Gaining traction: Power dividers versus diff locks

By: Steve Skinner

We visit Meritor’s assembly plant in Australia, and pass on advice from the American axle giant on how to maintain traction in slippery conditions without damaging the truck

Gaining traction: Power dividers versus diff locks
Steer axles all in a row, with both drum and disc brakes.


Here’s a question that will be a bit tricky for a lot of fleet managers and truck drivers.

What’s the difference between a power divider and a diff lock?

The answer is that the power divider locks the front axle to the rear axle on a tandem axle set; and the differential lock locks both wheels on the same axle, ensuring that they spin at the same speed.

In the cab of a truck you might have three switches for extra traction in the rough stuff: one for the power divider — which is also called an ‘inter-axle differential’; and two for the diff locks on each drive axle.

Knowing when to use each of them properly could prevent a driver getting bogged in a paddock or on the dirt; or prevent you slipping off the bitumen in ice or snow.

And a bit of knowledge could also avoid doing big damage to the axles.

Here is the official advice from Meritor on all this. It comes from a technical bulletin titled Driver Instruction Kit TP-95790, and it’s for the ‘Inter-Axle Differential’ (IAD, or power divider) and ‘Driver-Controlled Main Differential’ (DCDL, or diff lock).

What Meritor calls ‘tandem axles’ in Australia we usually call ‘bogey drives’ or ‘6x4s’.

All Meritor tandems have a power divider.

"Drivers may lock the IAD to improve traction when encountering adverse conditions where loss of traction may be possible," the technical bulletin says.

"The IAD lock may be used at all speeds and for long time periods depending on the road conditions such as rain, snow or gravel/dirt roads."

That’s a very different situation to the diff locks, which come as an option for the forward drive axle, the rear drive axle, or both.

They should only be used at very low speeds because steering can be badly affected.

"The DCDL is used during on and off highway operations that encounter slippery road conditions and/or uneven terrain," the bulletin says.

"It is used only at low speeds while travelling through an area with poor traction conditions and should be deactivated as soon as the vehicle is through the area.

"Drivers may lock both the IAD and the DCDL for maximum traction under icy, snowy or poor road conditions.

"However, when using both of these traction-enhancement devices or the DCDL alone, vehicle speed must remain at 25mph [40km/h] or less."


To find out how to use both the IAD and the DCDL, subscribe to ATN to get the November issue.


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