Ford Expedition Owner's Manual page 200

Tires, Wheels and Loading
VEHICLE CHARACTERISTICS
Four-wheel drive (4WD) system (if equipped)
A vehicle equipped with 4WD (when selected) has the ability to use all
four wheels to power itself. This increases traction which may enable you
to safely drive over terrain and road conditions that a conventional
two-wheel drive vehicle cannot.
Power is supplied to all four wheels through a transfer case or power
transfer unit. 4WD vehicles allow you to select different drive modes as
necessary. Information on transfer case operation and shifting procedures
can be found in the Driving chapter. Information on transfer case
maintenance can be found in the Maintenance and Specifications
chapter. You should become thoroughly familiar with this information
before you operate your vehicle.
On some 4WD models, the initial shift from two-wheel drive to 4WD
while the vehicle is moving can cause a momentary clunk and ratcheting
sound. These sounds are normal as the front drivetrain comes up to
speed and is not cause for concern.
WARNING: Do not become overconfident in the ability of 4WD
vehicles. Although a 4WD vehicle may accelerate better than
two-wheel drive vehicle in low traction situations, it won't stop any
faster than two-wheel drive vehicles. Always drive at a safe speed.
200
2012 Expedition (exd)
Owners Guide, 1st Printing
USA (fus)