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7A3-4
1977 PONTIAC SERVICE MANUAL
3. Soft upshifts and downshifts.
4. Slips in low, drive and reverse.
5. Transmission overheating.
6. Engine burning transmission oil.
If any one of the above complaints are encountered, the
modulator must be checked.
The vacuum modulator has three areas to be checked.
If any one of the three (3) areas fail to pass the prescribed
checks, the modulator must be replaced.
1. Load Comparison Check
To check load proceed as follows:
a. Insert one end of the comparison gage J 24466 into
the suspected modulator sleeve. Insert the opposite end of the
gage into a known, good modulator of the same part number
as the suspected modulator. This part number of the
modulator assembly is located on the vacuum end of the
modulator.
b. Holding the modulators in a horizontal position, see
Fig. 7A3-3, bring them slowly together under pressure. The
modulator in question, if bad, will reach the center line of the
comparison gage before the known good modulator lines up
with the outer gage line.
CENTER LINII
4885
Fig. 7A3-3
Modulator Load Comparison Check
If the modulator in question is good, both modulator
assemblies will be within the outer gage lines as the assemblies
are slowly brought together.
2. Vacuum Diaphragm Leak Check
Turn modulator so vacuum line stem points downward.
If the transmission oil comes out the vacuum diaphragm is
bad.
Gasoline and/or water vapor may settle in the vacuum
side of the modulator. If this is found WITHOUT the
presence of oil, the modulator MUST NOT BE CHANGED.
Check solution that comes out of the modulator for
evidence of lubricity. If the solution does not have the feel
of oiliness it can be assumed the solution is a mixture of gas
and/or water. The only way transmission oil can be on the
vacuum side of the modulator is by a leak in the vacuum
diaphragm.
If oil is found, the modulator must be replaced. If oil is
not found in the vacuum side of the modulator but the
transmission oil level is continually low, and no external leaks
are found, there is a possibility that a pin hole leak exists.
3. Inspection for External Damage
-
a. Check for dents or cracks in modulator.
b. Check modulator valve sleeve alignment. Roll
modulator on a flat surface to determine if the sleeve is
concentric to the modulator can. If the sleeve is bent, runout
will be visible, and modulator must be replaced.
If the modulator passes the above checks, the following
items should also be checked as a possible cause of the
problem:
1. Check freeness of modulator valve in transmission
case.
2. Check the vacuum line from the manifold to
modulator for holes, cracks or dents. Check the rubber hose
connections at the modulator and at the intake manifold for
leaks.
CAUSES OF IMPROPER VACUUM AT
MODULATOR
1. Engine
-
a. Tune up.
b. Loose vacuum fittings.
c. Vacuum operated accessory leak (hoses, vacuum
advance, etc.).
d. Engine exhaust system restricted.
2. Vacuum line to modulator
-
a. Leak
b. Loose fitting
c. Restricted orifice or incorrect orifice size.
d. Carbon build-up at modulator vacuum fitting.
e. Pinched line.
f. Grease in pipe (no or delayed upshift-cold).
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