Overview - X-Drive BMW X3 E83

xDrive is a new four-wheel-drive system that delivers continuously variable input torques to the front and rear axles. xDrive comprises Dynamic Stability Control (DSC) and an electronically controlled multi-plate clutch in the transfer case. The DSC triggers the electronically controlled multi-plate clutch to supply continuously variable and power-oriented input torques to the front and rear axles. The rear axle is always powered. All of the input torque is applied to the rear axle when the multi-plate clutch is separated. xDrive communicates permanently with the DSC and receives from it, for instance, the following information

1. Whether the accelerator is pressed or released.
2. Whether the engine torque is increasing or decreasing.
3. Whether the car is driving straight ahead or in a curve.

xDrive continuously evaluates this information to detect whether the car can respond to the driver's requirements. xDrive intervenes to counter the threat of any tendency for wheelspin, oversteer or understeer. xDrive regulates input torque distribution between the two axles to meet driving demands. DSC only ever engages (by reducing engine power output and selective braking of individual wheels) should xDrive need assistance to keep the car on course. The input torque is delivered to the axle that has better traction when road conditions change, such as on snow, ice or a loose road surface.

TOPPING UP/CHANGING TRANSFER CASE OIL

Note. Use only the approved gear oil in the transfer box. See TRANSFER CASE APPROVED OIL . Failure to comply with this requirement will result in serious damage to the transfer box. Only change the oil when the transfer box is at normal operating temperature.

TRANSFER CASE APPROVED OIL

CHECKING/CORRECTING TRANSFER CASE OIL LEVEL

Note. Numbers in text refer to numbers in figures.

Undo oil filler plug (1). Check transfer case oil level. If necessary, pour in gear oil up to lower edge of opening for oil filler plug (1). (Scheme 27) Replace sealing ring.

Scheme 27

CHANGING TRANSFER CASE OIL

Note. Numbers in text refer to numbers in figures.

Place oil collecting apparatus underneath. Remove oil drain plug (2). Drain and dispose of gear oil. Replace sealing ring, screw in oil drain plug (2) and tighten down. Undo oil filler plug (1). Pour in gear oil up to lower edge of opening for oil filler plug (1). (Scheme 27) Replace sealing ring.

The innovative xDrive four-wheel drive is a system that controls and regulates the distribution of driving torque to the front and rear axles. The measured variables of DSC are used by xDrive but are also influenced by modified handling performance. The multi-disc clutch is the heart of the xDrive. By using the controlled multi-disc clutch, it is possible to resolve the conflict between traction and handling performance. This is achieved through the fact that torque distribution is not determined by a fixed gear ratio in the xDrive as was the case in the previous systems. Instead, the distribution of driving torque is dependent on the locking torque of the controlled multi-disc clutch in the transfer case and on the transferable torque to the front and rear axles. (Scheme 28)

Scheme 28

ATC 400/ATC 500 Transfer Case

The ATC 400 is installed in the E83 and the ATC 500 in the E53 MU. They differ in that the ATC 500 is splined to the front propeller shaft and the ATC 400 uses a four bolt flange. In addition, there is one more disc in the multi-disc clutch of the ATC 500 and the distance between the input shaft and the output shaft to the front axle is 19 mm greater than in the ATC 400. (Scheme 29)

Scheme 29

Power Flow

When the multi-disc clutch in the transfer case is disengaged, no driving torque is transmitted to the front axle. All of the driving torque is then distributed to the rear axle. This is because the input shaft (1) is splined providing a permanent connection to the rear axle propeller shaft output flange (2). The multi-disc clutch couples the rear axle propeller shaft output flange to the front propeller shaft output (3). The driving torque on the front axle is increased or decreased by regulating the locking pressure of the multi-disc clutch, providing a stepless coupling of the front axle to the drivetrain. This depends on driving situations and road conditions. When the multi-disc clutch is fully engaged, the front and rear axles turn at the same speed. Driving torque distribution (front/rear) is based on available traction at each axle. For example, when traction is identical on the front and rear axles and a driver accelerates from a stop in first gear at full throttle, the rear axle is capable of sustaining greater driving torque as the vehicle weight shifts from the front to the rear. Another example is when the front axle is on a high traction surface and the rear axle is on ice. In this case, virtually all of the available driving torque is transmitted to the front axle. Based on available traction, virtually no driving torque can be supported by the rear axle. Obviously, when more driving torque is transmitted to the front axle, driving torque on the rear axle is proportionally reduced due to lack of traction. (Scheme 30)

Scheme 30

Note. On a vehicle equipped with an automatic transmission, when driving onto brake analyzers, move the selector lever to the "N" position . On a vehicle equipped with a manual transmission, do not press the accelerator pedal once on the brake analyzer. This keeps the transfer case clutch open and the vehicle cannot be pulled off the analyzer.

Adjusting Levers

When the disc cam is rotated, it forces the adjusting levers apart. The ball ramps create a precision axial movement which compresses and increases pressure on the multi-disc clutch. This is completely variable up to a full lock. (Scheme 31)

Scheme 31

Servomotor with Motor Position Sensor

The servomotor with worm gear are powered to rotate the disc cam. The servomotor is a permanent magnet (1) DC motor which contains a Hall sensor (2) to detect the position and the adjusting speed of the motor shaft. This is proportional to the degree of multi-disc clutch engagement. (Scheme 32)

Scheme 32

Coding Resistor

Because of mechanical tolerances in production, the characteristic curve of the multi-disc clutch locking torque varies slightly. Once the actual locking torque has been measured on the clutch test bench, a resistor is attached to the servomotor; the resistor's value is a reference to the locking torque characteristic. Each time the engine is started, the transfer case control unit measures the resistance value once and the optimum program map for the transfer case fitted is selected. (Scheme 33)

Scheme 33

Transfer Case Electronic Control Unit

The transfer case control unit (VGSG) is installed in the X3 on the rear floor panel under the luggage compartment trim. In X5, it is located underneath the rear bench seat on the left. (Scheme 34) The transfer case control unit (VGSG) is on the PT-CAN Bus. VGSG shares information with DSC for overall xDrive control and has diagnostic communication via the OBD connector. (Scheme 35)and (Scheme 36).

Scheme 34

Scheme 35

Scheme 36

The transfer case control unit (VGSG) regulates the locking pressure of the multi-disc clutch in the transfer case. The transfer case control unit receives information on the required clutch locking pressure from the DSC control unit. The processing, control and electronics required for this are integrated in the transfer case control unit. This information is converted and output as a corresponding rotary motion of the servomotor. In order to position the servomotor and compensate for wear, a reference run is carried out each time the ignition is switched off. The servomotor position is determined by a Hall sensor integrated in the servomotor. During the reference run, the clutch is engaged and disengaged completely (once). While the clutch is actuated, the current consumption is measured for the servomotor position. This allows the VGSG to determine the beginning and end of the clutch actuating procedure. A clutch and oil wear calculation is also processed and stored in the VGSG. It increases the locking pressure as necessary in order to reduce friction. In the event of DSC failure, the VGSG incorporates a fallback level (strategy) for activating the transfer case clutch in order to maintain the four-wheel drive function.

TCC

Regulation of the transfer case clutch (TCC) locking pressure allows stepless coupling of the front axle to the drivetrain. The driving torque on the front axle can be increased or decreased depending on the driving situation and road conditions. Obviously, when more driving torque is transmitted to the front axle, driving torque on the rear axle is proportionally reduced due to lack of traction. The advantages of variable distribution of driving torque to the front and rear axles are

1. Optimum utilization of the cornering and longitudinal wheel forces on the front and rear axles.
2. DSC brake interventions only become necessary at a significantly later stage, an increase in comfort refinement.
3. Compared with an "open" differential transfer case and DSC, xDrive significantly improves driving torque distribution when traction on the front and rear axles is notably different. The DSC control unit influences control of the transfer case clutch. Even when DSC is deactivated, TCC remains active for the purpose of maximum traction and driving dynamics.

Permanent four-wheel drive is only completely deactivated in three control situations

1. During very tight cornering with low engine torque to allow speed compensation between the front and rear axles (e.g. parking).
2. At speeds greater than 112 mph.
3. When the vehicle dramatically understeers.

The transfer case clutch control logic is described in three main modules

1. Pre-control.
2. Traction control/driving dynamics control.
3. Tire tolerance logic.

Pre-control

The pre-control logic (shared from DSC) reflects the driver's command and is calculated based on

1. Accelerator Pedal Value
2. Engine Torque
3. Engine RPM

In normal driving, the clutch is operated with minimum slip so that permanent four-wheel drive with a driving torque distribution of 40 percent on the front axle and 60 percent on the rear axle is available. Even when the traction for the front and rear axles is dramatically different, the pre-control ensures that the system responds very quickly.

In the case of the open transfer case, the brake is applied after slip is detected on the rear axle. This takes approximately one half of a second in reaction time. Sixty-two percent of the driving torque is supported on the two rear brake discs and only 38 percent of the driving torque can be transferred to the front axle. In other words, wheel slip must be sensed first before driving torque is transferred through the transfer case by applying the rear wheel brakes. In contrast to an "open" transfer case (differential), the xDrive does not require brake intervention on the rear axle because no slip can occur (permanent through connection). The transfer case clutch is engaging the front axle as the vehicle is accelerating. This takes significantly less time (approximately 1/10th of a second).

Scheme 37

Traction Control/Driving Dynamics Control

Traction control monitors the slip conditions on the front and rear axles. The wheel speeds, yaw rate and transversal acceleration serve as the input signals. The function of traction control/driving dynamics control is to achieve optimum traction and to keep the vehicle stable. In the event of an oversteer tendency, the transfer case clutch is completely engaged and the maximum supportable driving torque on the front axle is transmitted. This helps to "pull' the front of the vehicle until stability is achieved. (Scheme 38)

In the event of an understeer tendency, the clutch can be fully disengaged if necessary. In this example, the front axle is separated from the drivetrain and the driving torque can only be transmitted to the rear axle. This helps to "push" the rear of the vehicle until stability is achieved. (Scheme 39)

Scheme 38

Scheme 39

Tire Tolerance Logic

The tire tolerance logic detects different tread circumferences on the front and rear axles. This occurs when

1. Mixed tires are used.
2. Space saving spare tire is installed.
3. Tires are used that have been worn down to different levels.

Normally, tire circumference deviations result in drivetrain torque bias (unwanted variations). The tire circumference can fluctuate up to 1 percent or more as a result of mixed tires or wear. The tire tolerance logic decides depending on the driver's command and driving situation whether the slip is to occur in the transfer case clutch or at the contact area between tire and road. If the slip is permitted in the transfer case clutch, the locking pressure set by the pre-control is reduced in order to keep the work loss low. In the driving dynamic control situation, the clutch is locked slightly more than normal, the four wheel drive is always guaranteed when required. For maximum xDrive performance, tires (and wheels) of the same diameter should be installed on the vehicle.

See also:
• TRANSFER CASE APPROVED OIL