To clarify:
Since the 2003 model year, BorgWarner updated the Auto mode for the automatic full-time ControlTrac four-wheel drive system. Auto mode no longer needs wheelslip to start transferring torque to the front wheels. It can transfer torque before wheel slip happens.
Additional lines of code were written and added to the control firmware (software). This additional code gives the system true artificial intelligence and allows it to use automated reasoning to detect different road surfaces and terrain conditions. The system analyzes driver input (steering, throttle, braking), wheel speed, and surface conditions to predict if the wheel or wheels will lose traction.
If the system calculates a high probability that wheel slip is eminent, it will transfer an "x" percentage of torque ("x" representing a variable integer) before wheel slip occurs. The goal is to stop wheel slip before it happens, or lessen the severity of wheel slip if it still occurs.
Furthermore, in Auto mode only, up to 100 percent of the engine’s theoretical torque output can sent to the front wheels if the system deems necessary. This behavior is called "torque biasing", and during a torque biasing event, the rear wheels receive 0 (zero) torque from the engine, for a brief moment in time. Effectively the Expedition is front-wheel drive, but only for as long as is needed. Once the rear wheels regain traction, the 4x4 system will start feeding torque back to them.
The front axle/front differential is always engaged in Auto mode, whether it receives torque or not. The system simply engages the center multi-disc differential (inside the transfer case), which transfers torque in 10 percent increments.
Engine electronic traction control (reduces engine torque via engine management) and four-wheel electronic traction control (brake locks the differentials via ABS) both operate in Auto mode.
AdvanceTrac electronic stability control (ESC) and roll stability control (RSC) are also active in Auto mode.
Both the ControlTrac four-wheel drive system and traction control system use steering, braking, throttle, and ABS wheel speed sensors to operate. Both the 4x4 system and traction control are networked into each other so the 4x4 system knows what the traction control system is doing and the traction control system knows what the 4x4 system is doing. They work together as a team.
Typically ControlTrac will begin transferring torque before traction control does.
