Copyright by Goodheart-Willcox Co., Inc.
Chapter 9 Automatic Transmission Control Components 189
Transmission Oil Filter
The transmission oil filter, or oil strainer, is 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. Most filters are
made of a disposable filter paper or felt. Some filters are
fine mesh screens, which can be cleaned and reused.
The filter is located inside the oil pan. It must be posi-
tioned where it will always be submerged in the automatic
transmission fluid. If the filter were not submerged with
fluid, air would be drawn into the hydraulic system, affecting
operation. Usually, the filter is attached to the bottom of the
valve body. It may be held in place by screws, clips, or bolts.
Some filters have a built-in pickup tube that goes directly up
to the oil pump. Filters used with deep transmission pans
may have an extended pickup tube.
Transmission Oil Cooler
The action of the transmission—the torque converter,
in particular—heats the transmission fluid. In addition, the
transmission’s close coupling with the engine serves to add
heat to the fluid. This heat must be removed so the fluid
and the transmission do not become overheated. Fluid sub-
jected to overheating will quickly lose its lubricating ability.
Further, it will break down into sludge and varnish, which
will plug passageways and destroy the transmission.
In addition, excessive heat destroys the nonmetallic
materials used in the clutch and band friction linings
and in the seals. As a result, transmission fluid that is
overheated can damage these materials or fail to cool the
friction materials properly. Then, not only are the materials
destroyed, but the transmission fluid ends up with particu-
lates from the damaged materials. These particulates can further
plug transmission passageways and destroy the transmission.
In general, fluid temperature should be kept below
275°F (135°C). Running at elevated temperatures greatly
reduces the life of the transmission fluid. If the transmission
is being worked hard, such as in trailer towing, tempera-
tures could exceed 300°F (150°C). Under such conditions,
the fluid and filter must be changed frequently.
Main Oil Cooler
Fluid in older transmissions was sometimes air-cooled.
The converter housing was open, so air could pass through
and absorb heat directly from fins on the torque converter.
This design has been abandoned.
Fluid in modern automatic transmissions is cooled in
the oil cooler. The main oil cooler consists of a heat
exchanger built into a side or bottom tank in the engine
cooling-system radiator. Cooler lines connect the transmis-
sion to the oil cooler, as shown in Figure 9-4. Some vehi-
cles have a direct air cooler installed in front of the air
conditioner condenser and do not use a radiator cooler.
The oil cooler is immersed in cooled engine coolant.
In operation, hot transmission fluid is pumped from the
transmission to the oil cooler, usually directly from the
torque converter. As the fluid passes through the cooler, its
heat is transferred to the engine coolant. The cooled trans-
mission fluid then returns to the transmission. The cooled
fluid may be routed through the transmission lubrication
system before returning to the oil pan.
Auxiliary Oil Cooler
When a vehicle is used to tow a trailer or subjected to
other extreme operating conditions, auxiliary oil coolers,
or external oil coolers, may be installed. Auxiliary coolers
consist of direct, air-cooled heat exchangers that connect to
the existing fluid cooling system. They may be installed to
completely replace the main oil cooler in the radiator, or
they may be helper units that further cool fluid that has
passed through the radiator oil cooler. Auxiliary coolers are
usually mounted ahead of the radiator.
Valve Body
The valve body is a complex casting made of cast iron
or aluminum. It contains many internal passageways and
components, including hydraulic valves. The valve body
serves as the control center for the transmission’s hydraulic
system. It is precisely machined, and valve body compo-
nents are manufactured to exact tolerances. Some toler-
ances are as small as 1 ten-thousandth of an inch (0.0001”),
or 25 ten-thousandths of a millimeter (0.0025 mm).
Oil passageways, which are sometimes called worm
tracks, are cast into the valve body during the casting pro-
cess. After casting, holes are drilled for valves, and one side
of the valve body is machined flat. This flat area is intended
to closely fit a similarly machined area on the transmission
case and, in some cases, other transmission parts. The close
fit prevents oil leaks that could lead to faulty transmission
operation.
A one-piece valve body is shown in Figure 9-5. This
component would bolt directly to the bottom of the trans-
mission case. Some valve bodies are made in two sections. Such
split valve bodies must be assembled before being installed in
the transmission. Auxiliary valve bodies are often bolted to