1. Four-Wheel-Drive

2. Why Four-Wheel-Drive? zAdvantages yImproved traction and handling yAble to transmit torque to all four wheels zDisadvantages yHigher initial cost and weight yRequires special service and maintenance

3. z4X4 = Four wheeled vehicle, four wheels can receive torque. z4X2 = Four wheeled vehicle, two wheels can receive torque. Definitions

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

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

6. zTransfer case - An auxiliary transmission mounted to the side or rear of the main transmission. yNo differential action is provided. 4WD Definitions

7. 4WD Components zTransfer case yTransmits power from the transmission to the front and rear drive shafts

8. 4WD Components zTransfer case yWill typically be a gear or chain type

9. Types of Transfer Cases zDrive chain design yHas less weight to improve fuel economy yIs usually used with planetary gearsets yIs the most common design

10. Types of Transfer Cases zGear-type yUses only gear-sets to transfer power yStronger & more durable

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

12. zLimited-slip center differential. Controls differential operation through a disc or cone-type clutch. yLater designs are computer operated. 4WD Definitions

13. 4WD Components zDrive shafts yConnect to the front and rear differentials

14. 4WD Components zUniversal joints or CV-joints yAre used to connect front axles to the wheel hubs

15. zPart-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. yThey do not corner well on dry pavement. ySelection 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 zElectric switch or shift lever yAllows the driver to select which axles receive power yPower can be directed to: xall four wheels xtwo wheels xnone (neutral) ySome vehicles have a low-speed range that can be selected

17. zOlder designs may have different drive- axle gear ratios. yThis will result in a push-pull type action if the vehicle is driven on dry pavement. yThis phenomenon is called drive-line windup. Part-Time 4WD

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

19. Part-Time 4WD zLocking Hubs yManual or automatic operation

20. Part-Time 4WD zLocking Hubs ymanual operation

21. Part-Time 4WD zLocking Hubs yautomatic operation - vacuum or electro- magnetically operated.

22. Part-Time 4WD zLocking 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. zUsed with an open differential

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

24. Transfer Case Modes of Operation zRanges available with a part-time transfer case: yNeutral

25. Transfer Case Modes of Operation zRanges available with a part-time transfer case: yTwo-wheel-drive—high

26. Transfer Case Modes of Operation zRanges available with a part-time transfer case: yFour-wheel-drive—high

27. Transfer Case Modes of Operation zRanges available with a part-time transfer case: yFour-wheel-drive—low

28. zFull-time four-wheel drive (4wd) - Constantly provides power to all four wheels. These systems typically do NOT have a provision for “low” operating range. yAxles and differentials are constantly locked. yUsually equipped with a center “differential” instead of a transfer-case. zAll-wheel drive - see above 4WD Definitions

29. 4WD Systems zFull-time systems yCannot be selected out of four-wheel- drive yMay have an open or limited-slip center differential.

30. Full-time 4WD zOpen Center Differential - If the wheels on either final differential loose traction, maximum torque is delivered to the axle with the LEAST traction. zFor this reason, limited-slip center differentials are far more common today.

31. Full-time 4WD zNewer computer-controlled designs allow for different operational modes. zFord Automatic Four-wheel-drive (A4WD) yNavigator and Expedition y4WD AUTO - full power to the rear wheels. The front wheels receive power as required. y4WD High - Full power to both wheels, no center gear-reduction. y4WD Low - Low gear ratio in center differential for (2:1 ratio) for slow, high torque driving

32. zOn-demand four-wheel drive - Automatically controlled 4WD. One axle is driven until slip is detected or anticipated. yComputer controlled yuses wheel speed sensors and/or axle speed sensors to detect slippage. yWhen slippage is detected, torque is sent to the opposite axle. The amount sent depends on the amount of slippage. 4WD Definitions

33. zOn-demand four-wheel drive 4WD Definitions

34. zViscous coupling in a center differential 4WD Definitions

35. zViscous coupling in a center differential 4WD Definitions

36. On-demand four-wheel drive zFord’s control-trac system ynormal operation - 96% of torque is sent to the rear wheels. yUpon 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 zFour-wheel drive systems without a center differential. xTire sizes must all be the same (this holds true for any 4WD application) xTread wear must be similar xTire changes on an all-wheel drive must be done in sets of four xGear ratios should be the same in the front and rear differentials.

38. Front Axle and Hub Service Guidelines zMost front axles are serviced like those in 2WD systems zFollow the recommended procedure for servicing 4WD hubs zSome hubs are not serviceable and must be replaced

39. Effects of Vehicle Modifications zInstalling larger tires and lift kits yAlters vehicle handling by changing unsprung weight yIncreases steering effort yCauses a rough ride yMay cause vehicle to wander and vibrate yPossible driveshaft working angle issues yPossible Vehicle Speed sensor issues xMay necessitate computer programming

40. Maintenance Tips zCheck the fluid level often zCheck for fluid contamination any time the vehicle has been driven through deep water zUse only the recommended lubricant zAlways follow the proper towing procedures