Modes of Operation of All Wheel Drive System

All Wheel Drive

  • Most all wheel drive vehicles do not offer a two-wheel drive or four-wheel drive selection.

  • All wheel drive (AWD) systems enhance vehicle performance in poor traction conditions such as snowy roads, dry pavement, or emergency maneuvers.

  • Torque is automatically transferred to the axle that has the most traction.

  • AWD systems can be based on either front-wheel drive or rear-wheel drive platforms.

All Wheel Drive Systems

Center Differential

  • Splits torque between the front and rear axles

  • Prevents driveline windup, reducing tire wear and component damage

  • Is located in the transfer case between the front and rear driveshafts

  • Often includes a viscous clutch to maintain traction when one axle slips

Viscous Clutch

  • Is a self-contained unit located inside the front transfer case housing

  • Contains thin steel clutch plates housed in a sealed drum

  • Has driving plates splined to the clutch hub and driven plates splined to the drum

  • Uses silicone fluid to transfer torque between plates

  • Splits engine torque based on the traction needs of the differential

Haldex Clutch

  • Acts as a coupler between the front and rear differentials

  • Is mounted in front of the rear differential

  • Receives torque from the front differential

  • Consists of three main parts:

    • Hydraulic pump

    • Wet multi-disc clutch

    • Electronically controlled module (with solenoid, pressure, and temperature sensors)

  • Engages the rear differential when there is a predetermined speed difference between the front and rear wheels

Modes of Operation

Electronically Controlled Differential - Normal

  • Uses a Power Transfer Unit (PTU) driven by the transaxle differential carrier

  • The PTU transfers torque to the driveshaft connected to the rear differential

  • An electronically controlled clutch connects the driveshaft to the rear differential

  • During normal operation, the system functions as a front-wheel drive vehicle

  • Power flows from the front differential to the front wheels

  • The PTU drives the rear driveshaft, but no torque is sent to the rear differential

Slip

  • The control module monitors the difference between front and rear wheel speeds

  • When a speed difference is detected, the electromagnetic clutch in the PTU engages

  • The clutch transfers torque from the rear driveshaft to the rear differential

  • Once wheel speeds equalize, the clutch pack releases, returning the system to normal operation

Torsen Center Differential - Normal

  • A mechanical, torque-sensing differential with no electronics, clutches, or viscous fluid

  • Uses a helical gearset to distribute torque

  • Located in the transfer case attached to the rear of the transmission

  • Transmits torque to both front and rear driveshafts

  • Under normal conditions, the vehicle operates as rear-wheel drive

  • Power splits automatically between the front and rear differentials

Slip

  • When rear wheels slip, internal helical gears react instantly

  • Torque is redirected to the axle with the most traction

  • Provides automatic torque distribution without electronics or fluid

  • Enhances traction and vehicle stability

Haldex Clutch - Normal

  • Consists of a hydraulic pump, wet multi-disc clutch, and electronic control module

  • Under normal conditions, vehicle operates in front-wheel drive

  • Power transfers from the front differential to the front wheels

  • The PTU drives the rear driveshaft, but no torque is transferred to the rear differential

Slip

  • When front wheels slip, the input shaft spins faster than the output shaft

  • The pump generates oil flow, activating the multi-disc clutch

  • Oil pressure engages the clutch pack, transferring torque to the rear wheels

  • Oil returns to the sump through a controllable valve that regulates pressure

  • In high slip conditions, the pump delivers higher pressure for stronger clutch engagement

Viscous Clutch - Normal

  • Sealed assembly containing two sets of steel plates immersed in silicone fluid

  • Under normal conditions, both sets of plates rotate at the same speed

  • Torque splits between front and rear wheels (typically 35% front / 65% rear)

  • Provides continuous torque transfer while allowing minor speed differences between axles

Slip

  • Speed difference between front and rear driveshafts creates friction between the steel plates

  • Friction heats the silicone fluid, causing it to expand

  • Expansion locks the plates together, transferring torque to the axle with less traction

  • When wheel speeds equalize, the fluid cools, allowing the driveshafts to rotate at different speeds again

  • Provides automatic torque redistribution during slip conditions