Ready for Review
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
►
Automatic transmissions are now standard equipment in vehicles.
Automatic transmissions contain planetary gear sets.
Automatic transmissions have four or more forward gear ratios and one reverse ratio.
Typical forward gear ratios are as follows: first gear, 3:1; second gear, 1.75:1; third gear, 1:1; and fourth
gear, 0.80:1.
Adding more gear ratios to the transmission boosts fuel economy.
Automatic transmissions select and shift gears without driver input and can couple or uncouple from the
engine as necessary.
Types of automatic transmissions include conventional, transaxle, dual clutch, continuously variable,
dual shaft, and hybrid (series or parallel).
Conventional automatic transmissions have planetary gear sets, connect to the engine via torque
converter, and use holding devices to stop one or more parts of the planetary gear.
Transaxle transmissions add a differential and final drive gear to a standard transmission.
Dual-clutch transmissions use two wet clutches instead of a standard torque converter.
Continuously variable transmissions use two moveable pulleys rather than typical gears.
Honda and Saturn vehicles use dual-shaft transmissions that have a main shaft and a countershaft, as
well as multidisc clutch packs.
Hybrid vehicles often use series transmissions, in which an electric motor supplements power to the
transmission.
A hybrid vehicle may use a parallel hybrid transmission, in which either the engine or an electric motor
can power the transmission.
Automatic transmission gears are typically helical (most commonly used), spur (straight cut), or hypoid.
Planetary gear sets use either helical- or straight-cut gears and are comprised of three components: planet
carrier, sun gear, and ring gear.
Bands, multidisc clutches, and one-way clutches are the three basic holding devices used in
transmissions.
Automatic transmission fluid (ATF) transfers heat from the internal transmission components to the
transmission cooler.
ATFs are oil-based or synthetic and may contain these additives: rust and corrosion inhibitors, friction
modifiers, seal conditioners, detergents, antifoam, and viscosity modifiers.
Components of an automatic transmission include the front pump; flex plate and ring gear; front pump
seal, bushings, and pump drive; case, extension housing, and pan; gaskets and seals; and parking pawl
assembly.
The front hydraulic pump ensures a continuous pressurized supply of fluid to the transmission.
The four types of automatic transmission front pumps are gear/crescent, rotor, vane, and variable
displacement.
On automatic transmissions, the flywheel is replaced with a flex plate, with a ring gear wrapped around
the outside edge.
Gaskets may be made of paper, fiber, cork, or neoprene (which may be reusable).
Types of internal seals include square cut, lip, locking, O-ring, and Teflon.
Compound planetary gear sets are used to create all the ratios needed for modern vehicle travel.
Two types of compound planetary gear sets are Simpson and Ravigneaux.
Thrust washers and thrust bearings are used to control forward and backward motion of transmission
components.
Bushings should be checked during transmission overhaul; the front pump bushing or the output shaft
bushing will likely need replacing.
Automatic transmission brake bands use friction material and servo pressure to stop the drum from
spinning.
Multidisc clutches are used to couple planetary gear members to input shaft, output shaft, or the case.
Clutch release may be assisted via a large coil spring, a pack of small springs, or a diaphragm spring.
Check clutch pack clearance when rebuilding an automatic transmission.
A torque convertor works with the transmission to select the proper gear to meet the speed and power
demands of the vehicle.
► A single-stage torque converter contains an impeller, a turbine, and a stator.
► Torque multiplication is a state that occurs when the impeller is turning substantially faster than
the turbine. The greater the difference in speed the greater the torque multiplication.
► A torque converter clutch is used to lock the turbine to the impeller when the vehicle is operating under
light loads, to improve fuel economy.