Volvo VNL 780 Operator's Manual page 389

The differential lock should be used on icy or slippery surfaces. As soon as the slippery
surface has been passed, the differential lock should be disengaged.
Differential locks must not, under any circumstances, be engaged when in a wheel-spin
situation. Engage the locks ahead of the slippery area. If already slipping, stop the wheels,
engage the locks and then continue. If the differential locks are engaged when the wheels
are spinning, the differential will be damaged and the rear axle drive unit could possibly fail.
All efforts must be made to avoid spinning the wheels at high speeds on slippery surfaces.
This is true whether the differential locks are engaged or not. Excessive wheel spin may
result in failure of the cluster gears and other components found within the rear axle
housing(s). If you are unable to obtain traction, engage the differential locks as described
below. If you are still unable to move the truck, seek assistance from a qualified tow operator.
Engaging the differential locks must always be done as follows "PRE-TRIP INSPECTION
AND DAILY MAINTENANCE" page 217 for switch location):
1 Depress the clutch pedal. Wait for the drive wheels to stop spinning.
2 Engage the differential lock.
3 Engage a suitable gear.
4 Carefully release the clutch pedal.
W3001991
OPERATION 379