Copyright by Goodheart-Willcox Co., Inc.
186 Automatic Transmissions and Transaxles
Oil passage
Front oil seal
Oil pump
Transmission
fluid
Valves and valve body
Figure 9-1. The oil pump is often located at the front of the
transmission case. It is driven by lugs on the rear of the torque
converter. Bolts hold the pump to the case. Some older
transmissions also have a rear oil pump, which is driven by the
output shaft. (Mercedes-Benz)
Introduction
In the last chapter, you studied the mechanical parts of
the automatic transmission. These parts are used to transfer
power from the engine to the drive shaft in the most effi-
cient manner. In this chapter, you will study the hydraulic
parts that make the mechanical parts operate. The hydrau-
lic parts control the mechanical parts to get maximum
power transfer through the transmission with the least use
of fuel. If the hydraulic parts do not operate properly, the
maximum benefit of the mechanical parts is lost. Knowledge
of hydraulic control systems is vital to transmission diagno-
sis and service.
This chapter describes the transmission devices that cre-
ate, direct, and control hydraulic pressure. Since all hydrauli-
cally controlled transmissions use the same basic devices,
studying this chapter will give you a fundamental understand-
ing of how all automatic transmissions operate. This chapter
covers driver-controlled valves and related linkage. It also
briefly describes the electrical components used by all new
and many older transmissions. Some sections review informa-
tion that was covered in earlier chapters.
Studying this chapter will give you the grounding needed
to understand the transaxle controls covered in the next chapter,
as well as electronic controls systems detailed in Chapter 11.
Studying this chapter will also enable you to move on to the
diagnosis and service chapters later in this book.
Control System Overview
Automatic transmission control systems are made of
many separate components. The major automatic transmis-
sion control parts include:
Transmission oil pump—produces the fluid pressure
that operates other hydraulic components and lubri-
cates the moving parts of the transmission. It is driven
by lugs on the torque converter.
Transmission oil filter—removes particles from the
automatic transmission fluid before the fluid is circu-
lated by the oil pump.
Transmission oil cooler—needed to dissipate heat from
the transmission fluid, which is generated by the trans-
mission and picked up from the engine. The transmission
oil cooler is often mounted in the engine cooling-sys-
tem radiator. Separate external oil coolers, or auxiliary
oil coolers, may also be installed on the vehicle.
Valve body—a complex casting that serves as the con-
trol center for the transmission’s hydraulic system. It
contains many internal passageways and compo-
nents, including hydraulic valves.
Hydraulic valves—used in the valve body to control
transmission fluid pressure, as well as the direction
and rate of fluid flow. Specifically, they control the
application of clutches and bands. In general,
hydraulic valves help control the entire operation of
the automatic transmission.
Shift linkage—operates the manual valve, which is
one of the hydraulic valves. This forms a direct
mechanical connection between the driver and the
transmission. Shift selector levers can be mounted on
the floor or on the steering column. The shift linkage
also operates the parking gear mechanism.
Electrical components—used in the transmission to
send signals to other vehicle components or to par-
tially control the transmission’s hydraulic system.
Transmission Oil Pump
The transmission oil pump, sometimes called the front
pump, has several basic functions. It generates system pres-
sure for the entire hydraulic control system and keeps the
moving parts of the transmission lubricated. It also keeps the
torque converter filled with transmission fluid for proper
operation. (Most of the fluid in an automatic transmission is
inside the converter.) Finally, it circulates transmission fluid to
and from the oil cooler for heat transfer.
The transmission oil pump is a positive displacement
pump. It is driven by the engine, through lugs on the back
of the torque converter. The relative position of the oil
pump in the transmission is shown in Figure 9-1.