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TRANSFER CASES

CHAPTER 8Page 175

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OBJECTIVES Describe operation and

controls of transfer cases Compare components of Full

time and part time 4WD Discuss purpose of interaxle

differential and variations Discuss purpose and operation

of viscous coupling in 4WD systems

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Purpose of Transfer Cases

Transfers torque to both front and rear drive axle assemblies (Gear or Chain)

Normally connected to each axle by two drive shafts.

Provides Direct drive (high) or torque multiplication (Low)

Is a “auxiliary transmission”Page 187

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TYPICAL 4WD

SYSTEM 2 WHEEL

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CLASSIFICATIONS Three Types

– Part-Time: Has neutral, 2WD High, 4WD High and 4WD Low.

– Full Time: 4WD High and 4WD Low, 4WD High Lock and 4WD Low lock

– Part-Time/Full time: 2WD High, Full time 4WD High and part time 4WD Low. Page 187

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NEUTRAL Power flows

into Transfer case with no output.

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2 WHEEL HIGH

Power flow to rear wheels only.

No reduction

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4 WHEEL HIGH Power flows to

both front & rear axles.

No reduction

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4 WHEEL LOW Power flows to

both front & rear axles.

In reduction

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TRANSFER CASE DESIGNS

GEAR DRIVEN (pg. 191) CHAIN DRIVEN (pg. 192) PLANETARY GEAR DRIVES (Pg.

193) ELECTRONICALLY CONTROLLED

PLANETARY GEARSETS (Pg. 194)

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GEAR DRIVEN POWER FLOW

Page 191

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OPERATION Most transfer cases have two

levers. One for 2wd/4wd and one for range (Hi, Low)

Oil Pump “Shift on Fly” Uses magnetic

clutch to bring shafts, differentials axles to same speed. Electric motor makes actual shift when components are synchronized. (Page 192)

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OPERATION Some Transfer cases are two

speed i.e. allow a low range and a high range

Some Transfer cases are single speed i.e. only between 2WD and 4WD

Some are computer controlled

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CHAIN DRIVEN Today’s transfer cases are

made of aluminum, have chains and planetary gearsets to reduce weight and improve efficiency

Simple Planetary Gearsets are made up of three gears (Pg. 188)

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Chain Driven Power Flow

Planetary Gearset allows for different gear positions.

Power flow through most transfer cases equipped with planetarys is the same.

Shift mechanism moves planetary gearset along splined section of the input shaft. (Power flow Pg. 189-190)

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CENTER DIFFERENTIALS Prevents Driveline wind-up Used in Full Time 4WD and

AWD systems (Can disengage operation on some cars to lock all four wheel direct)

Can be external between transmission and axles (page 177 and 182)

Can be located inside transfer case (page 183 and 198)

Page 198

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VISCOUS COUPLINGS Drum filled with thick silicone

fluid Houses several thin steel discs Splits engine torque according

to the needs of each axle. Prevents wheel scuffing along

pavement. Used on 4WD and AWDPage 199