FORD 2013 F-550 v.1 - Manual (Page 340)

Four-Wheel Drive System (If Equipped)
WARNING: Do not become overconfident in the ability of
four-wheel drive vehicles. Although a four-wheel drive vehicle
may accelerate better than two-wheel drive vehicle in low traction
situations, it will not stop any faster than two-wheel drive vehicles.
Always drive at a safe speed.
A vehicle equipped with four-wheel drive (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. Four-wheel drive vehicles allow you to select different drive
modes as necessary. You can find information on transfer case operation
and shifting procedures in the Transmission chapter. You can find
information on transfer case maintenance in the Maintenance chapter.
You should become thoroughly familiar with this information before you
operate your vehicle.
On some four-wheel drive models, the initial shift from two-wheel drive
to four-wheel drive 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.
Wheels and Tires 339
2013 F-250/350/450/550 (f23)
Owners Guide gf, 1st Printing
USA (fus)