Volvo FM Driver's Handbook Manual page 405

Differential lock 5
5 Engage the differential lock between the
front wheels.
The control lamp on the instrument panel
will come on.
6 Release the clutch pedal
7 Drive away from the slippery area
8 Release the accelerator
9 Disengaging the differential locks
T4015830
The differential lock between the front
wheels will be disengaged if the differential
lock between the rear wheels or the front-
wheel drive is disengaged.
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