Pressure Switch Check - Chevrolet 10 Series 1970 Service Manual

Using a short piece of rubber tubing apply a ir p ressu re
to the vacuum pipe by blowing into the tube and ob serve
fo r leak bubbles. If bubbles
m odulator.
NOTE:
Do not use any method other than human
lung p ow er fo r applying a ir p ressu re, as p r e s ­
sures o v e r 6 p si may damage the modulator.
3. Bellows Comparison Check
Using a com parison gauge, as shown in F igu re 6T,
com pare the load of a known good H yd ra-M atic modulator
with the assem bly in question.
a. In stall the m odulator that is known to be acceptable
on eith er end of the gauge.
b. Install the m odulator in question on the opposite end
o f the gauge.
c. Holding the m odulators
b rin g them together
m odulator s le e v e end just touches the line in the
cen ter of the gauge.
m odulator s le e v e end and the gauge line should then
be 1/16" or le s s . If the distance is g re a te r than
this amount, the m odulator in question should be
replaced.
4. Sleeve A lignm ent Check
R o ll the main body of the modulator on a fla t surface
and o b serve the s le e v e fo r con cen tricity to the can. If the
s le e v e is concentric and the plunger is fr e e , the modula­
to r is acceptable.
Once the modulator assem bly passes a ll of the above
tests, it is an acceptable part and should be re-u sed.

PRESSURE SWITCH CHECK

P re s s u re switch in the Turbo H ydra-m atic 400 tra n s­
m ission may be e a s ily checked with a continuity te s te r
which uses eith er the v e h ic le ' s battery or a separate
source to p ow er the test lamp o r m eter.
CAUTIO N: T o preven t damaging the p ressu re
resistan ce that it does not supply the switch
with m ore than .8 amp o f current at 12 volts.
T e s t lam ps which use s iz e
bulb, w ill not damage the switch.
appear, replace
in a horizon tal position,
under p ressu re until either
The gap between the opposite
1893 o r s m a ller
t h e
1. Disconnect e le c tric a l w ire harness fro m tra n sm is­
sion e le c tr ic a l connector. If using a continuity tester
which has a s e lf contained pow er source, connect
one te s te r lead to p ressu re switch term in al o f tran s­
m ission connector (see F ig. 7T), and connect other
lead to ground. T e s te r should show current flow ing
through the switch (sw itch closed).
I f the truck' s battery is to be used to pow er a
sim ple light bulb type te s te r, connect one lamp lead
to p ressu re
nector and connect other lead to a " Hot" term in al
o f the truck' s w irin g harness.
battery cables a re connected.
2. Set parking brake, apply s e rv ic e brakes, start en­
gine and le t it idle. M ove transm ission s e le c to r to
" D riv e " and
through switch to ground (switch closed).
3. With brakes s till applied, m ove transm ission s e le c to r
to " R e v e r s e " (which supplies transm ission o il p r e s ­
sure to p ressu re switch) and te s te r should show no
current flow ing
open).
4. If above test indicates a d efective p ressu re switch
circu it, the tran sm ission bottom pan must be r e ­
m oved to s e rv ic e the pressu re switch o r lead w ire
assem bly.
CLUTCHES AND TRANSMISSIONS 7-55
TERMINAL
F ig . 7 T — Pressure S w itc h
switch term in al o f transm ission con­
Check that tru ck' s
tester should show cu rren t flow ing
through switch to ground (switch
CHEVROLET TRUCK SERVICE MANUAL