GMC 4000 Series Service Manual page 135

Sec. 4A
Page 128
REAR AXLE A N D CONTROLS
HIGH SPEED
With shift control button on gearshift lever
up, in "Hi" position, rear axle power shift cham­
ber is subjected to vacuum through the control
valve. With vacuum present on one side of the
shift chamber diaphragm, the atmospheric p res­
sure on the opposite side of the diaphragm forces
the diaphragm to move toward the vacuum side.
Movement of the diaphragm is transmitted to the
rear axle sliding clutch shift fork through the
shift rod, and the axle gears are shifted into high
speed. At the same time, vacuum is removed
from the speedometer adapter causing the spring
to move the diaphragm and rod, shifting speed­
ometer into high speed.
At regular intervals, the following lubrication,
inspection, and maintenance procedures should be
accomplished and corrective measures taken when­
ever necessary.
LUBRICATION
Lubrication intervals, method of filling and
draining, also type of lubricant and capacities for
all axles are covered in LUBRICATION (SEC. 0).
Examine housing cover, pinion oil seal re­
tainers, and axle shaft flanges for lubricant leaks.
Tighten bolts or nuts, or replace gaskets and seals
as necessary to correct leaks.
MOUNTING
Check for axle misalignment. Select a point
at one side of vehicle and a corresponding point at
opposite side. Measure distances between points
selected and identical points at each end of axle
assem bly. If distances are not equal, axle m is­
alignment is indicated and rear spring should be
checked for correct installation.
Refer to "REAR SPRINGS" (SEC. 4B) for spring
U-bolt tightening instructions, also for torque rod
inspection and maintenance.
Figure 6 — Clearance Betw een A x le Shaft Fla n g e a n d Stud Nut
G M C S E R V I C E M A N U A L
SHIFTING
pulls on control button, which is attached to valve
through a cable. This raises the cable trunnion and
operating lever up as shown in figure 5. The ten­
sion springs are therefore moved "over center"
causing poppet lever to rotate, lifting one poppet
valve and lowering the opposite valve.
p resses on control button. This moves the cable
trunnion and operating lever downward, as shown
in figure 5. The tension springs are therefore
moved "over center" causing poppet lever to ro­
tate, lifting one poppet valve and lowering the
opposite valve.
M AINTENANCE O N VEHICLES
AXLE SHAFT FLANGE MOUNTING
caps or stud nuts and dowels.
tervals. Tighten 1/2" nuts to 50-60 foot-pounds
torque, or 5/8" nuts to 90-110 foot-pounds torque.
correct taper must be used. There should always
be a slight clearance between nuts and driving
flange when nuts are drawn up (fig. 6). No clear­
ance at this point indicates excessive wear at
studs, dowels, or holes in flange. Replacement of
worn parts is the only remedy.
specified, play between flange and studs will cause
bent or broken studs, also worn tapered holes in
flange of axle shaft.
with unit in vehicle in following manner; however,
conventional alignment instruments can be used if
available.
clear floor. Block up axle under each spring seat.
if necessary, then check wheels for looseness and
tighten wheel nuts if necessary. Refer to "REAR
HUBS AND BEARINGS" (SEC. 4C).
tires at bottom. Measure across tires at chalk
marks with a toe-in gauge.
marks are positioned at top of wheel. Measure
across tires again. If measurement at top is 1/8"
or more sm aller than previous measurement noted
at bottom of wheels, axle housing has sagged and
is bent. If measurement at top exceeds bottom
dimension by 1/8", axle housing is bent at ends.
When shifting from "Lo" to "Hi" the operator
When shifting from "Hi" to "Lo" the operator
Axle shaft flanges are retained to hubs by hub
1. Check tightness of stud nuts at regular in­
2. The studs must be straight and dowels of
3. If stud nuts are not tightened to torque
BENT HOUSING CHECK
A check for bent axle housing can be made
1. Raise rear axle with a jack until wheels
2. Check wheel bearing adjustment and adjust
3. Place a chalk mark on outer side wall of
4. Turn wheels half-way around so that chalk