ABS/ATC and Off-Road Operation

First let me say that Automatic Traction Control (ATC) was never meant to replace locking axles. ATC is great for counteracting wheel slip on paved surfaces, but if you go off-road then Diff Lock (cross-locked axles) may be required to keep you from getting stuck, or to pull yourself out of slippery terrain. At times you will see both of these products used on the same vehicle, especially today on tractors.

ATC is a function of the Anti-lock Braking System (ABS). So make certain that if you have tandem drive axle, you are using a 6S/6M setup. There are two important features for off-road with Diff Locks

Switch for Off-Road ABS: This switch desensitizes the anti-lock system and allows for a degree of lockup when braking off road.  

The theory here is that the slowed activation will allow more material to build up ahead of the tire, increasing traction potential. You can never turn off ABS completely, on vehicles where it is required.

Switch to Deactivate ATC: This switch turns the system off and is highly recommended if you also spec locking axles.  ATC senses a speed difference between outer ends and applies brakes to faster spinning end, which can defeat the progress. Also, ATC includes and engine de-rate for preserving brake lining, another nemesis of Diff Lock activity. The ATC switch icon reads TC or ATC on the switch (depending on truck model), but they are same switch.

The speed at which ATC re-engages is programmable by the shop on most models with multiplexing. Default is typically around 25 mph. 

You can utilize both of these features in concert, if required.