Four-wheel drive vehicles provide improved traction over two-wheel drive vehicles by transmitting power to all four wheels. There are several types of 4WD systems including part-time 4WD, full-time 4WD, and on-demand 4WD. Key components include the transfer case, drive shafts, differentials, and locking hubs or axles. Proper maintenance such as checking fluid levels and using the correct lubricant is important for 4WD vehicles.

1. Four-Wheel-Drive

2. 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

3. 4X4 = Four wheeled vehicle, four wheels
can receive torque.
4X2 = Four wheeled vehicle, two wheels
can receive torque.
Definitions

4. 4WD Designs
RWD vehicles equipped with an added
transfer case, a front drive shaft, a front
differential, and front drive axles

5. 4WD Designs
 FWD vehicles with an added transfer case, a rear drive
shaft, and a rear axle

6. Transfer case - An
auxiliary
transmission
mounted to the side
or rear of the main
transmission.
No differential
action is provided.
4WD Definitions

7. 4WD Components
 Transfer case
Transmits power from the transmission to the front and rear
drive shafts

8. 4WD Components
 Transfer case
Will typically be a gear or chain type

9. 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

10. Types of Transfer Cases
Gear-type
Uses only gear-sets to transfer power
Stronger & more durable

11. Center
differential -
serves the
same purpose
as a transfer
case but allows
for differential
operation.
4WD Definitions

12. Limited-slip center
differential. Controls
differential operation
through a disc or
cone-type clutch.
Later designs are
computer operated.
4WD Definitions

13. 4WD Components
Drive shafts
Connect to the front and rear differentials

14. 4WD Components
 Universal joints or CV-joints
Are used to connect front axles to the wheel hubs

15. 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.
4WD Definitions

16. 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

17. Older designs may have different drive-
axle 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

18. Part-Time 4WD Shift Controls
Manual Shift lever
Some vehicles
have a
low-speed range
that can
be selected

19. Part-Time 4WD
Locking Hubs
Manual or automatic operation

20. Part-Time 4WD
Locking Hubs
manual operation

21. Part-Time 4WD
Locking Hubs
automatic operation - vacuum or electro-
magnetically operated.

22. 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

23. Part-Time 4WD
Locking Axles - Toyota’s Automatic
Disconnecting Differential (ADD)

24. Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Neutral

25. Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Two-wheel-drive—high

26. Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Four-wheel-drive—high

27. Transfer Case Modes of
Operation
 Ranges available with a part-time transfer case:
Four-wheel-drive—low

28. 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 Definitions

29. 4WD Systems
Full-time systems
Cannot be selected
out of four-wheel-
drive
May have an open
or limited-slip center
differential.

30. 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.

31. 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

32. 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

33. On-demand four-wheel drive
4WD Definitions

34. Viscous coupling in a center differential
4WD Definitions

35. Viscous coupling in a center differential
4WD Definitions

36. 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.

37. 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.

38. 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

39. 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

40. 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