FORD 2011 F-150 v.2 - Manual (Page 316)

WARNING: If the parking brake is fully released, but the brake
warning lamp remains illuminated, the brakes may not be
working properly. See your authorized dealer.
4WD systems
4WD (when you select a 4WD mode) uses all four wheels to power the
vehicle. This increases traction, enabling you to 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. On 4WD
vehicles, the transfer case allows you to select 4WD when necessary.
Information on transfer case operation and shifting procedures can be
found in this 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.
Normal characteristics
On some 4WD models, the initial shift from two-wheel drive to 4x4 while
the vehicle is moving can cause some momentary clunk and ratcheting
sounds. This is the front drivetrain coming up to speed and the
automatic locking hubs engaging and is not cause for concern.
Sand
When driving over sand, try to keep all four wheels on the most solid
area of the trail. Avoid reducing the tire pressures but shift to a lower
gear and drive steadily through the terrain. Apply the accelerator slowly
and avoid spinning the wheels.
Avoid excessive speed because vehicle momentum can work against you
and cause the vehicle to become stuck to the point that assistance may
be required from another vehicle. Remember, you may be able to back
out the way you came if you proceed with caution.
Driving
316
2011 F-150 (f12)
Owners Guide, 2nd Printing
USA (fus)