Automatic Transmission Fundamentals

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Automatic Transmission Fundamentals

• Chapter 73

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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

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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

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Automatic Transmission Partsand 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

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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

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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

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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

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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

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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

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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

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Driving and Holding Devicesand 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

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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

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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

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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 servos
  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

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Hydraulic System and Fluid Pump

• Hydraulic system
• 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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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