GMC 1976 ZEO 6083 Maintenance Manual page 458

7- 10
TRANSMISSION
Detent Regulator Valve
When the detent valve shifts, the detent regulator
is freed to allow drive oil to enter the detent passage
and thus becomes regulated at 70 psi . Detent pres
sures will also flow into the modulator passage which
flows to the shift valves . Detent pressure acting on
the modulator valve limits the low limit of modulator
pressure to the value of detent pressure .
Rear Servo and Accumulator Assembly
The rear servo piston applies the rear band for
engine braking in Lo range 1st gear . It applies the
band in Reverse to hold the reaction carrier to pro-
vide the reverse gear ratio .
On the 1-2 shift in Drive or Super, the rear ac-
cumulator piston serves as an accumulator for the
intermediate clutch to provide a smooth shift.
Front Servo
The front servo piston applies the front band to
provide engine braking in 2nd gear in Super and Lo
ranges . The front accumulator piston cushions the
apply of the direct clutch, and in conjunction with a
series of check balls controlling orifices, is a part of
the timing for the release of the direct clutch .
TROUBLE DIAGNOSIS
FLUID LEAKAGE PRECAUTIONS POINTS OF POSSIBLE OIL LEAKS
The precautions that must be observed to prevent
fluid leaks are as follows:
1 . Use new gaskets and O-ring seals whenever
there is a disassembly.
2. Use a very small amount of petrolatum to hold
gaskets and thrust washers in place during assembly,
or to seal gaskets. Never use gasket paste or shellac.
3. Make sure that composition cork and paper
gaskets are not wrinkled or creased when installed.
Make sure that gaskets have not stretched or shrunk
during storage.
4. Make sure the square type O-ring seals are
installed squarely and are not twisted during assem-
bly .
5. Make sure that mating surfaces of castings are
flat and smooth, free of deep scratches, chips, and
burrs.
6. Torque bolts to proper torque .
To prevent the apply of the front band in Neu-
tral, Drive and Reverse ranges, oil is directed from
the manual valve to the release side of the servo
piston .
In Drive the servo release oil from the manual
valve is used to charge the servo in preparation for
the apply of the direct clutch .
Direct clutch oil is directed to the front servo
accumulator piston where spring force plus direct
clutch pressure stroke the piston up against the force
of servo release oil. This lowers the clutch apply
pressure for a smooth engagement .
The release of the direct clutch and the exhaust-
ing of the front servo accumulator is slowed down by
the three check balls and three orifices which permits
a soft return of the drive load to the intermediate
roller clutch and also allows engine rpm to increase
during a detent 3-2 downshift in preparation for the
lower gear ratio, which results in a smooth shift and
better acceleration .
IMPORTANT: Oil flow circuits and power flow
charts are the same as used in 1974 and are not
included in this manual .
When checking for oil leaks, first determine
whether leak originates from transmission or engine .
The original factory fill fluid in the transmission is
formulated with a red aniline dye to assist in locating
leaks. However, the fluid color will change after a
short time in service. Red dye appearing in the leak-
ing oil will give positive identification as to the loca-
tion of the leak .
If oil leak is found to be in transmission, check
for leak in following areas:
REAR END
It will be necessary to remove converter cover to
determine location of leaks at rear end. To correct
leaks at rear end, it will be necessary to remove trans-
mission from vehicle.
1 . Pump oil seal leak - Check pump oil seal to
make certain it is correctly installed and not da-
maged.