Chevrolet 10 1971 Series Chassis Service Manual page 313

returned as defective were good. For this reason, the
following procedure is recommended for checking Turbo
Hydra-Matic 400 modulator assemblies in the field before
replacement is accomplished.
1. V acuum D iap h rag m Leak Check
Insert a pipe cleaner into the vacuum connector pipe
as far as possible and check for the presence of trans­
mission oil. If oil is found, replace the modulator.
Gasoline or water vapor may settle
NOTE:
in the vacuum side of the modulator. If this is
found without the presence of oil, the modulator
should not be changed.
2. Atm ospheric Leak Check
Apply a liberal coating of soap bubble solution to the
vacuum connector pipe seam, the crimped upper to lower
housing seam, and the threaded screw seal (Fig. 5T).
Using a short piece of rubber tubing apply air pressure
to the vacuum pipe by blowing into the tube and observe
for leak bubbles. If bubbles
modulator.
Do not use any method other than human
NOTE:
lung power for applying air pressure, as pres­
sures over 6 psi may damage the modulator.
3. Bellows C om parison Check
Using a comparison gauge, as shown in Figure 6T,
compare the load of a known good Hydra-Matic modulator
with the assembly in question.
a. Install the modulator that is known to be acceptable
on either end of the gauge.
b. Install the modulator in question on the opposite end
of the gauge.
c. Holding the modulators in a horizontal position,
bring them together under pressure until either
modulator sleeve end just touches the line in the
center of the gauge. The gap between the opposite
modulator sleeve end and the gauge line should then
Fig. 7T -V acu u m M odulator
appear, replace the
CLUTCHES AND TRANSMISSIONS 7-37
Fig. 8T— Bellows Comparison G auge
be 1/16" or less. If the distance is greater than
this amount, the modulator in question should be
replaced.
4. Sleeve A lig n m e n t Check
Roll the main body of the modulator on a flat surface
and observe the sleeve for concentricity to the can. If the
sleeve is concentric and the plunger is free, the modula­
tor is acceptable.
Once the modulator assembly passes all of the above
tests, it is an acceptable part and should be re-used.
PRESSURE SWITCH CHECK
Pressure switch in the Turbo Hydra-matic 400 trans­
mission may be easily checked with a continuity tester
which uses either the vehicle's battery or a separate
source to power the test lamp or meter.
To prevent damaging the pressure
CAUTION:
switch, the tester used must have sufficient
resistance that it does not supply the switch
with more than .8 amp of current at 12 volts.
Fig. 9T — Electrical Connector For TCS (Two Term)
CM Model Has Detent Terminal O nly
10 30 CHEVROLET TRUCK SERVICE MANUAL