Copyright by Goodheart-Willcox Co., Inc.
204 Automatic Transmissions and Transaxles
Converter lockup
solenoid Overdrive cancel
Vacuum
modulato
Downshi
solenoid
Figure 9-28. Some older transmissions have solenoids installed
on the outside of the case. (Nissan)
Coil
winding
Return
spring
Drain Drain
Hydraulic
valve plunger
Line pressure
Wire
terminal
Figure 9-29. A typical solenoid consists of a coil winding, a
plunger, and a return spring. The winding, when energized, pulls
the plunger against return spring pressure. This causes trans-
mission fluid to be exhausted and activates the particular con-
trol circuit. (Chrysler)
Kickdown switch
Downshift solenoid
Accelerator pedal
Figure 9-30. Simplified diagram of a typical detent solenoid
circuit is shown here. Pressing the accelerator to the floor
closes contacts on the kickdown switch and energizes the
solenoid. The valve opens and fluid is exhausted, reducing
hydraulic circuit pressure. (Nissan)
Lockup solenoid
Line pressure
(from 1-2 shift valve)
Converter pressure
Drain
Line pressure
(from pump)
To torque converter
(lockup on)
To oil
cooler
To torque
converter
(lockup off)
Lockup
relay
valve
Figure 9-31. The lockup solenoid operates the hydraulic
portion of the torque converter lockup relay valve on many mod-
ern transmissions. (Chrysler)
valve is unseated, causing transmission fluid in the connect-
ing line to be exhausted. Loss of this fluid reduces circuit
pressure. This causes the lockup relay valve to move in such
a way as to apply the clutch. The first lockup solenoids
were often operated by vacuum and pressure switches.
Today, most lockup solenoids are operated by the on-board
computer.
Summary
Transmission oil pumps use gears, rotors, or vanes that
cause the hydraulic system to become pressurized. The oil
pump is usually driven by lugs on the rear of the torque
converter. Some older transmissions have a second oil
pump that is driven by the transmission output shaft.
Transmission oil filters are used to remove particles
from the automatic transmission fluid. Fluid being drawn
into the oil pump must first pass through the filter. This
keeps fluid contaminants from entering the pump or the
other hydraulic components.
The transmission oil cooler removes heat from trans-
mission fluid. Fluid that has been overheated does not
lubricate properly. Further, it breaks down into sludge and
varnish, which plug passageways. The oil cooler is located
in the engine cooling-system radiator.
The valve body contains the manual valve, shift valves,
and other hydraulic system valves. Valve bodies usually