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© 2017 National Automotive Parts Association Customers like the safety and convenience that All- Wheel Drive (AWD) provides. This segment of the market provides increased revenue opportunities for the repair facility that has the trained technicians and proper tools to perform service on the unique components of these systems. In This Issue u Center Differentials u Electric Differentials u Audi Quattro u Audi Torque Vectoring u Subaru Symmetrical AWD u BMW xDrive u BMW Dynamic Performance Control u Chrysler LX AWD u GM AWD Systems u GM Versatrak u New 2017 NAPA Autotech Class u Tech Tips u Review Questions All-Wheel Drive Technology JANUARY 2017 Basically, what differentiates AWD from 4WD is that AWD is activated without driver intervention. All-wheel drive (AWD) has to be able to control power application to each differential during both low traction conditions and steering maneuvers. Most AWD systems are “on-demand” and require seamless transitions from two-wheel drive (2WD) to AWD. Center Differentials The heart of the AWD system is the center differential that is located in the transfer case or transmission. The center differential allows for speed differences between the front and rear drive shafts to eliminate binding during cornering. If a center differential is not used, the AWD system will either use electrical or viscous type clutches at the front and/or rear differential(s). Center differentials typically use a viscous coupler made of steel clutch plates with alternating radial and axial grooves cut into them. The plates are placed in a sealed container of silicone fluid. The externally splined plates are splined to the drum and the internally splined plates fit over the output shaft. A difference in the speeds between the front and rear drivelines creates heat as the clutch plates sheer. Torque progression begins when the expansion of the silicone reaches a certain magnitude, locking the plates together. When the driveline speeds equalize, the silicone fluid contracts to allow the plates to slip. Clutch activated center differentials can use a Gerotor pump to apply the discs. The pump operates based on the difference in speed between the front and rear drive shafts. If either shaft is rotating faster than the other, the difference in speed of the gears creates a pumping action. The clutch progressively engages until the front and rear drive shaft speeds match each other.

All-Wheel Drive Technology January 2017 · All-Wheel Drive Technology January 2017 ... The DCCD AWD system has three automatic modes: ... modulating the electromagnetic clutch

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Page 1: All-Wheel Drive Technology January 2017 · All-Wheel Drive Technology January 2017 ... The DCCD AWD system has three automatic modes: ... modulating the electromagnetic clutch

© 2017 National Automotive Parts Association

Customers like the safety and convenience that All-

Wheel Drive (AWD) provides. This segment of the market provides increased revenue opportunities for the repair facility that has the trained

technicians and proper tools to perform service on the

unique components of these systems.

In This Issueu �Center�Differentialsu �Electric�Differentialsu Audi Quattrou Audi Torque Vectoringu Subaru Symmetrical

AWDu BMW xDriveu BMW Dynamic

Performance Controlu Chrysler LX AWDu GM AWD Systemsu GM Versatraku New 2017 NAPA

Autotech Classu Tech Tipsu Review Questions

All-Wheel Drive Technology J a n u a r y 2 0 1 7

Basically, what differentiates AWD from 4WD is that AWD is activated without driver intervention. All-wheel drive (AWD) has to be able to control power application to each differential during both low traction conditions and steering maneuvers. Most AWD systems are “on-demand” and require seamless transitions from two-wheel drive (2WD) to AWD.

Center�DifferentialsThe heart of the AWD system is the center differential that is located in the transfer case or transmission. The center differential allows for speed differences between the front and

rear drive shafts to eliminate binding during cornering. If a center differential is not used, the AWD system will either use electrical or viscous type clutches at the front and/or rear differential(s).

Center differentials typically use a viscous coupler made of steel clutch plates with alternating radial and axial grooves cut into them. The plates are placed in a sealed container of silicone fluid. The externally splined plates are splined to the drum and the internally splined plates fit over the output shaft. A difference in the speeds between the front and rear drivelines creates heat as the clutch

plates sheer. Torque progression begins when the expansion of the silicone reaches a certain magnitude, locking the plates together. When the driveline speeds equalize, the silicone fluid contracts to allow the plates to slip.

Clutch activated center differentials can use a Gerotor pump to apply the discs. The pump operates based on the difference in speed between the front and rear drive shafts. If either shaft is rotating faster than the other, the difference in speed of the gears creates a pumping action. The clutch progressively engages until the front and rear drive shaft speeds match each other.

Page 2: All-Wheel Drive Technology January 2017 · All-Wheel Drive Technology January 2017 ... The DCCD AWD system has three automatic modes: ... modulating the electromagnetic clutch

The Torsen center differential provides a 2:1 torque distribution between the front and rear drive shafts, enabling the tires with good traction to receive double the torque. The Torsen center differential works on the principle of the irreversibility of a worm gear. During a turning maneuver, the worm gears are capable of turning the pinion gears, but when one axle spins faster than the case, it tries to reverse the operation and turn the worm gears. This locks the center differential and transfers torque to the axle that is not spinning.

The electrical locking center differential is controlled by a module that applies and releases the clutch pack to control torque distribution to the drivelines. Typically, the system will rely on wheel speed sensors (WSSs) to determine if activation of the clutch pack is required.

A variation of the electronically controlled clutch pack is the Haldex clutch, which uses a centrifugal electro-hydraulic actuator to apply the clutch pack. The pump is activated to create a hydraulic pressure that is applied to the multi-disc clutches. The engagement can be pulse width modulated (PWM) to engage the driveline at various rates.

Electric�DifferentialsElectrically controlled differentials use an electrically activated clutch pack to lock the drive shaft to the differential pinion gear. Some systems use two-stage engagement, while others use PWM to provide infinite slip rates.

The electric eAxle differential (developed by GKN Automotive) is used to provide power to the rear wheels of a normally front wheel drive (FWD) vehicle. The eAxle uses an electromechanically actuated dog clutch to connect and disconnect the rear axle.

Audi QuattroThe Torsen central differential is utilized on Quattro packages. The system is mainly rear wheel drive (RWD) with FWD assist. The T3 ((C) type) system uses a planetary carrier to provide a 40/60 torque bias between the front and the rear axles. When one drive shaft starts to spin faster than the other, the pinion gears start to rotate. As one pinion gear is forced up and the next one is forced down, they push against the friction plates and lock them together to send torque to the axle with traction.

The Allroad Quattro equipped with Ultra Technology uses a multi-plate clutch located at the rear of the transmission to couple and decouple the transmission and the rear driveshaft. A worm gear rotates a ring gear to move the balls up and down the ramp and apply pressure to the multi-plate disc.

Audi Torque VectoringThe Audi Sport Differential is essentially an electronically locking differential. This system works in conjunction with the dynamic stability control system to aid in cornering. As the vehicle turns into a corner, a small amount of braking is applied to the two inside wheels, and the differential transfers more torque to the outside rear wheel.

Subaru Symmetrical AWDThe WRX STI uses a limited-slip, planetary gear-type center differential, coupled with an electronically controlled center limited-slip differential. This provides a 41:59 torque split.

The Driver Controlled Center Differential (DCCD) allows the driver to adjust center differential locking. The DCCD utilizes both the electromagnetic multi-plate transfer clutch and mechanical locking of the limited slip differential to distribute torque to the front and rear wheels. The DCCD AWD system has three automatic modes:

n Auto mode: provides optimal performance for all conditions.

n Auto (-) mode: Active Sport setting shifts the torque bias to the rear differential and opens the center limited-slip differential to improve steering feel for performance driving.

The Allroad Quattro uses a multi-plate clutch located at the rear of the transmission.

The�Torsen�style�center�differential�uses worm gears.

Page 3: All-Wheel Drive Technology January 2017 · All-Wheel Drive Technology January 2017 ... The DCCD AWD system has three automatic modes: ... modulating the electromagnetic clutch

© 2015 National Automotive Parts Association

n Auto (+) mode: Used to tighten the limited-slip center differential for improved traction on dirt, gravel, or snow by transferring more torque to the front wheels.

Manual mode offers six selectable settings, allowing the driver to vary the front-to-rear torque distribution to their favorite handling characteristics.

BMW xDrive

The xDrive system is a permanent, AWD system that distributes a 40:60 ratio torque split between the front and rear axles, under normal driving conditions. During parking and high speed driving, all torque is sent to the rear wheels. The ratio can be changed variably as road surface or driving conditions change. xDrive can direct up to 100% of drive forces to the axle with the greatest traction.

xDrive uses information from Dynamic Stability Control system sensors to monitor road conditions. The center differential uses a servo controlled wet clutch. When needed, the servo motor pushes on a pressure plate to lock the wet clutch. If an understeer condition is detected, the clutch is disengaged so more torque is transferred to the rear wheels. If oversteering is detected, the clutch locks fully to transfer more torque to the front wheels.

BMW Dynamic Performance ControlMost vehicle stability programs use the anti-lock brake system to stop the spinning wheel from rotating. The Dynamic Performance Control (DPC) system sends additional torque to the wheels that have traction. This means that nothing is slowed down. The rear differential is fitted with two sets of planetary gears and two electronically controlled multi-plate clutches (one set for each axle.) In addition, the clutch packs are controlled by individual motors. The clutches are individually activated to supply more torque to the wheel with the most traction.

Chrysler LX AWDAWD Chrysler LX vehicles use a Borg Warner 44-40 transfer case and a front axle actuator. The Drivetrain Control Module controls system operation without driver intervention by modulating the electromagnetic clutch assemblies to vary the amount of torque applied to the front axle. Wheel spin is monitored by individual wheel speed sensors (WSSs). In addition, the system utilizes bus network information from the rain sensor, ambient temperature sensor, and the longitudinal and lateral acceleration sensors.

GM AWD SystemThe AWD system on 2007 and newer GM vehicles use a fulltime

Getrag transfer case and a Clutch Control Module (CCM). Although the transfer case provides power to the rear driveline at all times, the rear differential is disengaged until slippage warrants engagement.

When the front WSS inputs vary, due to the tires spinning, the CCM activates the differential mounted solenoid. When energized, the electromagnetic clutch is applied and transfers torque to the rear differential. The solenoid also has an internal temperature sensor that is used to calculate the temperature of the clutch. A fault with the temperature sensor may prevent activation of the electromagnetic clutch.

GM VersatrakThe GM Versatrak system is normally FWD, but can bias the rear axle torque up to 44% as needed. This system uses a power take off (PTO) unit without a center differential.

The rear differential uses two Gerotor pumps that actuate two sets of wet clutches. If there is a difference in speed between an axle and the case, the pump gears will rotate at different speeds and pull fluid from the sump, pressurize it, and send it to the piston to lock the clutch pack. The AWD actuator is pulse width modulated (PWM) controlled by the PCM to regulate the flow of fluid to the Gerotor pumps.

New 2017 NAPA Autotech ClassNew for 2017 is the NAPA Autotech class on Powertrain Diagnostics – All Wheel Drive (Course No: 6508). This class focuses on diagnosing and servicing AWD systems. There are many classes to choose from. Enroll today by visiting one of the sites:n www.napaautotech.com n www.napatraining.com n www.napaautocare.com

© 2017 National Automotive Parts Association

The BMW xDrive system.

CCM�mounted�at�the�rear�differential.

Page 4: All-Wheel Drive Technology January 2017 · All-Wheel Drive Technology January 2017 ... The DCCD AWD system has three automatic modes: ... modulating the electromagnetic clutch

Tech Tipsu One of the first items to check when diagnosing an issue

with the AWD system is tire size. Confirm that all four tires are the same brand, size, and circumference. Even though the tire sizes match, this does not mean they all have the same rolling circumference. Mismatched tire sizes will cause rapid wear of center differentials, viscous couplings, U-joints, and axle differentials.

u The viscous fluid can degrade due to excessive heat. Degraded fluid causes torque binding as the vehicle warms up. Degradation of the fluid can be caused by aggressive driving, or different tire diameters. In addition, improper towing can cause viscous coupling failure. AWD vehicles must be transported on a flatbed, towing with a dolly can cause catastrophic damage.

u The viscous coupler is serviced as an assembly. The fluid cannot be refilled.

u Torque bias ratio (TBR) is a term used to describe the “locking effect” of the differential. It indicates how much more torque is sent to the high traction wheel (or axle) than is sent to the spinning wheel. For example, when a differential has a TBR of 3:1, 3 times the torque the spinning wheel can maintain (75% of the total torque) will go to the higher traction wheel. If a wheel is spinning and can support 100 lb-ft (135.6 N·m), the other wheel will get 300 lb-ft (406.7 N·m).

u Electrically controlled differentials can be used in AWD systems that do not utilize a center differential. However, many AWD systems that use the center differential will also incorporate the electrical differentials to improve performance.

u A common issue on the Subaru Symmetrical AWD center differential is bearing noise. When replacing the bearings, be sure to note the location of all washers.

u The BMW xDrive system uses a classification resistor that is attached to the motor. The resistor is used to compensate for mechanical tolerances in the transfer case. The resistance value is learned by the TCM so it can make corrections to the shift strategies based on the inherent mechanical differences of the transfer case. If the TCM is replaced, there is a scan tool function to allow the new TCM to learn the value of the resistor.

u The DPC system transaxle fluid requires replacement at specified service intervals. The transfer case holds very little fluid and can break down quickly. The TCM may set code P54C6 for worn fluid.

u If a vehicle with the DPC has been driven with a non-engaging drive system, the transfer box will require calibration after the repairs are completed. The calibration procedure will require the use of a scan tool.

u The front axle actuator on the Chrysler LX AWD is serviceable separate of the differential, but the actuator itself is serviced as a complete assembly. A common issue is the actuator rod sticking and not allowing the axles to engage or disengage.

u The CCM is also referred to as the rear differential control module (RDCM).

u The CCM connector has a two digit code stamped on it. If the module is replaced, this code must be entered into the new module.

u When diagnosing the Versatrak system, normal AWD solenoid duty cycle is 18% when no slip is occurring.

review Questions 1. Technician A says the center

differential is used to prevent drive shaft binding during corners. Technician B says, the viscous coupler fluid must be replaced every 60,000 miles. Who is correct?

a. Technician Ab. Technician Bc. Both Technician A and Technician Bd. Neither Technician A nor

Technician B

2. Technician A says the Torsen center differential operates on the principle of the irreversibility of a worm gear. Technician B says during a turning maneuver, the worm gears of the Torsen center differential are capable of turning the pinion gears. Who is correct?

a. Technician Ab. Technician Bc. Both Technician A and Technician Bd. Neither Technician A nor

Technician B

3. Technician A says AWD vehicles should be towed on a dolly. Technician B says to confirm that all tires of an AWD vehicle are the same rolling circumference. Who is correct?

a. Technician Ab. Technician Bc. Both Technician A and Technician Bd. Neither Technician A nor

Technician B

Answers: 1. a, 2. c, 3. b.

© 2017 National Automotive Parts Association