Falls,
Here is a copy/paste of an atricle that appeared in Car and Driver on the Escape 4wd system:
Contrary to what you'll be told by the dashboard markings, your Ford Outfitter salesperson, and even Ford's press materials, the Escape's Control Trac II system does not offer fully locked four-wheel drive. Rather, this on-demand all-wheel-drive setup only sends power to the rear axle when the front wheels slip. Here's how it works:
In the "4x4 Auto" mode, 100 percent of the torque is sent to the front wheels until a wheel slips. When that happens, a rotary blade coupling (RBC) generates enough pressure to activate a multiplate clutch, like that found in an automatic transmission, which sends torque (as much as 100 percent of it) to the rear. At the heart of the RBC is a "fan" with three blades in a chamber filled with a silicone fluid like that used in viscous couplings. The fan is shaped kind of like the warning symbol for radiation. When the front wheels slip, this fan spins through the fluid, heating and expanding it, which generates the pressure to activate the clutch. Then the clutch--not the viscous fluid -- bears the burden of transmitting the torque.
When set to "4x4 On," an electromagnet energizes a small clutch pack, locking a ball ramp to the input shaft. Now when the front wheels spin, they turn the ball ramp, which overrides the RBC and pressurizes the same multiplate clutch pack. In the "4x4 On" mode, the rear axle is engaged quicker and more securely. The front and rear axles are, however, never locked together, except during front-wheel slippage. That way, there is never any crabbing or binding in tight turns, and there's no need for a center differential.
