The two stage switch allows the center differential to be locked, either by itself or along with the rear axle differential.
Center Differential Lock Engagement
When the shift valve is pulled to the first detent, intake manifold vacuum is directed to the engagement side of the center differential lock vacuum servo. The diaphragm in the servo is pulled and the center differential lock is engaged via the linkage.
At the same time, the warning lamp on the indicator panel is illuminated by the center differential lock switch.
Vacuum is also directed to the disengagement side of the rear axle differential lock.
Center and Rear Differential Lock Operation
Both diaphragms are pulled, engaging both differential locks through the appropriate linkages. At the same time, both warning lamps on the indicator panel are illuminated.
When the shift valve is pushed all the way in, vacuum is directed to the disengagement sides of both servos, and the locks are released.
Differential action can once again occur in the center differential and in the rear axle.