GMC 1976 ZEO 6083 Maintenance Manual page 472

7- 2 4 TRANSMISSION
CAUTION :
Do not use any method other
than human lung power for applying air
pressure, as pressures over 6 psi may dam-
age the modulator .
3 . Bellows Comparison Check. Make a compari-
son gauge, as shown in Figure 12, and compare the
load of a known good Hydra-matic modulator with
the assembly in question . Modulator Tension Tester
such as J-24466 may also be used for this purpose.
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
be 1/16" or less . If the distance is greater than this
amount, the modulator in question should be re-
placed .
4. Sleeve Alignment 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 modulator is
acceptable .
Once the modulator assembly passes all of the
above tests, it is an acceptable part and should be
re-used.
DOWNSHIFT SOLENOID CIRCUIT CHECK
NOTE :
Before checking the downshift solenoid
circuitry, make certain that the transmission
downshift switch is properly adjusted .
1 . With transmission shift lever in Park, turn
ignition switch to "ON" positon, but do not start
vehicle. Leave ignition switch "ON" throughout
checking procedure.
2. With the engine cover removed, slowly ad-
vance throttle linkage to wide open position . One
click should be heard from transmission .
3 . Allow throttle to return to closed position . One
click should be heard from transmission .
4. If system performed as described above, down-
shift circuit is operating properly . If system does not
perform as described above, proceed to step 5.
5. Use test light to check orange wire at connec-
tor on side of transmission case . Test light should
Figure 12-Vacuum Modulator Test Gauge
light with throttle wide open and go out when throt-
tle is released .
a. If system operates as described above, but
did not perform properly during steps 1-3, replace
solenoid after first checking to see that internal wir-
ing is operational .
b. If light fails to light with throttle in wide
open position, the circuit is open, proceed to step 6.
c . If light lights with throttle closed, the cir-
cuit is shorted. Proceed to step 9.
6. At transmission downshift switch, use test
light to check from the top terminal at switch with
throttle wide open .
7. Check lower feed wire at transmission down-
shift switch with test light.
a. If test light lights, replace transmission
downshift switch . Recheck system .
8. Check 10 amp . (gauges and transmission con-
trol fuse) in fuse panel.
tem.
a. If test light lights, recheck system .
b. If test light fails to light, proceed to step 7.
b. If test light fails to light, proceed to step 8.
a . If necessary to replace fuse, recheck sys-
b. If fuse is all right, it will be necessary to
locate the open in the wiring . Test the circuit con-
tinuity from the lower wire at the downshift switch
to the battery.
9. At the transmission downshift switch, use test
light to check from the disconnected top terminal at
switch with throttle closed .
a. If test light fails to light, orange wire is
shorted. Correct shorting condition.
b . If test light lights, proceed to step 10 .