Diff locks as well as electronic traction control

Drivers of 4WDs with electronic traction control sometimes wonder whether they can also run locking cross-axle differentials … especially after discovering that electronic systems are still far from perfect in harder terrain compared to a mechanical setup that's either on or off and not wondering what to do next. The latest newsletter from ARB, maker of the best-known aftermarket lockers, discusses this question.  I thought it was interesting and helpful so I'm posting this slightly abridged version. You can download the newsletter as a pdf from ARB.

Today’s factory equipped traction control systems come in two common forms:

Throttle Aperture Control
This type of traction control typically uses ABS brake sensors at each wheel to determine when a wheel has lost traction and has ‘spun up’ faster than the other wheels. It then attempts to get the wheel to slow enough to regain its traction and roll with the road at the same speed as the vehicle. To do this, it interacts with the Engine Control Unit (ECU) to instantly reduce maximum throttle input level (that is, to simulate a decrease in the full travel of the  throttle pedal, and reduce motor torque that has broken the wheel of traction). This can be effective on level roads, but this type of traction control can work against you if you need the extra throttle to get up a hill or through some deep sand or mud. For this reason you will most often find this type of traction control in road cars. 

Brake Bias Control
This also typically uses ABS sensors to detect loss of traction, but instead of limiting motor output, it uses a high pressure pump integrated with the ABS brake system to override the braking system and apply additional braking force regardless of the position of the brake pedal. By applying braking force to only the over-spinning wheel it attempts to slow it  enough to regain its traction and prevent the driving force of the other wheels from all being exhausted through the one over-spinning wheel. This can be effective on road and in mild off road situations, and this is the same system that enables vehicles to offer such things as hill descent control.

The downside of brake bias control is that it activates after traction loss is detected, so you have already lost some degree of control of the vehicle before it is able to help.

Another downside is that they tend to get confused by the traction loss situation that exists in loose sand, mud or snow. As the wheelspin changes randomly from wheel to wheel, the traction control system follows with a corresponding braking pattern and the overall effect tends to be a general slowing of the vehicle. The other major downside of brake bias control is that it depends entirely on the brakes for its function, and so use in heavier off road situations could result in overheating and premature wear or damage to the brakes. For this reason, these systems are equipped with temperature sensors to alert the driver when they have been automatically deactivated.

Because these traction control systems do not involve the vehicle’s differential(s) in any way, an ARB air locker can be harmoniously installed without the need for any alterations to the existing systems.  In many ways a combination of systems offers the best of both worlds because just as your factory traction control was not really intended for heavy off road use, where an ARB air locker was, an air locker is not intended for high traction on-road use, and so simply switching them off allows the factory traction control to instantly take over for milder driving conditions.