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7A2-4
1977 PONTIAC SERVICE MANUAL
Atmospheric Leak Check
Apply a liberal coating of soap bubble solution to
crimped upper to lower housing seam. Using a short piece
of rubber tubing, apply air pressure to vacuum pipe by
blowing into tube and observe for leak bubbles. If bubbles
appear, replace modulator.
NOTE: Do not use any method other than human lung
power for. applying air pressure as pressures over 6 psi
may damage modulator.
Spring Comparison Check
Using J 24466, or a comparison gage as shown in Fig.
7A2-4, compare the load on a known good modulator with
assembly in question:
a. Install modulator that is known to be good on either
end of the gage.
b. Install modulator in question on opposite end of the
gage.
c. Holding modulators in a horizontal position, bring
them together under pressure until either modulator sleeve
end just touches line in center of gage. The gap between
opposite modulator sleeve end and gage line should then be
1/16 inch or less. If distance is greater than this amount, the
modulator in question should be replaced.
Sleeve Alignment Check
Roll main body of modulator on a flat surface and
observe the sleeve for concentricity to cam. If sleeve is
concentric and plunger is free, modulator is acceptable.
Once modulator assembly passes all of the above tests,
it is an acceptable part and should be re-used.
TRANSMISSION CLUTCH PLATES
DIAGNOSIS
1. Lined Drive Plates-
a. Dry plates with compressed air and inspect lined
surface for:
1. Pitting and flaking.
2. Wear.
3. Glazing.
4. Cracking.
5. Charring
6. Chips or metal particles imbedded in lining. If a line
drive plate exhibits any of the above conditions, replacement
is required. Do not diagnose drive plates by color.
2. Steel Driven Plates-
Wipe plates dry and check for heat discoloration. If
surface is smooth and an even color smear is indicated, plate
should be reused. If severe heat spot discoloration or surface
scuffing is indicated, plate must be replaced.
3. Clutch Release Springs-
Evidence of extreme heat or burning in area of clutch
may have caused springs to take a heat set and would justify
replacement of springs.
CAUSES OF BURNED CLUTCH PLATES
OR BURNED BAND
1. FORWARD CLUTCH
ENDS TO B E SQUARE
'T
"lTHIN
/64"
7
13/32" TO
3/8"
-
ROUND OR FLAT STOCK
SCRIBE CENTER A N D
OUTER LINES
4349
Fig.
7A2-4
Spring Comparison Gage
a. Check ball in clutch housing damaged, stuck or
missing.
b. Clutch piston cracked, seals damaged or missing.
c. Low line pressure.
d. Pump cover oil seal rings missing, broken or
undersize; ring groove oversize.
e. Case valve body face not flat; porosity between
channels.
2. INTERMEDIATE BAND
a. Intermediate servo piston seals damaged or missing.
b. Low line pressure.
c. Case valve body face not flat; porosity between
channels.
3. DIRECT CLUTCH
a. Restricted orifice in vacuum line to modulator (poor
vacuum response).
b. Check ball in direct clutch piston damaged, stuck or
missing.
c. Defective modulator spring.
d. Clutch piston seals damaged or missing.
e. Case valve body face not flat; porosity between
channels.
f. Clutch installed backwards.
NOTE: Burned clutch plates can be caused by incorrect
usage of clutch plates. Also engine coolant in
transmission fluid can cause severe damage, such as
large pieces of composition clutch plate material peeling
off.
GOVERNOR PRESSURE CHECK
1. Install Pressure Gage, at line pressure tap location
(Fig. 7A2-2).
2. Disconnect vacuum line to modulator.
3. With vehicle on hoist (rear wheels off ground), foot
off brake, in Drive, check line pressure at 1000 RPM.
4. Slowly increase engine RPM to 3000 RPM and
determine if a line pressure drop occurs (7 PSI or more).
5. If no pressure drop occurs:
a. Inspect Governor-
1. Stuck valve.
2. Free Weights.
3. Restricted orifice in governor valve.
b. Governor Feed System-
1. Check screen in control valve assembly.
2. Check for restrictions in feed line.
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