Volvo FM Handbook page 173

162 Driving
3 Press the lower part of the bogie switch a third time.
The air bellows for the bogie axle are inflated so that
axle loading is distributed evenly again. The bogie
lamp goes out.
1 Press the lower part of the bogie switch once.
If the speed is less than 30 km/h when it is pressed:
The drive axle pressure increases to max 30% above
the axle pressure limit. The bogie lamp flashes.
When the speed increases to more than 30 km/h, the
drive axle load drops to the axle load limit again.
2 Press the lower part of the bogie switch once more.
The function "Optimize traction" is re-activated. The
function distributes the load between the axles to
provide the best road grip possible with regard to the
load. The bogie lamp goes out.
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