41FourFour- and AllAllWheel Drive
Prepared by
Martin Restoule Algonquin CollegeChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-1
Objectives Identify the advantages of four- and allwheel drive. Name the major components of a conventional four-wheel drive system. Name the components of a transfer case. State the difference between a transfer, open, and limited slip differential.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-2
Objectives State the major purpose of locking/unlocking hubs. Name the five shift lever positions on a typical 4WD vehicle. Understand the difference between four- and all-wheel drive. Know the purpose of a viscous clutch in all-wheel drive.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-3
4WD Systems Have a separate transfer case. Give the driver a choice between twoand four-wheel drive. Can have manual or automatic locking hubs. May or may not use an interaxle differential or viscous clutch.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-4
4WD SystemsA transfer case
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-5
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.
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-6
AWD SystemsA viscous clutch assembly
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-7
A Typical 4WD Vehicle
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-8
4WD Operational Modes 2-high (2H) Vehicle operates like a normal two-wheel drive vehicle.
4-high (4H) The front axle is connected to the driveline.
4-low (4L) The transfer case provides a lower gear ratio.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-9
The Transfer Case The transfer case itself is constructed similarly to a standard transmission. It uses shift forks to select the operating mode, plus splines, gears, shims, bearings, and other components found in manual and automatic transmissions. The outer case of the unit is made of cast iron, magnesium, or aluminum.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-10
The Transfer Case The front and rear drivelines are connected to a interaxle differential inside the transfer case. The interaxle differential allows for different front and rear driveline shaft speeds. Driveline windup, developed as a result of different front and rear axle gear ratios, is dissipated by the interaxle differential. continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-11
The Transfer Case The power flow through transfer case in 2H, 4H, and 4L modes.
Chapter 41
Copyright 2007 by Nelson, a division of Thomson Canada Limited.
continued
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Axle Disconnects Rather than using locking wheel hubs some 4WD vehicles use a vacuum or electric motor axle disconnect system The motor moves a collar that connects or disconnects one front axle from the front differential. When the collar is disconnected, the front ring gear and pinion remain stationary, reducing wear.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-13
Axle DisconnectsA vacuum operated axle disconnect
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-14
Axle DisconnectsAn electric motor axle disconnect
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-15
4WD Design Variations Open centre differential Uses differential action to compensate for differences in speed between the axles.
Limited-slip centre differential Allows more torque to be delivered to the axle with the most traction.
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-16
Viscous Clutches or Couplings Typically used in AWD vehicles. 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.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-17
Viscous Clutches or CouplingsA typical viscous clutch assembly
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-18
Haldex Clutch Has three main components: Hydraulic pump. Wet multi-disc clutch. Electronically controlled valve. Uses hydraulic pressure to engage the clutches to engage the rear wheels.
continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-19
Haldex ClutchA Haldex clutch assembly
Chapter 41
Copyright 2007 by Nelson, a division of Thomson Canada Limited.
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Summary The heart of most conventional 4WD systems is the transfer case. The interaxle is placed in the transfer case to operate in the same fashion as the differentials at the axles. Many vehicles require that the front hubs be in a locked condition to operate as 4WD vehicles.continuedChapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-21
Summary Components of 4WD vehicles can be serviced in basically the same manner as the same components on a 2WD vehicle. All-wheel drive vehicles may use a viscous clutch, rather than a transfer case, to drive the axle. Some AWD vehicles have a third differential, called an interaxle differential, instead of a transfer case.Chapter 41 Copyright 2007 by Nelson, a division of Thomson Canada Limited. 41-22
