Chevrolet 1977 light duty truck, Service Manual

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The use o f a "Black Light" to locate the point at
which the oil is leaking is helpful. C o m p a rin g the oil
from the leak to that on the engine or transmission
dipstick, w hen viewed by black light, will determ ine the
source o f the leak - engine or transmission.
Oil leaks a round the engine and transmission are
generally carried toward the rear o f the vehicle by air
stream. For example, a transm ission oil filler tube to case
leak will sometimes a p p e a r as a leak at the rear o f the
transmission. In d eterm in in g the source o f a leak,
proceed as follows:
1. Degrease underside o f transmission.
2. R o a d test to get unit at operatin g temperature.
3.
Inspect for leak with engine running.
4. W ith engine off, check for oil leaks due to the
raised oil level caused by d ra in back.
Possible Points of Oil Leak
1. Transm ission Oil P an Leak.
a. Attaching bolts not correctly torqued.
b. Im properly installed or d a m a g e d pan gasket.
c. Oil p a n gasket m o unting face not flat.
2. Extension Housing.
a. Attaching bolts not correctly torqued.
b. R ear seal assembly dam ag ed or im properly
installed.
c. Square seal, extension to case, d a m a g e d or
im p ro p erly installed.
d. Porous casting. See su b p a ra g ra p h C.
3.
Case Leak.
a. Filler pipe " O " ring seal d a m a g e d or missing;
m isposition o f filler pipe bracket to engine.
b. M odulator assembly " O " ring seal dam ag ed
or im p ro p erly installed.
c. D e te n t cable c o n n e c to r " O " rin g seal
d a m a g e d or im properly installed.
d. G o v e rn o r cover not tight, gasket d a m a g e d or
leak between case face and gasket.
e. Speedom eter g ear " O " ring dam aged.
f. M anual shaft seal d a m a g e d or im properly
installed.
g. Line pressure tap plug loose.
h. Vent pipe (refer to item 5).
i. Porous casting. See S u b p a ra g ra p h C.
4. Leak at F ro n t o f Transmission.
a. F ro n t pu m p seal leaks.
1. Seal lip cut. Check converter hub, etc.
2. Bushing moved and d am aged, Oil return
hole plugged.
3.
N o oil return hole.
b. Front pu m p attaching bolts loose or bolt
w asher type seals d a m a g e d or missing.
c. F ro n t pu m p housing " O " ring dam ag ed or
cut.
d. C onverter leak in weld area.
e. Porous casting (pump).
5. Oil Comes Out Vent Pipe.
a. Transm ission over-filled.
b. W a te r in oil.
c. Foreign m aterial between p u m p and case or
between p u m p cover and body.
d. Case - porous n ear converter bosses. Front
pum p cover or housing oil channels shy or stock near
breather. See S u b p a ra g ra p h C.
e. Pum p to case gasket mis-positioned.
OIL PRESSURE CHECK
While vehicle is stationary (service brake on),
engine speed set to 1200 rrnp, transmission oil pressure
gauge attached as shown in Fig. 7A-10B, and vacuum
m odulator tube
d is c o n n e c te d
the transmission line
pressure tap should read 167 psi in drive, 166 psi in LI
or L2. and 254 psi in reverse.
While vehicle is stationary (service brake on),
engine speed set to m ain tain 12 inches hg. absolute
m anifold pressure, transmission oil pressure gauge
attached, and vacuum m odulator tube
c o n n e c te d
the
transmission line pressure tap should read 85 psi in
drive, 105 psi in LI or L2, and 129 psi in reverse.
CASE PO R O SITY REPAIR
Fxternal oil leaks caused by case porosity can be
successfully repaired with the transm ission in the vehicle
by using the following recom m ended procedures:
1. Road test and bring the transmission to
o perating tem perature, approximately 180 degrees F.
2. Raise vehicle on a hoist or jack stand, engine
running, and locate source o f oil leak. Check for oil leaks
in Low. Drive, and Reverse.
3. Shut engine off and thoroughly clean area to be
repaired with a suitable cleaning solvent and a brush -
air dry.
A clean, dry soldering acid brush can be used to
clean the area and also to apply the epoxy cement.
4. Using instructions o f the m anufacturer, mix a
sufficient am ount o f epoxy to make the repair. Make
certain the area to be repaired is fully covered.
5. Allow cement to cure for 3 hours before starting
engine.
6. R oad test and check for leaks.
V A C U U M M O D U L A TO R D IA G N O S IS
A defective vacuum modulator can cause one or
more o f the following complaints.
1. Harsh upshifts and downshifts.
2. Delayed upshifts.
3. Soft upshifts and downshifts.
4. Slips in low, drive and reverse.
5. Transm ission overheating.
6. Engine burning transmission oil.
If any one of the above complaints are encountered,
the modulator must be checked.
Vacuum Diaphragm Leak Check
Insert a pipe cleaner into the vacuum connector pipe
as far as possible and check for the presence of
transmission oil. If oil is found, replace the modulator.
Gasoline or w ater vapor may settle in the vacuum
side o f the modulator. If this is found without the