6 GEARS December 2008

What’s New in Four-Wheel Drive?

Over the past two years, when I’m not working on the ATRA HotLine,

I’ve been writing articles, and cre-ating seminar and EXPO presen-tations related to understanding and diagnosing today’s domestic four-wheel drive systems. Most of those programs have been cen-tered around understanding and diagnosing four-wheel drive sys-tems in today’s domestic trucks.

In this article, we’re going to do something a little bit differ-ent. We’re going to take a look at a section of the four-wheel drive market we rarely talk about: the AWD systems used in today’s domestic passenger vehicles.

BorgWarner is now offer-ing its award winning Interactive Torque Management (ITM 3e®) system as an option for drivers who value performance, fuel econ-omy and stability. BorgWarner works closely with manufacturers such as General Motors, Ford, Daimler-Chrysler, Volkswagon/Audi, Honda, Toyota, Nissan, Hyndai/Kia, BMW and Fiat, to name just a few.

In addition to providing the all-wheel drive components, BorgWarner supplies the electron-ics, software controls and vehicle integration support.

KEEP THOSE TRANNIES ROLLING

Figure 1: Mechanical components of the AWD system

Figure 2: AWD control module uses the CAN bus system to communicate with other modules.

EW EWEW DRIVE?RIVDRIVE?D ?R ?DRIVED by Pete Huscher

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8 GEARS December 2008

What’s New in Four-Wheel Drive?

The ITM 3e® system is just one of the latest interactive all-wheel drive sys-tems available as part of BorgWarner’s growing iTrac® torque management systems portfolio. BorgWarner has a full line of active torque management devices focused on the needs of the ever expanding all-wheel drive market, such as the new NexTrac® all-wheel drive system being launched in this year’s Hyundai Santa Fe, and the Cadillac CTS’s ITM® all-wheel drive system also being launched this year.

BorgWarner worked closely with Daimler-Chrysler’s engineers to bring an all-wheel drive system to Chrysler’s 2007 Pacifica. BorgWarner and Daimler-Chrysler are working together again to bring the ITM 3e® system to the 2008 Dodge Avenger.

A Closer Look at the ITM 3e® System

The ITM 3e® system uses sophis-ticated controls and algorithms to sense wheel slip, and can seamlessly convert to all-wheel drive within milliseconds. This enhances vehicle handling by opti-mizing vehicle traction to both the front and rear wheels when needed.

While driving at highway speeds, the ITM 3e® system reduces torque transfer to the rear wheels, which improves fuel economy when com-pared to a full-time all-wheel drive sys-tem. The ITM 3e® system is designed to work in conjunction with other on-

board systems such as the antilock brake system (ABS) and the electronic ride stability system, to provide improved traction and stability when needed.

Electronically Controlled All-Wheel Drive System

Unlike other all-wheel drive sys-tems that rely on pumps or a viscous fluid to transfer torque to the non-driving wheels, the ITM 3e® system requires no front-to-rear slip to activate the all-wheel drive system. This system transfers torque solely in response to accelerator pedal position and other vehicle system inputs.

This electronically controlled all-wheel drive system doesn’t incorporate traction control, nor does it aid in side-to-side traction. The antilock brake sys-tem (ABS) is responsible for side-to-side traction using brake intervention programming.

ITM 3e® ComponentsThe electronically controlled all-

wheel drive system includes (figure 1):• An all-wheel drive ECM

(AWD ECM)• A power transfer unit (PTU) or

transfer case• A two-piece driveshaft• An electronically controlled

coupling (ECC)• A rear differential assembly• Axle shafts

All-Wheel Drive ECMThe AWD ECM communicates

with the ABS control module and the PCM over the CAN C bus system. The AWD ECM uses the wheel speed sensor inputs from the ABS module, to monitor relative wheel speeds and determine the vehicle’s actual speed.

The AWD ECM uses this informa-tion along with the accelerator pedal position to determine how much torque to transfer to the rear wheels. The AWD ECM then sends an electrical signal to the ECC that’s proportional to the amount of torque required. This allows the torque transfer to the rear wheels to be fine-tuned to the amount of traction actually needed (figure 2).

Power Transfer Unit (PTU)

The power transfer unit (PTU) or transfer case is mounted to the right side of the transmission (figure 3). The right axle half-shaft passes through the power transfer unit and drives the right front wheel. The power transfer unit transfers torque from the driving wheels to the rear differential through the driveshaft and electronically con-trolled coupling (ECC).

Two-Piece DriveshaftThe two-piece driveshaft connects

the power transfer unit to the elec-tronically controlled coupler. A center bearing supports the driveshaft, and a

Figure 3: ITM 3e power transfer unit (PTU)

The power transfer

unit (PTU) or transfer

case is mounted to

the right side of the

transmission. The

right axle half-shaft

passes through the

power transfer unit

and drives the right

front wheel.

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10 GEARS December 2008

What’s New in Four-Wheel Drive?

center plunge-type joint compensates for movement between the engine/power transfer unit and the electroni-cally controlled coupling.

The Electronically Controlled Coupling (ECC)

The electronically controlled cou-pling, or ECC, is attached to the rear differential. The purpose of the ECC is to transmit torque to the rear wheels through a two-stage clutch system. The low torque, electromagnet-powered clutch is controlled by the AWD ECM.

A cam-and-ball mechanism ampli-fies the force of the low torque clutch, applying the wet multi-disc main clutch that transmits torque to the rear wheels. The wet clutch is similar to clutch packs in automatic transmissions (figure 4).

Rear Differential Assembly

The rear differential assembly of the ITM 3e® system consists of the electronically controlled coupling and the rear differential. The rear differ-ential uses the torque provided by the electronically controlled coupling to drive the rear wheels. The electronical-ly controlled coupling isn’t serviceable and must be replaced as an assembly (figure 5).

Axle ShaftsThe axle shafts are inside the rear

differential assembly. The purpose of the axle shaft is to transfer torque from the differential carrier to the wheels.

How Does the ITM 3e® System Work?

The electrically controlled all-wheel drive system includes these elec-trical components (figure 6):

• The AWD electronic control module (AWD ECM)

• The antilock brake system (ABS) and wheel speed sen-sors

• The electronically controlled coupling (ECC)

The system has four operating modes:

• Open loop• Close loop• Wheel speed dependent• Vehicle dynamics enhance-

ment

The AWD ECM also interfaces with the ABS/stability ride control and traction control systems. This interface allows the ABS to influence rear wheel torque transfer through the ECC and helps the driver maintain control of the vehicle.

Open Loop OperationIn high power demand situations,

the all-wheel drive system immedi-ately starts engaging the electronically controlled coupling, transferring a high percentage of torque to the rear wheels. This prevents front wheel slip, as power is transmitted to all four wheels. This mode of operation is called open-loop operation because the AWD ECM doesn’t use inputs from the ABS sys-tem to control the torque transfer to the rear wheels.

Closed Loop OperationIn closed-loop operation, the AWD

ECM uses inputs from the ABS con-trol module to determine the proper torque transfer needed. Power to the rear wheels is also modulated under these conditions:

• Front wheels slipping on ice while backing up will transfer torque to the rear wheels.

• Loss of traction at freeway speeds, as in hydroplaning, will transfer torque to the rear wheels.

Both open-loop and closed-loop modes are always active, with the closed-loop mode layered on top of the

Figure 4: ECC (electronically controlled coupling)

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GEARS December 2008 11

open-loop mode to increase torque to the rear wheels when needed, to main-tain traction in extreme situations.

Wheel Speed Dependent Operation

Wheel speed dependent mode is independent of the open-loop and closed-loop modes, using the difference in wheel speed to determine when the vehicle is in a tight turn. This condition is indicated by a large difference in the side-to-side wheel speeds. This causes the AWD ECM to reduce torque to the rear wheels, which prevents binding the driveline.

Vehicle Dynamics Enhancement

Additional AWD ECM calibrations control the torque being sent to the rear wheels for improved handling at speeds between 25 and 65 mph. At these speeds, the system increases torque to the rear wheels during cornering. Above 70 mph and under normal driv-ing conditions, the AWD ECM strategy provides minimal torque transfer to the

rear wheels to increase fuel economy.Now you know what’s new in

four-wheel drive systems. With a bet-ter understanding of how the new ITM 3e® AWD system operates, you should have no problem keeping those trannies rolling.

I’d like to thank Carol Dupke and Erica Nielson from BorgWarner-Morse Tec for their contribution to this arti-cle.

Figure 6: Electronic components of the ITM 3e AWD system

The rear differential assembly of the ITM 3e® system consists of the electronically

controlled coupling and the rear differential.

Figure 5: Rear differential assembly

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