-36 A pressure relief valve (a). When fluid pressure acting on the area of the ball exceeds the spring force, the ball will move off of its seat and allow excess pressure to escape (b).

Valve Hydraulic Forces Think of a valve as a hydraulic actuator. The hydraulic force exerted by a valve is simply the valve land area multiplied by fluid pressure. The valve area is determined using the formula for the area of a circle, πr2. FIGURE 6-37 This valve spool has four possible hydraulic reaction faces. The areas are calculated like those of any other circular surface using the formula πr2.

CONTROLLING FLUID PRESSURE
Fluid pressure in an automatic transmission is controlled by a variable pressure regulator valve. Fluid pressure must be high enough to apply a clutch or band tightly enough to prevent slippage. Excessive fluid pressure will produce heat and fluid foaming as well as more drag on the engine. Remember that fluid horsepower is a product of pressure and flow.

In most transmissions, the pressure regulator valve is positioned close to the outlet of the pump. This valve is usually arranged so fluid pressure is at one end and a spring is at the other end. An additional passage from the pump enters the valve at one valley, and a passage leading back to the pump inlet is located at an adjacent valley

FIGURE 6-38 When fluid pressure at the right end of the regulator valve gets high enough, the valve will move toward the left and allow excess pressure to return to the pump suction passage. (Courtesy of Chrysler Corporation)

SEALING FLUID PRESSURE
The fluid passages run throughout the valve body, transmission case, shafts, and tubes of the transmission. Remember that fluid transmits pressure equally through a passage, regardless of its size or shape. The many passages in the valve body and the transmission case look like a bunch of worm tracks (a nickname used by transmission rebuilders.

FIGURE 6-39 This valve body uses upper (at bottom) and lower sections (at top) that are separated by the separator plate. Note how the separator plate can restrict a passage so it becomes a port or orifice for flow into the other section. (Courtesy of Chrysler Corporation)

FIGURE 6-40 This intermediate shaft has fluid passages to transfer lubricating oil to the planetary gearsets. (Courtesy of Chrysler Corporation)

FIGURE 6-41 Static seals prevent fluid from passing between two stationary surfaces. Dynamic seals keep fluid from passing through when one of the surfaces is moving. (Courtesy of Toyota Motor Sales USA, Inc.)

FIGURE 6-42 The sealing member of a metal-clad lip seal makes a dynamic seal with the rotating shaft while the metal case forms a static seal with the transmission case. (Courtesy of Chrysler Corporation)

FIGURE 6-43 Sealing rings are used to seal the passages between stationary and rotating members. For example, the seal rings at the right keep the fluid flows from the pump to the front clutch from escaping. (Courtesy of Chrysler Corporation)

FIGURE 6-44 Fluid pressure forces a sealing ring outward in both directions to make firm contact with the side of the groove and outer diameter of the bore.

FIGURE 6-45 Metal seal rings (bottom) have plain or hooked ends. Teflon rings (top) are either uncut, scarf cut, or butt cut.

FIGURE 6-46 Clutch and servo piston seals are usually O-rings, lathe-cut rings, or lip seals.

FIGURE 6-47 Two lip seals (blue) form fluid-tight seals between the front clutch piston and its bore. (Courtesy of Chrysler Corporation)

Special Notes on Elastomers The rubber seal materials used for seals are called elastomers because of their elastic nature. A dynamic seal must remain flexible and maintain its size and shape in order to work properly. Plain rubber is not used in a transmission because it is adversely affected by heat and contact with ATF. Natural rubber has an operating range of –58°F (–50°C) to 212°F (100°C). Higher temperatures cause rubber materials to harden, and exposure to transmission fluid causes it to swell excessively. A hardened seal may leak and cause a pressure loss, which in turn will cause clutch slippage and failure.

FIGURE 6-48 This chart compares the ability of seal materials to withstand heat and fluid.Note that natural rubber is very low in both cases.

FIGURE 6-49 The code letters cast into the surface of this seal identify the manufacturer of the seal.

APPLYING FLUID PRESSURE
An automatic transmission shifts when hydraulic pressure applies or releases a clutch or band. Hydraulic pressure causes the clutch or servo piston to stroke, taking up the clearance, and then squeezes the parts together. Both bands and clutches are released by spring pressure. Band servos can also be released by hydraulic pressure.

FIGURE 6-50 A typical clutch piston area is determined by subtracting the area of the inner diameter from the area of the outer-circle diameter.

FIGURE 6-51 Some clutch pistons use a middle seal so the piston will have two working areas.

Determining Piston Force The force exerted by a piston is simply the piston area multiplied by fluid pressure. Like a valve, piston area is determined using the formula πr2.

MODIFYING FLOW AND PRESSURE
During a shift , it is desirable to provide a gradual pressure increase to the clutch pistons or band servos. This improves the shift quality, producing the desirable shift feel. Shift feel is a “seat-of-the-pants” response the driver experiences during shifts. As the piston is stroking to take up the clutch or band clearance, the pressures in the circuit are relatively low.

But the instant the clearance disappears and the piston stops moving, the pressure rises very rapidly, causing a sudden an possibly very harsh application. Shift quality can be adjusted mechanically by changing the friction material or hydraulically by controlling fluid pressure. There are many variables that affect shift quality

FIGURE 6-52 Shift quality can be modified by changing any of these variables.

FIGURE 6-53 An orifice will cause a pressure drop as fluid flows through (a); when the flow stops, the pressure on both sides of the orifice will be the same (b).

FIGURE 6-54 In neutral, the accumulator piston is moved to the top of its stroke by line pressure (top). Front servo apply pressure strokes the accumulator piston downward, delaying the pressure rise at the servo. (Courtesy of Chrysler Corporation)

FIGURE 6-55 When a clutch applies, fluid pressure rises gradually until the circuit is filled and the piston strokes to take up the clearance; then there is a rapid increase to line pressure (a). An accumulator can be placed into the circuit to slow this pressure rise and soften clutch application (b).

AUTOMATIC TRANSMISSION FLUID
Automatic transmission fluid (ATF) is one of the most complex fluids used in a vehicle. It has to: Transfer hydrodynamic energy in the torque converter; Transfer hydrostatic energy at the clutch and servo pistons as well as the valve body; Help transfer sliding friction energy as the clutches and bands apply; Transfer excess heat away from high-temperature locations such as the torque converter’s friction surfaces, gears, and bushings; Lubricate the various moving parts.

FIGURE 6-56 The first special ATF was developed in the late 1940s. Since that date, advanced, special fluids have been developed by domestic as well as foreign vehicle manufacturers.

FIGURE 6-57 The markings on ATF containers show the fluid types. (b and c are courtesy of Pennzoil)

FIGURE 6-58 The dynamic coefficient of friction for these two fluids is almost the same, but the static friction is very different.

FIGURE 6-59 As the transmission warms up, the fluid level increases from the cold mark at 70ºF to the hot mark at 180ºF.

SUMMARY The hydraulic system applies the band and clutches, transmits force and motion, maintains fluid flow to the torque converter, and provides lubrication and cooling to the moving parts of the transmission. Pumps produce the fluid flow in a transmission and the restriction to the flow results in the system pressure. The mainline pressure is controlled by a variable pressure regulator. Other valves and orifices are used to modify mainline pressure for various purposes.

SUMMARY The flow of fluid through a transmission is controlled by valves that are moved by hydraulic pressure, spring force, centrifugal force, and engine vacuum, electrically or manually. Seals are used to confine the fluid to the appropriate passages. Metal, rubber, Teflon, and various synthetic materials are used for this purpose. Automatic transmission fluid is the lifeblood of an automatic transmission, and only the specified fluid should be used.

REVIEW QUESTIONS The important functions of the automatic transmission’s hydraulic system are to _________ the clutches and bands, transmit _________ and _________, maintain _________ _________ through the torque converter, and cool and _________ the moving parts. Pressure is defined as _________ pushing on a specified area and is commonly measured in _________. Hydraulic systems usually have a _________ valve or a _________ _________ to prevent damage from excessive pressure. The automatic transmission pump turns whenever the _________ is running. Regulated pump output pressure is called _________ pressure. Pump output volume changes relative to engine _________. The main transmission filter is designed to trap _________, metal, and other foreign _________.

REVIEW QUESTIONS A micron is approximately _________ of an inch.
The position of a valve can be controlled by: (List five) Identify a transmission valve that is controlled mechanically. by centrifugal force. by vacuum. Older automatic transmissions use a _________ and a _________ _________ to control shift timing and quality. Electronically controlled automatic transmissions use a _________ _________ _________ and a _________ _________ _________ to help the ECU determine when the shift should occur. A typical application for a ball check valve is to act as a _________ relief valve. Three types of seals are used to seal clutch pistons. Which one provides the best pressure retention?

REVIEW QUESTIONS Clutches are applied by a _________ in the clutch assembly and bands are applied by a _________ piston. If a clutch piston has a diameter of 5 in. and 80 psi of pressure is applied, the apply force is _________ pounds. The rate that pressure is applied to a clutch will affect shift _________ and shift _________. Two devices that can be used to control shift feel and quality are an _________ and an _________. Driving conditions that produce higher temperatures are _________ towing, driving up _________, and stop-and- _________ driving. Always use the automatic transmission fluid that is recommended by the _________ when adding or replacing the fluid.

CHAPTER QUIZ Fluid under pressure can be used to transmit motion and force because fluids cannot be compressed. fluid conforms to the shape of its container. fluids under pressure will apply pressure equally in all directions. All of these

CHAPTER QUIZ 2. Student A says that fluid pressure is measured in kilopascals or pounds per square inch. Student B says that 1 psi is equal to about 7 kPa. Who is correct? Student A Student B Both A and B Neither A nor B

CHAPTER QUIZ 3. If a fluid pressure of 50 psi is exerted on a piston area of 10 in2, it will generate a force of _________ lb. 10 50 100 500

CHAPTER QUIZ 4. Student A says that to increase hydraulic force, the amount of fluid flow should be increased. Student B says that to increase hydraulic force, the length of the piston stroke should be increased. Who is correct? Student A Student B Both A and B Neither A nor B

CHAPTER QUIZ 5. The pressure regulator valve
directs flow in the valve body. regulates flow to match the driving conditions. regulates pressure to match transmission requirements. controls the pump speed.

CHAPTER QUIZ 6. Positive displacement pumps
maintain a constant pressure. move a constant volume of fluid. vary the flow with rpm. vary the flow with gear changes.

CHAPTER QUIZ 7. Fluid is forced through the pump intake by
pump suction. inner case pressure. atmospheric pressure. gravity.

CHAPTER QUIZ 8. Student A says that some automatic transmissions use an internal/ external pump. Student B says that some automatic transmissions use gerotor or a vane-type pump. Who is correct? Student A Student B Both A and B Neither A nor B

CHAPTER QUIZ 9. Variable displacement pumps change the output volume based on torque converter demands. transmission temperature. gear range and shift demands. engine speed.

CHAPTER QUIZ 10. Student A says that a depth filter can trap smaller particles than a surface filter. Student B says that depth filter has more filter capacity than a surface filter. Who is correct? Student A Student B Both A and B Neither A nor B

CHAPTER QUIZ 11. Excessive mainline pressure loss could be caused by
too much fluid. a plugged filter. the wrong transmission fluid. a plugged cooler line.

CHAPTER QUIZ 12. The part of a spool valve where pressure is applied to cause the valve movement is called the land. bore. valley. face.

CHAPTER QUIZ 13. The throttle valve is designed to produce a fluid pressure signal that is proportional to the speed of the vehicle. the gear selector position. the load on the engine. All of these

CHAPTER QUIZ 14. A governor’s output pressure increases gradually and in proportion to engine speed. vehicle speed. gear range. road conditions.

CHAPTER QUIZ 15. A shift valve is moved to the upshift position by
throttle valve pressure. a spring. governor pressure. Any of these

CHAPTER QUIZ 16. The metal balls are used in a valve body to
allow fluid flow in only one direction. close one passage while fluid is flowing in another. relieve excess pressure. Any of these

CHAPTER QUIZ 17. Student A says that an accumulator in a hydraulic circuit is used to increase the fluid flow through the circuit. Student B says that an accumulator in a hydraulic circuit is used to cushion a shift by absorbing some of the fluid flow. Who is correct? Student A Student B Both A and B Neither A nor B

CHAPTER QUIZ 18. An orifice in a fluid passage
reduces the pressure in a servo while it is in the applied position. causes a servo to apply faster than normal. causes a pressure drop in the circuit while there is fluid flow. reduces the chance that a leak will develop.

CHAPTER QUIZ 19. Which type of seals require special tools to properly install and resize? Lip O-ring Teflon square cut

CHAPTER QUIZ 20. Automatic transmission fluid contains additives designed to change the friction characteristics of the fluid. clean the transmission. reduce the rate of oxidation. All of these

CHAPTER QUIZ 21. Clutches are applied by hydraulic pressure and released by hydraulic pressure. a spring. centrifugal force. Neither A nor B

CHAPTER QUIZ 22. Bands are applied by hydraulic pressure and released by hydraulic pressure. a spring. centrifugal force. Both A and B

CHAPTER QUIZ 23. Student A says that all transmission fluids are the same and a universal type of fluid can be used in any automatic transmission. Student B says that all transmission fluids are not the same and only the fluid recommended by the manufacturer should be used. Who is correct? Student A Student B Neither A nor B Both A and B

CHAPTER QUIZ 24. When automatic transmission fluid is overheated,
it will turn pink. nothing will happen. varnish will form. tar will form.

CHAPTER QUIZ 25. Using the wrong fluid could
shorten the life of the transmission. cause shift-quality problems. cause the clutches to slip excessively. All of these

OBJECTIVES After studying Chapter 6, the reader should be able to:

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