Chevrolet 70 1969 Series Shop Manual page 141

REAR SUSPENSION 4A-2
Figure 2 — Two-Speed Electric Shift Unit Installed
(Eaton Axle Shown)
S H IFT UNIT
The shift unit and automatic switch, assembly
is mounted on differential carrier (fig. 2). This
unit, controlled by the control switch, shifts the
axle into L O W or HIGH range.
When the control switch button is in HIGH
range, wiring carries current to one field of the
unit motor. The armature and drive screw turn in
a clockwise direction and move the nut down (fig. 3).
When the nut has traveled a sufficient distance
to wind the torsion spring, a contact bumper on the
nut breaks an electrical connection on the auto­
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
Figure 3— Position of Drive Screw in High and Low Range
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 raised to
approximately 90 to 140 pounds, depending upon size
of axle.
The additional pressure is used to shift the
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 L O W range, the
motor is energized so that the motor armature and
drive screw rotate to drive nut in opposite direc­
tion for shift into L O W range, in the same manner
as previously described for HIGH range operation.
S P E E D O M E T E R A D A P T E R
The speedometer adapter is mounted to back
of speedometer and is electrically connected to
control switch. When the control switch button is
placed into L O W range position, an electro-magnet
shifts the adapter mechanism to compensate for
the difference in gear reduction between HIGH and
L O W range in the axle. When shift button is in
HIGH 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.
W IR IN G S Y S T E M
Reference should be made to optional equip­
ment wiring diagrams in Section 12. 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.
R e f e r to Section
connections.
INTER-AXLE DIFFERENTIAL
CONTROLS
A differential lock is installed as a part of
forward axle to lock the inter-axle differential in
the event a wheel or wheels on one of the driving
axles are slipping. Lock is shifted by apower shift
system, controlled by a control lever on instru­
ment panel in cab (fig. 4).
CHEVROLET SERIES 70-80 HEAVY DUTY TRUCK SH O P M A N U A L
12 for c i r c u i t breaker