13.6.Four-Wheel Drive and All-Wheel Drive

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startAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

OBJECTIVES:After studying Chapter 99, the reader should be able to:

Prepare for ASE Manual Drive Train and Axles (A3) certification test content area F (Four-Wheel Drive Component Diagnosis and Repair). Explain the difference between a mode shift and a range shift. Describe the purpose and function of the center differential.Continued 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman

OBJECTIVES:After studying Chapter 98, the reader should be able to:

Explain the purpose and function of a viscous coupling. Describe the difference between fourwheeldrive and all-wheel drive. Explain how the front axle is disconnected when two-wheel drive is selected.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman

2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

KEY TERMS:annulus gear bevel gear differential electronically controlled clutch assembly input member interaxle differential (center differential) internal gear

locking hubs low rangemode shift

output memberContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

KEY TERMS:part-time four-wheel drive planet carrier planet Pinions range shift reaction member ring gear

sun gearviscous coupling

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman

2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

FOUR WHEEL DRIVE SYSTEMSTwo-wheel drive vehicles use engine torque to turn either the front or the rear wheels. A differential is required to allow the drive wheels to travel different distances and speeds while cornering or driving over bumps or dips in the road. A four-wheel-drive vehicle requires two differentialsone for the front wheels and one for the rear wheels. NOTE: The term 4 x 4 means a four-wheeled vehicle that has enginetorque applied to all four wheels (four-wheel drive). A 4 x 2 is a fourwheeled vehicle that has torque applied to two wheels (two-wheel drive).

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Four-wheel-drive vehicles require more than just two differentials. The front and the rear wheels of a four-wheel-drive vehicle also travel different distances and speeds whenever cornering or running over dips or rises in the road.There are three different methods used to allow for front-to-rear driveline speed variation.

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Method 1Locking Hubs Engine torque from the transmission is applied directly to the rear differential through the transfer case.The transfer case permits the driver to select a low-speed, highpower gear ratio inside the transfer case while in four-wheel drive.Figure 991 Many light trucks and sport utility vehicles use a transfer case to provide engine torque to all four wheels and to allow a gear reduction for maximum power to get through mud or snow. (Courtesy of Dana Corporation, Perfect Circle Products)

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

CAUTION: Check the owners manual or service manual for therecommended procedure to follow when changing from one position to another in the transfer case. Some vehicles require that the vehicle be stopped before selecting between two- and four-wheel drive and between high and low range.

These positions and their meanings include: 4H four-wheel drive with no gear reduction in the transfer case. 4L four-wheel drive with gear reduction. Use of this position is usually restricted to low speeds on slippery surfaces. 2H two-wheel drive (rear wheels only) in high range, meaning no gear reduction in the transfer case.

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

The transfer case also applies power to the front differential. Power is then applied to the front wheels through the drive axles to the locking hubs. In normal 4H driving on hard surfaces, the front hubs must be in the unlocked position. The front hubs are locked whenever driving on loose road surfaces to absorb and allow for tire slippage due to the different tire speeds front to back.This type of four-wheel-drive system is called part-time fourwheel drive because it can only be driven in four-wheel drive on slippery surfaces. See Figure 992.ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Figure 992 Cutaway of a manually-operated locking hub.

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Method 2Autolocking Hubs Another method of locking the hubs on a part-time four-wheel-drive system is with a clutch arrangement built into the hub assembly. Whenever driving on smooth, hard road surfaces, the hubs free wheel and allow the front wheels to rotate at different speeds from the rear wheels.When the speed difference between the wheels and the front drive axle is great, the hubs will automatically lock and allow engine torque to be applied to the front wheels. Automatic-locking hubs are unlocked by disengaging four-wheel drive at the transfer case and driving in reverse for several feet. See Figure 993.ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Figure 993 Manual locking hubs require that the hubs be rotated to the locked position by hand to allow torque to be applied to the front wheels. Automatic locking hubs enable the driver to shift into four-wheel drive from inside the vehicle.

ContinuedAutomotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

CAUTION: Failure to unlock the front wheel hubs while driving on ahard road surface can cause serious driveline vibrations and damage to driveshafts, U-joints, and bearings as well as to the transfer case, transmission, and even the engine.

NOTE: One of the disadvantages of automatic-locking hubs is that theydo not transfer torque to the front wheels when the vehicle is in reverse.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman

2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

How To Tow a Four-Wheel-Drive Vehicle Without Doing HarmIf any of the drive wheels are on the ground, the wheels are turning the axles. Depending on the exact type of four-wheel-drive vehicle being towed, this rotation of the wheels can cause severe wear; therefore most experts suggest the following options:

Placing the vehicle on a flatbed or trailer. This keeps all four wheels off the ground and is the safest method for transporting a four-wheel-drive (or all-wheel- drive) vehicle without doing any harm. Hoisting the front wheels off the ground and placing the rear wheels on a dolly. This procedure also keeps all wheels off the ground and therefore prevents any damage being done to the power train as a result of towing.

See Figure 994.Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D. Halderman 2009 Pearson Education, Inc. 2008 Pearson Education, Inc. Pearson Prentice Hall - - Upper Saddle River, NJ 07458 Pearson Prentice Hall Upper Saddle River, NJ 07458

Figure 994 If a four-wheel-drive vehicle must be towed, it should be either on (a) a flatbed truck or (b) a dolly.

(a)

(b)Automotive Technology: