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How do power dividers work on a truck?

When a tractor unit has a 6×4 or 8×4 configuration it has two axles that are drive axles. Some units are fixed, while others have a power divider to enable the driver to decide where to send the engine’s power.

These are used when driving in snowy weather or off-road, or when one axle is likely to wheelspin.

Tandem drive

With tandem drive axle sets, both axles are driven by the engine at all times. The individual axles can travel at different speeds, and this happens when cornering (a small difference) or if one of the axles loses traction (potentially a large difference).

If one axle is lifted off the ground, all power will go through that axle and the wheels will spin, meaning there’s no power going through the axle that remains on the ground. This means that it’s possible to beach a tractor unit with tandem drive.

One axle is off the ground. This will spin unless the power divider is activated.

Tandem drive with power divider

The power divider works like a differential lock between the two axles meaning both axles will run at the same speed, even during loss of traction. Each axle’s differential means that the wheels on either end can run at different speeds to help with cornering.

Power dividers should be engaged when going uphill and downhill to reduce the risk of engine damage as they can reduce inadvertent wheelspin when going uphill and engine brake compression lockup when going downhill.

The power divider should be engaged when conditions are slippery, such as when driving off-road. They can be used at any speed.

Mercedes screen showing the power divider is activated

How to engage the power divider

Don’t engage the power divider when the wheels are spinning and traction is lost.

  1. Ensure that the truck is travelling in a straight line and below 60km/h
  2. Release the accelerator pedal
  3. Press the power divider switch
  4. Depress the accelerator pedal and continue driving normally

To disengage it, drive in a straight line, lift off the accelerator, press the power divider switch, then press the accelerator.

Tandem drive with power divider and diff lock (cross lock) activated

Mercedes screen showing that the power divider and diff’ locks are activated

This can also be called an interaxle cross lock. This locks the centre diff between the axles, as well as the diffs on the axles, meaning all wheels turn at the same speed. If one axle loses traction, the other axle will continue to turn at the same speed. This mode should not be used for tight cornering as it stresses the axle.

Cross locks are used when extra traction is required off-road.

How to engage the cross lock

  • Stop the truck completely
  • Press the power divider and cross lock switches
  • Move off slowly until you get to an area where there will be enough traction to not have to use the cross lock

How to disengage the cross lock

  • Stop the truck completely
  • Switch off the cross lock; you can leave the power divider engaged, if required
  • Move off slowly

Examples of power dividers in different trucks

Power divider and cross lock button in a MAN
Power divider and cross lock switches in a Freightliner – each of the two drive axles has its own control for cross lock which can be switched independently to the power divider.
Power divider and cross lock switch on a Mercedes.
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Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above

Posted in Advice, Heavy Vehicle