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4. Hydraulic System Parts and Operation. FIGURE 4–1 Fluid pressure is used to apply clutches and bands. The pressure and calculated volume index readings of the fluid in the unit can be monitored using a scan tool.
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4 Hydraulic System Parts and Operation
FIGURE 4–1 Fluid pressure is used to apply clutches and bands. The pressure and calculated volume index readings of the fluid in the unit can be monitored using a scan tool.
CHART 4–1 Selected samples of automatic transmission fluid and some applications. Always check service information for proper specified fluid when servicing automatic transmissions/transaxles.
CHART 4–1 (continued) Selected samples of automatic transmission fluid and some applications. Always check service information for proper specified fluid when servicing automatic transmissions/transaxles.
CHART 4–1 (continued) Selected samples of automatic transmission fluid and some applications. Always check service information for proper specified fluid when servicing automatic transmissions/transaxles.
FIGURE 4–2 The use of the factory-specific fluid is the recommend fluid to insure the best possible shifting and transmission operation.
FIGURE 4–3 Multi-vehicle, or universal fluid, is designed to meet the specifications of many types of fluids, making it popular with independent shops that service many makes and models of vehicles.
FIGURE 4–4 Aftermarket additives are available that can convert friction-modified ATF into highly friction-modified ATF.
FIGURE 4–5 Fluid pressure is transmitted undiminished in all directions. Note that the pressure is equal throughout the system.
FIGURE 4–6 A 100 lb force applied on an input piston that has an area of 1 sq. in. will produce a fluid pressure of 100 PSI.
FIGURE 4–7 A simple memory triangle can be used to help remember the commonly used hydraulic formulas.
FIGURE 4–9 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.
Frequently Asked QuestionWhat Is a Front Pump?The pumps used in automatic transmissions are driven by the torque converter and are located at the front of the transmissions. In the early days of automatic transmissions, manufacturers equipped the transmission with a pump that was driven from the output shaft and was called the rear pump. The purpose of this pump was to supply fluid under pressure to the unit when the vehicle was coasting down a hill with the engine at idle speed. It also made it possible to push start the vehicle. When a unit was equipped with a rear pump, it was common terminology to refer to the pump at the front as the “front pump.” This term is still heard today long after the rear pump has been deleted from automatic transmissions.
FIGURE 4–10 A variable displacement vane pump in maximum and minimum output positions. The slide is moved to the high output position by a spring. Decreased pressure comes from the pressure regulator valve.
FIGURE 4–11 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 output of secondary stage is vented.
FIGURE 4–12 A surface filter traps particles that are too big to pass through the openings in the screen.
FIGURE 4–13 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.
FIGURE 4–14 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.
Frequently Asked QuestionHow Large Are Dirt Particles?Some depth filters trap particles as small as 10 μm. The space between a bushing and the shaft is about 0.001 to 0.003 inch (0.025 to 0.076 mm), but if the shaft is loaded to one side by gear pressure, this clearance might be only the width of two or three oil molecules. A hard, abrasive particle in this area will produce wear that, in turn, will produce small metal particles that cause more wear. Dirt or other particles that enter the valve body may cause a valve to stick in its bore. This can cause a no-shift problem or a partial shift with low pressure. A recent study of the fluid from eight different transmissions used for less than 3,000 miles (4,800 km) showed the following:• 1 to 20 particles in the 50-μm size• 800 to 8,000 particles in the 15-μm size• More than 50,000 particles in the 5-μm size
FIGURE 4–15 A spool valve resembles a spool for thread (top).
FIGURE 4–16 A spool valve and its bore. Note the names of the various parts.
FIGURE 4–17 When pressure on the face of the pressure regulator valve overcomes spring force, the valve moves to open the exhaust port.
FIGURE 4–18 The pressure control solenoid controls the mainline pressure, which is in turn controlled by the powertrain control module (PCM) or the transmission control module (TCM), by applying pressure to the spring side of the pressure regulator valve.
FIGURE 4–20 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.
FIGURE 4–21 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.
FIGURE 4–22 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 4–23 Metal seal rings (bottom) have plain or hooked ends. Teflon rings (top) are either uncut, scarf cut, or butt cut.
FIGURE 4–24 Clutch and servo piston seals are usually O-rings, lathe-cut rings, or lip seals
FIGURE 4–25 Engine coolant from the engine block flows through the passages in the warmer/cooler, and then out through the thermo valve to the upper radiator tank. The thermostatic valve uses a wax element–type valve to control the flow of engine coolant through the case-mounted cooler/warmer. The thermostatic valve improves the ATF warm-up times and maintains ATF temperature within the optimum operating range between 170°F and 180°F (77°C and 82°C).
FIGURE 4–26 The life of automatic transmission fluid drops drastically when the temperature increases above normal.
FIGURE 4–27 Automatic transmission fluid is routed from the torque converter, where most of the heat is generated, to the radiator where it is cooled. The fluid then returns to the transmission/transaxle to lubricate the bearings and bushings.
Frequently Asked QuestionWhat Is a “Turbulator”?A plain-tube cooler is not very effective for cooling fluid because fluid tends to increase its viscosity and slow down as it cools. The cooler oil then tends to become stationary on the outer, cooler areas of the cooler while the hotter, thinner-viscosity fluid flows through the center. The turbulator in well-designed oil coolers continuously mixes the fluid. The ATF in contact with the outer part of the cooler tubes are in contact with the relatively cool coolant in the radiator tank. The screen-like turbulator causes turbulence in the fluid flow to ensure constant mixing and thorough cooling of all the fluid. SEE FIGURE 4–28.
FIGURE 4–28 Cold fluid tends to stick to the walls of a plain tube cooler (top). The turbulator causes fluid turbulence to promote mixing so all of the fluid cools (bottom).
Tech TipDon’t Tow a Vehicle with the Drive Wheels on the GroundA vehicle with an automatic transmission should not be towed or pushed very far because there will be no lubricating fluid flow when the engine is not running. The gear sets and bushings will run dry, wear, and overheat or burn out without a constant flow of lubricating oil. Most manufacturers recommend towing only when absolutely necessary. They caution that towing should be limited to a few miles with a maximum speed of 20 to 25 mph (32 to 40 km/h). If possible, the drive wheels should be lifted off the ground or the driveshaft removed from a rear-wheeldrive (RWD) vehicle. Special cautions also should be taken when towing an all-wheel-drive (AWD) vehicle.
