GMC 4000 Series Service Manual page 150

SPEEDOMETER ADAPTER
SHIFT DIAPHRAGM
When shift control lever is moved to "HI"
position, cavity behind speedometer adapter shift
diaphragm (fig. 28) is subjected to atmosphere
through the shift control valve. Spring movement
of diaphragm is transmitted to the speedometer
adapter lever by the diaphragm rod, shifting the
speedometer adapter gears into high ratio. When
the control lever is moved to "LO" position, the
control valve admits vacuum to the cavity behind
the diaphragm and forces diaphragm and rod in­
ward, shifting the speedometer adapter gears into
low ratio.
REMOVAL AND DISASSEMBLY
1. Disconnect vacuum line from the shift
diaphragm body.
2. Remove two screws attaching diaphragm
bracket to speedometer adapter. Lift diaphragm
and bracket to unhook diaphragm rod from adapter
lever and remove diaphragm and bracket assembly.
G M C S E R V I C E M A N U A L
bracket. Remove body, spring, and diaphragm and
rod assembly from bracket.
INSPECTION
damage. Replace assembly if diaphragm is damaged.
spring. If spring is corroded or if tension is not
sufficient to return diaphragm to release position,
replace assembly.
ASSEMBLY AND INSTALLATION
spring, and body on diaphragm bracket and attach
with four screw s. Screws must be tightened suf­
ficiently to make the assembly air-tight with two
pounds of air pressure.
at speedometer adapter, hooking diaphragm rod
into adapter lever. Attach diaphragm bracket to
adapter with two screws and washers.
Tighten connection firmly.
See next page for Torque Specifications.
REAR AXLE AND CONTROLS
3. Remove four screws attaching body to
1. Examine diaphragm for cracks or other
2. Check condition and tension of diaphragm
1. Position the diaphragm and rod assembly,
2. Position diaphragm and bracket assembly
3. Connect vacuum line to diaphragm body.
Sec. 4A
Page 143
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