1. Automatic Transmission Fundamentals
Chapter 73

2. Objectives Identify the basic parts of an automatic transmission
Describe the operation of the major sections of an automatic transmission
Explain how automatic transmissions shift gears
Understand how an electronic automatic transmission works

3. Introduction Automatic transmission Front-wheel-drive vehicles
Shifts gears automatically
Does not require a manual clutch
Front-wheel-drive vehicles
Combine transmission with differential in transaxle
Most automatic transmissions use a torque converter
Some use a dual clutch arrangement

4. Automatic Transmission Parts and Power Transmission
Automatic transmission consists of several parts
Torque converter, input shaft, transmission pump, valve body, planetary holding members, etc.
Methods of transmitting power
Fluid, friction, and gears
Torque converter transmits power using fluid
Planetary holding members use fluid and friction
Gears transmit power and change speed and torque

6. Flexplate and Torque Converter
Replace the flywheel
Flexplate is fastened to crankshaft
Torque converter
Allows vehicle to idle at a stop sign
Slips during initial acceleration to prevent stalling
Fluid coupling: compared to two fans
First fan (impeller): turns faster
Second fan (turbine): picks up energy and turns
Pump produces fluid flow to develop pressure

7. Torque Multiplication and Torque Converter Operation
Increases torque
Torque is multiplied whenever the impeller spins faster than the turbine
Operation
Impeller rotates at idle speed: fluid is thrown from impeller toward turbine
Centrifugal force of rotating torque converter also throws fluid to outside of housing

8. Torque Converter Stator and Operation
Makes torque increase possible
Stator between impeller and turbine
Redirects fluid flow
Split half-rings in centers of turbine and impeller blades
Direct fluid in a smooth pattern

9. Stator Clutch Operation
Locks in one direction and freewheels in other
Fluid strikes stator at a high angle: clutch locks
Speed of turbine catches speed of impeller: stator clutch freewheels
Converter
Becomes efficient at power transfer when engine reaches 2300 rpm
Turbine speed is 9/10 of impeller: no torque multiplication

10. Stall Speed and Lock-Up Converters
Point of maximum torque multiplication
Lower stall speed converters: more efficient
Lock-up converter
Pressure plate behind turbine locks it to back of converter housing
Provides mechanical link between crankshaft and transmission input shaft
Fluid is directed to one side of pressure plate and is exhausted from the other

11. Planetary Gears and Simple Planetary Gearset
Change gear ratios by holding and turning different members
All gears are in constant mesh
Load is distributed over several gears
Simple planetary gearset
Has sun gear, planetary pinions, carrier, and a ring gear
Several types: compound, Simpson, Ravigneaux, and tandem

12. Simple Planetary Operation
Basic gear rules
Two gears with external teeth in mesh rotate in opposite directions
Two gears in mesh, one with internal and one with external teeth, rotate in same direction
Forward gear reduction
Turn sun gear while holding ring gear
Holding sun gear while turning ring gear
Reverse
Use only the rear gearset

13. Compound Planetary Operation (Simpson)
Double reverse: Simpson geartrain low-gear operation
Results in forward operation
Ravigneaux operation: two sun gears, two sets of pinions, and a ring gear
Large and small sun gears
Six planetary pinions: three long and three short
Lepelletier geartrain
Combines different planetary arrangements

14. Driving and Holding Devices and Clutches
Planetary gearset
One member held and another is driven
Fluid clutch
Holds rotating member to input shaft
Bands and clutches operate when fluid pressure applied
Multiple disc clutches
Used for holding or driving
Steels: held against one element of clutch pack
Friction discs are splined to corresponding part

15. Clutch Operation
Hydraulic pressure not directed at the clutch: clutch releases
Friction discs and steels turn independently
Driving clutch engaged: fluid directed into clutch drum
Fluid pressure is applied to large piston on inside of drum
Piston is applied against discs to compress springs and lock clutch through pressure plate
Pressure is released: piston is pushed away

16. One-Way Clutches Characteristics
Holds part of planetary gearset from turning
Commonly used in drive low gear
Have inner and outer race and a set of springs and rollers
Sprag clutch: different-shaped locking device between inner and outer races
Mechanical diode: used in some torque converters and transmissions

17. Bands and Accumulator Steel straps with friction lining on inside
Single or double wrap
Double wrap bands are used for low and reverse
Servo operates a band
Fluid pressure is directed into the servo’s cylinder to apply the band
During shifts: some parts held and others driven
Shuddering or damage result if two components applied at same time
Accumulator has piston and reservoir that must fill before pressure applied to driving device

18. Hydraulic System and Fluid Pump
Makes fluid pressure that transmits power through the torque converter
Fluid pump does several things
Creates hydraulic pressure
Lubricates transmission parts
Fills torque converter
Circulates fluid throughout transmission
Pressure operates valves

19. Types of Pumps Three types Rotor type
Internal/external gear crescent type
Vane type

20. Transmission Valves and Pressure Regulator
Spool valves
Lands and valleys control fluid flow
Valves can be moved
Done by spring, lever or rod, or hydraulic pressure
Pressure regulator valve
Determines pressure in the system
Orifice restricts fluid flow
Also reduces pressure of moving fluid

21. Hydraulic Valve Body Valve body
Senses engine load and adjusts shift points and fluid pressure
Usually bolted to bottom of transmission inside of the pan
Spacer plate fits between transmission and valve body
Manual control valve is attached to shift lever
Shift quadrant tells the gear the transmission is in
Shift order always PRNDL or PRNDD2L

22. Transmission Automatic Shift Selection
Transmission selects correct gear range based on engine load and vehicle speed
Upshift: transmission shifts to higher gear
Downshift: transmission shifts to lower gear
Throttle pressure
Results when engine vacuum changes
Governor pressure
Results from increase in vehicle speed

23. Governor and Vacuum Modulator
Located on output shaft
Variable-pressure relief valve
Pressure is no greater than line pressure
Vacuum modulator valve
Controls throttle pressure
Has diaphragm and hose fitting attached to vacuum source at intake manifold

24. Kickdown Valve
Either manually operated by cable or electrically operated solenoid
Causes throttle pressure to go to highest point
Spring loaded: extends from the valve body side
Contacts lever that applies it from outside the transmission
Bushings are made of bronze alloy or steel with soft bearing surface
Thrust washers control end play
Snap rings maintain part position on a shaft

25. Automatic Transmission Fluid and Automatic Transmission Cooling
ATF is oil
Specially formulated for automatic transmissions
Cooling
Transmission develops heat during operation
Heat damages transmission fluid
Most transmissions have a fluid cooler
Results of radiator heat exchanger leaks:
Engine running: ATF migrates into radiator
Engine off: coolant migrates into transmission

26. Auxiliary Cooler/Heat Exchanger and Park Pawl
Added to motor homes and vehicles that pull trailers
Resembles small radiator
Hooked into cooler line in series
Installed before the radiator cooler
Park pawl
Lever that locks transmission output shaft when shift lever is in park

27. Electronic Automatic Transmissions and Operation
Shifts controlled by computer using engine load, vehicle speed, and other inputs
More precise control
Less expensive solenoids
Electronic transmission shifting
Driver shifts gears without assistance of a clutch
Manually overriding the computer
Computer decides shift points based on power output from the engine
Adaptive learning

28. Electronic Torque Converter Control
Torque converter clutch
Computer controlled
Comes on after engine is warm
Typical speed required for lockup is about 40 mph
Engages if brake switch closed and throttle position sensor signal does not show a closed throttle

29. Electronic Pressure Control
Electronic pressure control (EPC) types
On/off variable force solenoids
Pulse width modulated
Variable force solenoids
Electronic modulators
Pulse width modulation slides back and forth
Opening or closing a passage
Different types of solenoids are used
Depends on application

30. Transmission Shift Control
Electronic shift control
Line pressure only
No governor or modulator pressure
Forward gears are controlled by computer
Reverse only works when solenoids are off
Honda/Acura and Saturn use multiple-disc hydraulic clutches and shift solenoids
Gearing similar to manual transmission
Dual clutch transmissions (DCTs)
Being used by several manufacturers

31. Continuously Variable Transmission
Characteristics
Similar operation to variable-speed drill press
Infinite driving ratios
Increases fuel economy in the range of 25%
Engine can be run with constant rpm
Engine does not accelerate through each gear
Do not handle torque as well
Torque travels between steel cones and a steel chain
Special lubricant changes phase to a gassy solid

32. Hybrid Planetary Transmission Operation
Hybrid planetary transaxle
Three inputs and one output
Toyota hybrid system
Two motor/generators
Hybrid motor/generators
Operate as motors when powering vehicle
Generate electricity to recharge battery pack
Some use a third electric motor on rear axle
Double regenerative braking