All-Wheel Drive System xDrive With Transfer Box
All-wheel Drive System xDrive With Transfer Box
All-wheel drive system xDrive with transfer box (VTG)
Starting September 2009, the intelligent All-wheel drive system xDrive is offered in the 750i xDrive and 750Li xDrive in advanced version for the BMW 7-Series. In future, this advanced xDrive will also be adopted for other series.
In addition to outstanding traction, further improved driving dynamic values were a high priority during the development. During normal straight-ahead driving, the front and rear axles are powered with the normal 40 to 60 force distribution. The front wheels are thus supplied with not too much drive torque and can optimally handle the job of steering.
When turning and in stable cornering, the transfer box transfers up to 80 percent of the force with electronically-controlled multidisc clutch to the rear axle. This increases the maneuverability of the vehicle and effectively prevents understeer. This ensures spontaneous and precise cornering steering and an easily-controllable drive behavior.
Brief component description
The following components for the All-wheel drive system are described in the example F01:
Drive train
The new transfer box (ATC 350: ATC = Active Torque Control) has a spur gear unit for the power transmission, which is advantageous in terms of the installation space.
The classification resistor required up to now is dispensed with. The compensation of the manufacturing tolerances is achieved through an internal classification.
NOTICE: ATC 450 for E70, E71!
With the X5, the new transfer box (ATC 450: ATC = Active Torque Control) has a chain for the transmission of power.
Transfer box
The separate oil pump in the transfer box is dispensed with. This improves the efficiency.
Sufficient levels of lubrication and cooling are assured through the delivery of oil via the spur gear unit.
Mechanical design transfer box
The control unit for the transfer box (VTG) is attached below at the servomotor for the multidisc clutch.
The design of the multidisc clutch was changed slightly.
A motor position sensor is located in the servomotor. The motor position sensor detects the position of the servomotor and informs the transfer box control unit of the position.
A temperature sensor in the VTG control unit monitors the power output stage for the servomotor. This protects the servomotor from thermal overload. Any thermal overload of the servomotor is signalled by a Check Control message. The transfer box moves temporarily into its open position, i.e. the multidisc clutch is opened. Once the servomotor has cooled down, the multidisc clutch can be operated again (conditions: DSC must request once-only 0 Nm locking torque).
Centre console operating facility
The Hill Descent Control is operated via the centre console operating unit (HDC button). For X1 and X5 in the centre console control panel.
The lower switch block in the centre console operating unit is electrically connected to the controller (CON).
System functions
The networking of its own control unit is required for the activation of the transfer box.
In particular: Transfer box, Dynamic Stability Control as well as Integrated Chassis Management via FlexRay. The VTG control unit is a bus user on the FlexRay network.
The transfer box control unit activates the servomotor. Distribution of the drive torque is oriented upon the torque that can be supported by each axle.
The electronically controlled multidisc clutch smoothly distributes the drive torque as required to the front axle. The rear axle is always powered. This means that all of the drive torque is applied to the rear axle when the multidisc clutch is separated.
The drive torque is distributed as follows in normal driving with four-wheel drive:
- 40 % to the front axle
- 60 % to the rear axle
The transfer box control unit and servomotor cannot be replaced individually. They must be replaced together. In consultation with the central driving dynamics control function in the Integrated Chassis Management (ICM) integrated in the master control unit, the Dynamic Stability Control (DSC) stipulates the nominal value for all-wheel control of the xDrive function. The nominal value depends on the tendency of the vehicle to oversteer or understeer and the wheel slip. The nominal value is transmitted to the VTG control unit.
The VTG control unit regulates the locking torque at the multidisc clutch in the transfer box in response to the following factors:
- Demand for required locking torque (comes from the DSC control unit)
- Condition of the transmission oil (calculated in the VTG control unit).
- Multidisc clutch wear (calculated in the VTG control unit).
- Load on servo motor (calculated in the VTG control unit)
- Transmission oil temperature (calculated in the VTG control unit).
The VTG control unit supplies the following information to the DSC control unit:
- The locking torque currently set
- All calculated data
Note for Service department:
NOTICE: Follow the instructions for service functions!
The diagnosis system provides a service function for replacement of the transfer box or servomotor with VTG control unit:
Path: Service functions - Powertrain - VTG transmission control unit
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.