Copyright by Goodheart-Willcox Co., Inc.
Chapter 9 Automatic Transmission Control Components 193
Valve
Boost pressure
from other
transmission
valves
To oil pan
or pump
intake
To rest of
hydraulic
system
To oil
pump
Spring
Figure 9-9. The main pressure regulator valve controls hydraulic system pressure by exhausting fluid to the pan or pump intake.
Basic pressure settings are determined by a spring that holds the regulator valve closed until a certain pressure is reached. The
valve setting is modified by boost pressure from the manual valve and throttle valve.
Pressure Regulator Valve
The pressure regulator valve controls the pressure of
the overall transmission hydraulic system, or the system’s
line pressure. This valve is sometimes located on the
pump or in an auxiliary valve body, but it is most often
found in the main valve body. A few transmissions have a
separate regulator valve to control pressure in the torque
converter. A schematic of a pressure regulator valve is
shown in Figure 9-9. The pressure regulator is often
installed on the pump body, Figure 9-10.
In operation, pressurized fluid at the oil pump dis-
charge is directed to the pressure regulator valve before
going to the rest of the transmission hydraulic system. A
heavy spring holds the valve closed against oil pressure from
the pump. At idle, the regulator valve is usually closed.
When the engine begins running faster, it turns the
oil pump at a faster rate. Fluid pressure rises. When
hydraulic pressure exceeds some predetermined value, it
forces the valve to move against spring pressure. This
opens a passageway back to the oil pan or pump intake.
Hydraulic fluid is diverted back through this passageway,
and pressure falls back within the limit. This action keeps
line pressure constant.
Note that if the pressure were allowed to exceed the
set maximum, it would cause hydraulic system malfunc-
tions and damage to parts. Also note that the pressure limit
is established by the spring; thus, the stiffer the spring, the
higher the line pressure.
On many vehicles with slipper-type pumps, the pres-
sure regulator controls line pressure by increasing or
decreasing pump output. Refer to Figures 9-11 and 9-12.
Note the pressure decrease line from the pressure regulator
to the left side of the pump. Also note that the pump
assembly can pivot on a pin located at the center top of the
pump assembly. A spring called the priming spring pushes
the pump to the left. This is the full output position.
If high pressures are needed, or if the pump is turning
slowly, the pressure regulator remains closed and no oil
enters the pressure decrease line. See Figure 9-12. The
priming spring holds the pump in the full output position.
In the full output position, the pump intake side chambers
are much larger than the output side chambers and the
pump creates full pressure.
If pump pressure becomes excessive, the pressure
moves the regulator valve against spring pressure, allowing
line pressure to flow into the pressure decrease line. This
pressure pushes the pump against priming spring pressure,
moving the pump assembly to the right. In this position, the
pump intake side chambers are only slightly larger than the
output side chambers, and the pump output is decreased.
Manual Valve
A manual valve, located in the valve body, is operated by
the driver through the shift linkage. This valve allows the driver
to select Park, Neutral, Reverse, or different Drive ranges.
When the shift selector lever is moved, the shift linkage moves
the manual valve. As a result, the valve routes hydraulic fluid
to the correct components in the transmission, including other
hydraulic valves, clutches, and band servos.
Note that when Overdrive, Drive, or Second is select-
ed, the transmission takes over, shifting automatically to
meet driving conditions. In Low and Reverse, however, the
transmission is locked into the selected gear.
Shift Valves
Shift valves, which are located in the valve body,
control transmission upshifts and downshifts. Hydraulic