Four-Wheel-Drive
Why Four-Wheel-Drive?
Advantages
Improved traction and handling
Able to transmit torque to all four wheels
Disadvantages
Higher initial cost and weight
Requires special service and maintenance
Definitions
4X4 = Four wheeled vehicle, four wheels
can receive torque.
4X2 = Four wheeled vehicle, two wheels
can receive torque.
4WD Designs
RWD vehicles equipped with an added
transfer case, a front drive shaft, a front
differential, and front drive axles
4WD Designs
 FWD vehicles with an added transfer case, a rear drive
shaft, and a rear axle
4WD Definitions
Transfer case - An
auxiliary
transmission
mounted to the side
or rear of the main
transmission.
No differential
action is provided.
4WD Components
 Transfer case
Transmits power from the transmission to the front and rear
drive shafts
4WD Components
 Transfer case
Will typically be a gear or chain type
Types of Transfer Cases
Drive chain design
Has less weight to
improve fuel economy
Is usually used with
planetary gearsets
Is the most common
design
Types of Transfer Cases
Gear-type
Uses only gear-sets to transfer power
Stronger & more durable
4WD Definitions
Center
differential serves the
same purpose
as a transfer
case but allows
for differential
operation.
4WD Definitions
Limited-slip center
differential. Controls
differential operation
through a disc or
cone-type clutch.
Later designs are
computer operated.
4WD Components
Drive shafts
Connect to the front and rear differentials
4WD Components
 Universal joints or CV-joints
Are used to connect front axles to the wheel hubs
4WD Definitions
Part-time 4WD - Typically systems that
operate in two-wheel drive until the driver
commands four-wheel drive operation.
Torque is then divided (50/50 fixed)
between the front and rear drive-shaft.
They do not corner well on dry pavement.
Selection of two- or four-wheel-drive is done
by a shifter, electric switch and/or locking
hubs.
Part-Time 4WD Shift Controls
Electric switch or shift lever
Allows the driver to select which
axles receive power
Power can be directed to:
all four wheels
two wheels
none (neutral)
Some vehicles have a
low-speed range that can
be selected
Part-Time 4WD
Older designs may have different driveaxle gear ratios.
This will result in a push-pull type action if
the vehicle is driven on dry pavement.
This phenomenon is called drive-line windup.
Part-Time 4WD Shift Controls
Manual Shift lever
Some vehicles
have a
low-speed range
that can
be selected
Part-Time 4WD
Locking Hubs
Manual or automatic operation
Part-Time 4WD
Locking Hubs
manual operation
Part-Time 4WD
Locking Hubs
automatic operation - vacuum or electromagnetically operated.
Part-Time 4WD
 Locking Axles - final differential will be of an “open”
design. When one axle is disconnected from the wheel
the spider gears rotate freely, neither axle receives
torque.
 Used with an open differential
Part-Time 4WD
Locking Axles - Toyota’s Automatic
Disconnecting Differential (ADD)
Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Neutral
Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Two-wheel-drive—high
Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Four-wheel-drive—high
Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Four-wheel-drive—low
4WD Definitions
Full-time four-wheel drive (4wd) Constantly provides power to all four
wheels. These systems typically do NOT
have a provision for “low” operating
range.
Axles and differentials are constantly locked.
Usually equipped with a center “differential”
instead of a transfer-case.
All-wheel drive - see above
4WD Systems
Full-time systems
Cannot be selected
out of four-wheeldrive
May have an open
or limited-slip center
differential.
Full-time 4WD
Open Center Differential - If the wheels
on either final differential loose traction,
maximum torque is delivered to the axle
with the LEAST traction.
For this reason, limited-slip center
differentials are far more common today.
Full-time 4WD
Newer computer-controlled designs allow
for different operational modes.
Ford Automatic Four-wheel-drive (A4WD)
Navigator and Expedition
4WD AUTO - full power to the rear wheels.
The front wheels receive power as required.
4WD High - Full power to both wheels, no
center gear-reduction.
4WD Low - Low gear ratio in center
differential for (2:1 ratio) for slow, high
torque driving
4WD Definitions
On-demand four-wheel drive Automatically controlled 4WD. One axle is
driven until slip is detected or anticipated.
Computer controlled
uses wheel speed sensors and/or axle speed
sensors to detect slippage.
When slippage is detected, torque is sent to
the opposite axle. The amount sent depends
on the amount of slippage.
4WD Definitions
On-demand four-wheel drive
4WD Definitions
Viscous coupling in a center differential
4WD Definitions
Viscous coupling in a center differential
On-demand four-wheel
drive
Ford’s control-trac system
normal operation - 96% of torque is sent to
the rear wheels.
Upon slippage a computer energizes an
electro-magnetic clutch in the transfer case.
Up to 96% of torque can be sent to the front
wheels.
4WD Systems
Four-wheel drive systems without a center
differential.
Tire sizes must all be the same (this holds true for
any 4WD application)
Tread wear must be similar
Tire changes on an all-wheel drive must be done
in sets of four
Gear ratios should be the same in the front and
rear differentials.
Front Axle and Hub Service
Guidelines
 Most front axles are
serviced like those
in 2WD systems
 Follow the recommended
procedure for servicing
4WD hubs
 Some hubs are not
serviceable and must
be replaced
Effects of Vehicle
Modifications
Installing larger tires and lift kits
Alters vehicle handling by changing unsprung weight
Increases steering effort
Causes a rough ride
May cause vehicle to wander and vibrate
Possible driveshaft working angle issues
Possible Vehicle Speed sensor issues
May necessitate computer programming
Maintenance Tips
 Check the fluid level often
 Check for fluid contamination any time the
vehicle has been driven through deep water
 Use only the recommended lubricant
 Always follow
the proper
towing
procedures