A " n o d r ive " c o n d i t i o n m i g h t ex i s t i n a
replaced. A defective clutch disc usually requires
replacement of the entire clutch assembly.
transmission, even with correct oil pressure, because
of inoperative clutches or bands. Therefore, before
Hydraulic Control Unit
removing a clutch assembly or a servo unit from the
transmission, check it with air pressure.
Foreign matter in the hydraulic control unit is one
The front or rear clutches and kickdown (low)
of the major causes of an automatic transmission
servos may be tested by applying clean, moisture-free
malfunction. A particle of dirt or a piece of lint could
compressed air of 30 to 100 psi to the apply-passages.
cause a valve within the unit to partially stick or seize
Figure 2-79 shows a typical oil passage arrangement as
in the valve body. This would cause the transmission to
seen after the removal of the hydraulic control unit
operate improperly, depending upon which valve was
sticking. If a malfunction or troubleshooting indicates
a defect in the hydraulic control unit, the entire unit
Apply air pressure to the apply-passage and listen
should be disassembled, cleaned, and inspected. All
for a dull thud, which indicates that the clutch or servo
defective parts should be replaced.
is operating. In some cases the thud may not be heard in
the clutches; in this case, place the fingertips on the
While the inspection of parts is mostly visual, the
clutch housing, and movement of the clutch piston can
springs contained in the unit should be checked for
be felt when the clutch is applied. If, by applying air
proper length and pressure and for such defects as
pressure, the clutches and servos operate properly, but
distortion and broken coils. Upon disassembly, it is
allow slippage under road testing, then the trouble is
good practice to place the parts on a large piece of
probably in the band, band adjustment, or the clutch
hard-surfaced paper. The nomenclature of each
disc. If air can be heard escaping through the clutch or
removed part should be written adjacent to the part.
servo unit, a seal or gasket is defective and must be
This is to prevent interchanging parts that look alike. A
FRONT SERVO APPLY
LINE PRESSURE TO ACCUMULATOR
FRONT SERVO RELEASE
REAR PUMP SUCTION
REAR PUMP PRESSURE
Figure 2-79.--Typical transmission oil passages.