Scheme 53
BRIEF DESCRIPTION OF COMPONENTS
xDrive consists of the following components
- Transfer case with multi-plate clutch The electronically controlled multi-plate clutch is housed in the transfer case. The multi-plate clutch distributes the drive torque smoothly and as required between the front and rear axles.
- VTG actuator motor with incremental sensor and classification resistor The VTG actuator motor (VTG: transfer case) opens and closes the multi-plate clutch. The location of the actuator motor shaft and the adjustment rate are detected by the incremental sensor. The classification resistor ensures that mechanical tolerances in the transfer case are considered. Optimum function is thus ensured.
- Actuator lever The actuator lever converts the rotational motion of the VTG actuator motor into an axial motion.
- VTG control unit The VTG control unit regulates the locking torque at the multi-plate clutch in the transfer case 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) Multi-plate clutch wear (calculated in the VTG control unit) Load on VTG actuator 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 The locking torque is limited whenever needed in order to reduce the frictional work. > E53, E83 > E70
- DSC: Dynamic Stability Control With xDrive, DSC includes the following functions: All-wheel control Automatic Differential Brake (ADB-X) Hill Descent Control (HDC) All-wheel control Dynamic Stability Control (DSC) specifies the nominal value for xDrive all-wheel control. The nominal value depends on the vehicle's tendency to oversteer or understeer and wheel slip. The nominal value is transmitted to the VTG control unit. Automatic differential brake The automatic differential brake (ADB-X) simulates the function of conventional limited-slip differentials by selectively applying brakes on individual wheels (known from the E53 and the E46 All-wheel drive). Whenever a wheel displays a tendency to spin the ADB-X automatically brakes it down to a preset slip. This effect of this is to increase the drive torque on those wheels that have a higher friction coefficient on a lining. Hill Descent Control Hill Decent Control (HDC) is a cruise control on four-wheel drive cars for driving downhill (known from the E53 and the E46 All-wheel drive). HDC can be activated and deactivated with a separate button. When the HDC button is pressed, HDC automatically reduces road speed. The brakes are applied on all four wheels to reduce road speed to just above walking pace. HDC holds this speed constant (all DSC functions remain active). The car's road speed can be freely varied within fixed values by pressing down on the accelerator pedal, brake pedal or the buttons for the cruise-control system.
The following control units and switches are incorporated in xDrive functions
- DME or DDE: Digital engine electronics or digital diesel electronics The DME or DDE modifies the engine response as required by the DSC control unit (such as power reduction to prevent the driven wheels from spinning).
- Instrument cluster display System states are displayed as follows: DSC/xDrive telltale and warning lights light up: DSC/xDrive not activated DSC/xDrive telltale and warning lights light up and acoustic signal: DSC defective, ABS not affected, VTG control unit OK or DSC OK, VTG control unit defective -> All-wheel drive in emergency mode DSC/xDrive telltale and warning lights, ABS telltale and warning lights and general brake warning lamp light up and acoustic signal: Total failure of the DSC and/or malfunction of the VTG control unit -> All-wheel drive in emergency mode > E53, E83 The instrument cluster is the data interface (= gateway) between the K-bus and the PT-CAN.
- JBE: Junction box electronics > E70 The JBE is the data interface (= gateway) between the K-CAN and PT-CAN. The junction box comprises the JBE and the power distributor.
Emergency operation
Driving dynamic control and the ADB-X function are not possible in emergency operation. The VTG control unit has an integral regulator for emergency control. The regulator provides redundancy for controlling the multi-plate clutch in the DSC control units.
The regulator is used to attempt to maintain all-wheel drive for as long as possible when the DSC control unit malfunctions or important sensor signals drop out.
Substitute values are calculated when individual sensor signals drop out. Functions are operated by using the substitute values until it is no longer possible to control the all-wheel drive effectively. This can lead to the complete loss of all-wheel drive.