device, k n ow n a s a b l o c k e r va l ve , i s u s u a l l y
CAUTION
hydraulically operated. The blocker valve normally
If the retarder is used for a long period of time, it
requires the vehicle speed to be below a certain speed
tends to overheat the transmission oil. Usually a
before reverse can be selected.
warning light is mounted on the operator's
Some transmissions use a lockup clutch with the
instrument panel to indicate high oil temperature.
torque converter. This clutch is usually located
When the light comes on, the retarder should be
between the turbine and the pump (impeller) drive
disengaged to allow the oil to cool.
housing and is applied by oil pressure controlled by
the transmission hydraulic control units. When the
converter reaches the coupling stage, oil pressure is
TRANSMISSION MAINTENANCE
directed to the lockup clutch, causing it to engage.
The lockup clutch locks the turbine and pump
Maintenance and repair of automatic
together at coupling stage speed for a positive 1 to 1
transmissions may sound like a complicated job, but
ratio.
with the proper tools, test equipment, replacement
materials, and "know-how," these units can be
Transmissions with several forward speeds and a
reverse normally use a device known as the high and
m a i n t a i n e d p r o p e r l y. B e f o r e d i s a s s e m b l y o f a
low splitter. This device consists of a planetary gear
transmission due to internal malfunction, you should
set, and hydraulic control units are used to lock two
investigate the availability of a quick engine change
members of the planetary gear set together for direct
(QEC) for that unit, ensure sufficient parts and
drive, or to hold one member for low gear ratio. The
technical information are available, and ensure the
output from the splitter assembly is passed to the
urgency of need is there to justify the teardown. It is
other planetary units, and thereby gives several gear
quite time-consuming to disassemble an engine, make
ratios for each combination of planetary units.
a repair, and reassemble the engine only to find the
engine requires additional internal repairs.
A n o t h e r d ev i c e u s e d o n s o m e a u t o m a t i c
transmissions is known as the hydraulic retarder.
The automatic transmission, like any other
This device is used on heavy duty vehicles to assist
hydraulic system, depends, as a very important first
the brake system in slowing down the vehicle, such
step, on its oil supply. The oil must be of the correct
as when descending steep grades. The retarder,
type, must be at the proper level, and most important,
usually located at the front of the transmission
must be clean.
behind the torque converter, is controlled by a foot
Contaminated oil can clog oil lines and
pedal in the operator's area. Through linkage, the
passageways and restrict valve movement in the
pedal is connected to a control valve mounted on the
hydraulic control unit. A high or low oil level usually
side of the transmission that cuts off or supplies oil to
causes the transmission to overheat and results in
the retarder.
eventual failure. Some transmission failures have been
The retarder consists of a cavity or housing with
a direct result of oil leaks; therefore, all oil leaks should
vanes cast into the housing and a rotor with integral
be corrected as soon as possible after being detected.
blades. The rotor is connected to and turned with the
There have been instances when the transmission
output (turbine) shaft of the torque converter. When
was removed from the vehicle for repair unnecessarily;
the operator depresses the foot pedal, oil is directed
t h e d e f e c t c o u l d h ave b e e n c o r r e c t e d w i t h o u t
into the cavity surrounding the rotor. The oil source
transmission removal. For examin the transmission
for the retarder unit is from the torque converter. The
output shaft seal, speedometer drive pinion, oil pan
oil strikes the rotor blades in a direction opposite to
gasket, or oil cooler can normally be corrected without
that in which the rotor is turning. This produces the
transmission removal. On some transmissions the
same effect as turning a paddle wheel in the opposite
servo units and governors may also be repaired or
direction from that in which a stream of water is
replaced without removing the entire assembly. In all
turning the wheel. That is, the oil is churned by the
cases every possible effort should be made to make
action of the rotor and the vanes in the cavity. This
repairs without transmission removal from the vehicle.
churning of oil tends to put a drag on the output shaft
of the torque converter. Therefore, anytime the
The manufacturers of automatic transmissions
retarder is used, it tends to help the vehicle brake
normally recommend a step-by-step procedure for
system.
testing the transmission in the vehicle. These
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