Volvo FM Driver's Handbook Manual page 403

Differential lock 3
5 Move the front-wheel drive switch to its
bottom position.
The control lamp on the instrument panel
will come on.
T4015829
6 Engage the differential lock between the
front wheels.
The control lamp on the instrument panel
will come on.
7 Release the clutch pedal
8 Drive away from the slippery area
9 Release the accelerator
10 Disengage the differential lock
T4015830
The differential lock between the front
wheels will be disengaged if the differential
lock between the rear axles, the differential
lock between the wheels on the rear axles
or the front-wheel drive is disengaged.
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