CH43 four and all wheel drive STUDENT VERSION rev

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Chapter 43

Four- and All-Wheel Drive

Four-Wheel-Drive (4WD) and

All-Wheel-Drive (AWD)

• Increased popularity on modern SUVs and AWD cars

• Increase traction even with narrower tires

• Both add initial cost and weight

• 4WD vehicles have special maintenance requirements not found on 2WD vehicles

Typical 4WD Vehicle

Part-Time 4WD

• Basically RWD with a two-speed transfer case

• Axles connect/disconnect via locking hubs

• The front and rear axles split engine torque

• Should not be operated on dry surfaces

Full-Time 4WD

• All four wheels have power all of the time

• Use a center or interaxle differential or

viscous clutch that allows for different

speeds between axles

• Has a high and low speed range

• No 2WD option

• Does not use locking hubs

AWD Systems

• Do not have a separate transfer case

• Use a front-wheel drive transaxle equipped with a viscous clutch, center differential, or transfer (Haldex) clutch

• Operate continuously in four-wheel drive

Full-Time AWD

• Similar to full time 4WD systems

• Not designed for heavy off-road use

• Engine torque is divided according to

operating conditions

• Can be computer controlled

• May allow the driver to lock the front and rear driveshafts

Transfer Case

• Delivers power

to front and rear

assemblies

Interaxle Differentials

• Driveline windup can cause handling

problems

• The front and rear must rotate at different

speeds to eliminate windup

• The interaxle differential allows for different front and rear speeds similar to a differential allowing separate left and right speeds

Viscous Clutches or Couplings

• Are self contained and not serviceable

• Can be installed inside or outside the

transaxle housing

• Consist of two sets of steel plates in a drum

of thick fluid

• Split the engine torque according to the

needs of each axle

Axle Disconnects

• Some vehicles use standard front hubs and a vacuum operated axle disconnect

• May disconnect using an electric motor to allow smooth transitions

Four-Wheel-Drive Passenger Cars

• Most developed from FWD models

• No separate transfer case

• Not designed for off-road driving

Limited-Slip and Open Differentials

• Used on many 4WD passenger cars to help

prevent wheel scrubbing and windup.

• A LSD is used to join the right and left axle

shafts to the differential case.

• Allows traction even if one or two wheels have none.

• Systems with open differentials have a mode selector for lock up if needed.

Diagnosing 4WD and

AWD Systems

• Verify complaint.

• Inspect for mechanical and electrical faults:

– Halfshafts - Locking hubs

– Driveshafts and U-joints - Shift lever/linkage – Mode switch - Shift motor

– Electromagnetic clutch vacuum lines

– Matching tire sizes - Transfer case

– Wiring and connectors fuses