Figure 4-27.Stator assembly.
off the stator vanes at tremendous speeds. The greatest
torque multiplication occurs at stall speed.
When the turbine speed nears impeller speed,
torque multiplication drops off. Torque is increased in
the converter by sacrificing motion. The turbine spins
slower than the impeller during torque multiplication.
If the counterclockwise oil were allowed to
continue to the center section of the impeller, the oil
would strike the blades of the pump in a direction that
would hinder its rotation and cancel any gains in
torque. To prevent this, you can add a stator assembly.
The stator (fig. 4-27) is located between the pump
and the turbine and is mounted on a one-way clutch
that allows it to rotate clockwise but not counter-
clockwise. The purpose of the stator is to redirect the
oil returning from the turbine and change its rotation
back to that of the impeller. Stator action is only
needed when the impeller and turbine are turning at
different speeds. The one-way clutch locks the stator
when the impeller is turning faster than the turbine.
This causes the stator to route oil flow over the impeller
vanes properly. Then, when turbine speed almost
equals impeller speed, the stator can freewheel on its
shaft so not to obstruct flow.
Even at normal highway speeds, there is a certain
amount of slippage in the torque converter. Another
type of torque converter that is common on modern
vehicles is the lockup torque converter (fig. 4-28). The
lockup torque converter provides increased fuel
economy and increased transmission life through the
elimination of heat caused by torque converter
slippage. A typical lockup mechanism consists of a
hydraulic piston, torsion springs, and clutch friction
In lower gears, the converter clutch is released.
The torque converter operates normally, allowing
slippage and torque multiplication. However, when
shifted into high or direct drive, transmission fluid is
channeled to the converter piston. The converter piston
pushes the friction discs together, locking the turbine
and impeller. The crankshaft is able to drive the
transmission input shaft directly, without slippage.
The torsion springs assist to dampen engine power
pulses entering the drive train.
Figure 4-28.Torque converter with lockup clutch.