Spicer Single Drive Axles Service Manual: S170, S190, S590 - Manual (Page 66)

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Theory of Operation
Operate Wheel Differential Assembly
The Spicer wheel differential lock is driver-controlled and
operated by a carrier mounted air-actuated shift unit. In
operation, it positively locks the wheel differential to pro -
vide improved traction under adverse road conditions.
Control Systems for Differential Lock
Two systems may be used to control the differential lock
operation.
Transmission Low Range Interlock Control System
The wheel differential is locked manually with the transmis -
sion in Low Range. It is unlocked by the driver or unlocked
when the transmission is shifted out of Low Range .
Note: The interlock system is preferred for vehicles
equipped with an air-shifted, Low Range transmis -
sion. It is designed to ensur e the differ ential lock is
not left engaged (and to prevent accidental engage-
ment) when transmission is in high range.
Direct Driver-controlled System
The driver manually locks and unlocks the wheel differen -
tial, using a cab-mounted electric switch (or air valve). The
following description assumes the system includes a cab-
mounted electric switch and a solenoid valve as shown in
the illustration. An air valve may be substituted for these
components.
Operation is as follows :
1. With control switch in the “unlock” position, the
wheel differential functions normally.
2. When the control switch is placed in the “lock”
position, the air supply solenoid valve opens and
air pressure activates the shift cylinde r. The shift
fork is moved to engage the curvic clutches,
which, in turn, lock the wheel differential .
3. When the control switch is placed in the “unlock”
position, air pressure supply to the shift cylinder is
shut off and air pressure is released from the cylin -
der. A compression spring moves the shift fork to
disengage the curvic clutch and unlock the wheel
differential .