GMC 4000 Series Service Manual page 133

Sec. 4A
Page 126
REAR AXLE A N D CONTROLS
Figure 2 — Two-Speed Electric Shift Unit
Installed (Eaton Axle Show n)
matic switch so that motor is no longer energized
and the armature stops rotating. To make sure
that the nut cannot travel back on the screw due
to vibration, a ball screw detent spring, holds the
nut at the end of its travel on the screw.
The nut moves the spring winding lever down
pivoting on winding and actuating lever shaft, winds
the torsion spring for high. Thus an increased load
is put on the spring, and in this position the axle
is ready to snap into high speed ratio as soon as
the load on the axle gears is relieved.
The torsion spring is assembled in the unit so
that it is under approximately 50 to 90 pounds
pressure, depending upon the size of the axle.
When the spring winding lever is moved so that the
spring is wound, the pressure of the spring is ra is­
ed to approximately 90 to 140 pounds, depending
upon size of axle.
The additional pressure is used to shift the
P O S IT IO N IN H IG H
Figure 3 — Position of Drive Screw in H ig h a n d Low R a n ge
G M C S E R V I C E M A N U A L
T -2359 i
P O S IT IO N IN L O W -2441
t
axle. When the shift is completed, the ends of the
spring come together leaving the original tension
on the spring. Thus pre-load tension holds the axle
in either selected gear.
When the shift button is in "Lo" range, the
motor is energized so that the motor armature and
drive screw rotate to drive nut in opposite direc­
tion for shift into "Low" range, in the same manner
as previously described for "High" range operation.
SPEEDOMETER ADAPTER
The speedometer adapter is mounted to back
of speedometer and is electrically connected to
control switch. When the control switch button is
placed into "Lo" range position, an electro-magnet
shifts the adapter mechanism to compensate for
the difference in gear reduction between "High"
and "Low" range in the axle. When shift button is
in "Hi" range position, current is released from
electro-magnet, and a spring holds the adapter
mechanism in "High" range position.
Speedometer adapters used on these vehicles
are 12 volt type. Voltage of the unit is stamped on
the housing below the wire terminal. Care should
be used to select unit of same voltage as vehicle.
WIRING SYSTEM
Reference should be made to optional equip­
ment wiring diagrams in "Wiring Diagrams" book­
let. The ignition switch must be on before electric
shift mechanism will operate.
A separate circuit breaker in the circuit pro­
tects the shift circuit in the event of a short. Refer
to applicable "Wiring Diagrams" booklet for cir­
cuit breaker connections.
VACU UM SHIFT
Vacuum operated power shift system is used
on some vehicles equipped with 2-speed rear axle.
A typical system arrangement is shown in figure 4.
Vacuum power shift system utilizes engine
intake manifold vacuum and atmospheric pressure
for its operation. Units used in vacuum power shift
system are: Control Button, located on gearshift
lever; Control Valve, mounted on frame side rail
at left-hand side; Vacuum Check Valve, installed
in vacuum line at dash junction; Speedometer Adap­
ter and Adapter Shift Diaphragm, mounted at speed­
ometer head under instrument panel; Rear Axle
Power Shift Chamber, mounted on differential car­
rier; and interconnecting vacuum lines and fittings.
SYSTEM OPERATION
LOW SPEED
With shift control button in "Lo" position,
control valve opens one side of shift chamber at