Automatic Transmission
Fundamentals
Chapter 73
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Introduction
• Automatic transmission
– 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
© 2012 Delmar, Cengage Learning
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
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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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Flexplate and Torque Converter
• 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
© 2012 Delmar, Cengage Learning
Torque Multiplication and
Torque Converter Operation
• Torque converter
– 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
© 2012 Delmar, Cengage Learning
Torque Converter Stator and
Operation
• Torque converter stator
– 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
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Stator Clutch Operation
• Stator clutch
– 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
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Stall Speed and Lock-Up
Converters
• Stall speed
– 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
© 2012 Delmar, Cengage Learning
Planetary Gears and Simple
Planetary Gearset
• Planetary gears
– 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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Hydraulic System and Fluid
Pump
• Hydraulic system
– Makes fluid pressure that transmits power
through the torque converter
• Fluid pump does several things
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–
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Creates hydraulic pressure
Lubricates transmission parts
Fills torque converter
Circulates fluid throughout transmission
Pressure operates valves
© 2012 Delmar, Cengage Learning
Types of Pumps
• Three types
– Rotor type
– Internal/external
gear crescent
type
– Vane type
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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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Governor and Vacuum
Modulator
• Governor
– 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
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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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Auxiliary Cooler/Heat
Exchanger and Park Pawl
• Auxiliary cooler/heat exchanger
– 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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
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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
© 2012 Delmar, Cengage Learning
Transmission Shift Control
• Electronic shift control
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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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning