Automatic Transmission Operation
The following is a brief discussion of how an auto-
matic transmission operates. The principles discussed here
will be explained in more detail in later chapters.
When the engine is running and the transmission is in
Park or Neutral, the pump produces pressure to keep the
torque converter filled. No holding members are applied,
and no power reaches the planetary gears. The converter
impeller turns the turbine and input shaft, but the power
stops at the planetary gears. In Park, the parking gear holds
the output shaft stationary.
When the transmission is placed in Drive, oil flows
through the manual valve to one or more holding mem-
bers. The holding members apply, causing the planetary
gears to connect the input and output shafts. Engine power
goes through the converter impeller, through the turbine
and input shaft, and into the planetary gears. It exits the
planetary gears and tries to turn the output shaft. If the
brakes are applied, the turbine, input shaft, and gears do
not move. Fluid from the impeller striking the turbine cre-
ates friction and heat, which is removed by the cooler in
the radiator. Once the vehicle starts moving, the planetary
gears reduce input shaft speed and increase torque to get
the vehicle moving.
As vehicle speed increases, the hydraulic control sys-
tem moves various valves to change which holding mem-
bers are applied. This changes the rotation of the planet
gears and shifts the transmission into a higher gear. Power
continues to flow through the torque converter, input shaft,
planet gears, and output shaft.
As vehicle speed continues to increase, other valves
move to obtain higher gears until the transmission is at its
highest gear ratio. At some point, the control system
applies the lockup clutch for increased fuel economy. With
the lockup clutch applied, the impeller and turbine turn at
the same speed.
Chapter 1 Introduction to Automatic Transmissions and Transaxles 29
Low-reverse
servo
assembly
Seal
Figure 1-33. Lip seals are used to seal band servos, such as
the one shown here, or to seal clutch pack pistons. Lip seals are
moved outward by pressure and provide a tighter seal with less
resistance than would be possible with O-rings.
(DaimlerChrysler)
Pump
extension
Seal rings
Figure 1-34. Metal or Teflon® seal rings are used to seal
pressure passages. Seal rings usually provide a seal between
rotating and stationary parts. (General Motors)
Transmission
shifter cable
assembly
Steering column
ignition lock
cable assembly
Figure 1-35. The shifter linkage is used to transfer the driver’s
shift selection to the transmission or transaxle. It can be a
series of rods and links, or, as shown here, a cable
arrangement. (Ford)