2000-06 TRANSMISSION: xDrive (Transfer Box)
SI Techniques - E53 & E83
Introduction
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.
Mechanical Construction Of The Transfer Case
Figure 1: Identifying xDrive Transfer Box
This figure illustrates the xDrive transfer box, showing its overall assembly and connection points to the vehicle's drivetrain. It highlights the unit responsible for distributing power between the front and rear axles.
Figure 2: Identifying Mechanical Construction Of xDrive Transfer Box
This figure displays a detailed view of the transfer box's internal mechanical components. A key identifies specific parts:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Connection to the manual-shift gearbox or automatic gearbox | 2 | Connection to the rear axle |
| 3 | Classification resistor | 4 | Electric servomotor |
| 5 | E53: Connection to the front axle | 6 | E83: Connection to the front axle |
Figure 5: Identifying xDrive Transfer Case Multi-Plate Clutches
This figure shows the multi-plate clutch assembly within the xDrive transfer case. It details the components responsible for variable torque distribution. A key explains the parts:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Output to the rear axle | 2 | Disc set |
| 3 | Actuator lever | 4 | Drive from manual-shift gearbox or automatic gearbox |
| 5 | Chain | 6 | Output to the front axle |
| 7 | Control cam | 8 | Electric servomotor |
How It Works: The rear axle is always powered. The rear and front axles are rigidly connected when the multi-plate clutch is fully closed.
Figure 6: Identifying Power Flow Through xDrive Transfer
This figure illustrates the flow of power through the xDrive transfer case. It shows how input torque from the gearbox is distributed to the front and rear axles via the chain and clutch mechanism. A key identifies the components:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Drive from manual-shift gearbox or automatic gearbox | 2 | Multi-plate clutch |
| 3 | Output to the rear axle | 4 | Control cam |
| 5 | Actuator lever | 6 | Output to the front axle |
| 7 | Chain |
Electric Servomotor With Incremental Sensor And Classification Resistor
The electric servomotor closes and separates the multi-plate clutch. Its location and adjustment rate are detected by an incremental sensor. The classification resistor accounts for mechanical tolerances to ensure optimal function.
Figure 7: Identifying xDrive Electric Servomotor
This figure shows the electric servomotor component. A key identifies its parts:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Control cam | 2 | Electric servomotor |
| 3 | Classification resistor | 4 | Worm gear |
How It Works: The electric servomotor is a DC motor. The incremental sensor records the adjustment rate and position of the servomotor shaft, data necessary for controlling the multi-plate clutch.
Classification Resistor: This component helps manage variations in the multi-plate clutch's locking torque characteristic curve due to manufacturing tolerances. An actual value is recorded after assembly and compared to stored nominal values. A classification resistor is assigned for each nominal value. Once assigned, it's installed on the transfer case, and its resistance value is imported by the transfer case control unit, which then automatically sets the characteristic curve. This setting occurs upon the first engine start or is checked during subsequent starts.
Reference Run: A reference run is performed with the ignition OFF (terminal 15 OFF) to assign a suitable locking torque for the multi-plate clutch based on the electric servomotor's angular position. This process also considers wear effects. During this run, the multi-plate clutch is fully closed and separated once. The current consumption is measured at each angular position during these movements to determine the start and end points of the closing motion. The angular position is recorded by the integral incremental sensor in the electric servomotor and stored for use when the car restarts.
Actuator Lever
The actuator lever converts the rotational motion of the electric servomotor into an axial motion.
Figure 8: Identifying xDrive Actuator Lever
This figure shows the actuator lever mechanism. A key identifies its parts:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Multi-plate clutch | 2 | Actuator lever |
Figure 9: xDrive Actuator Lever Operation
This figure illustrates how the actuator lever operates. When the electric servomotor runs, the control cam turns, pushing apart two leverages of the actuator lever. Bearing ramps cause axial movement as the leverages separate. This axial movement pushes the plates together in the multi-plate clutch, closing it.
System Circuit Diagram
Figure 3: Identifying xDrive Components
This figure shows a system circuit diagram illustrating the interaction of various electronic components within the xDrive system. A key identifies the components:
| Key | Explanation | Key | Explanation |
|---|---|---|---|
| 1 | Transfer case control unit | 2 | Electric servomotor with control cam |
| 3 | Instrument cluster | 4 | Dynamic Stability Control (DSC) |
| 5 | Electronic transmission control (EGS) | 6 | Digital engine electronics (DME) or Digital diesel electronics (DDE) |
| PT-CAN | Powertrain controller area network |
Figure 4: xDrive Circuit Flow Diagram
This figure provides a flow diagram of the xDrive system, showing how the DSC triggers the electronically controlled multi-plate clutch to supply variable input torques to the front and rear axles. The rear axle is always powered, receiving all input torque when the multi-plate clutch is separated.
xDrive communicates with the DSC to receive information such as accelerator status, engine torque changes, and whether the car is driving straight or in a curve. xDrive uses this data to predict the car's response to driver demands and intervenes to counter tendencies like wheelspin, oversteer, or understeer by regulating input torque distribution between the axles.
DSC engages by reducing engine power or applying selective braking when xDrive needs assistance to maintain the car's course. Input torque is directed to the axle with better traction during changing road conditions (e.g., snow, ice, loose surfaces).
The document notes two transfer cases matched to specific cars: E83 with ATC 400 (Active Torque Control) and E53 with ATC 500. Differences include the number of plates in the multi-plate clutches, clearance between shafts, and the propeller shaft connection (inserted into the front axle differential on E53, flange-mounted on E83). The E53 transfer case is designed to be stronger to handle higher engine torques.
The Advantages: xDrive offers advantages through demand-oriented input torque distribution, including outstanding driving stability, optimal forward momentum, and excellent traction in all road situations. xDrive became standard equipment on E83 and E53 models from October 2003.
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. It distributes input torques continuously and variably between the front and rear axles based on demand.
Transfer Case Control Unit
The control unit in the transfer case regulates the locking torque at the multi-plate clutch in response to:
- Demand for required locking torque (from the DSC control unit)
- Condition of the transmission oil (calculated by the transfer case control unit)
- Multi-plate clutch wear (calculated by the transfer case control unit)
- Electric servomotor load (calculated by the transfer case control unit)
- Transmission-oil temperature (calculated by the transfer case control unit)
The transfer case control unit provides information to the DSC control unit, including the current locking torque and all calculated data. Locking torque is limited when necessary to reduce frictional work.
Installation Location
E83: The control unit is located on the floor panel below the luggage compartment trim.
Figure 10: Locating Transfer Case Control Unit (E83)
This figure shows the location of the transfer case control unit in the E83 model.
E53: The control unit is located on the floor panel on the left below the rear bench seat.
Figure 11: Locating Transfer Case Control Unit (E53)
This figure shows the location of the transfer case control unit in the E53 model.
Figure 12: Identifying Transfer Case Control Unit Harness Connectors
This figure shows the transfer case control unit (TCU) and its harness connectors. It details the pin assignments for the 6-pin and 18-pin connectors:
TCU CONNECTOR PIN ASSIGNMENT X2153, 6-PIN
| Pin | Type | Description |
|---|---|---|
| 1 | E/A | Powertrain CAN Low |
| 2 | E/A | Diagnosis bus |
| 3 | E | Terminal 15 (wake-up wire) |
| 4 | E/A | Powertrain CAN High |
| 5 | M | Terminal 31 (earth) |
| 6 | V | Terminal 30 (power supply) |
Key: E = Input, E/A = Input/Output, M = Earth, V = Supply. For current specifications, refer to the BMW diagnosis system.
TCU CONNECTOR PIN ASSIGNMENT X70006, 18-PIN
| Pin | Type | Description |
|---|---|---|
| 1 | E | From the incremental sensor, indicates the direction of rotation of the electric servomotor |
| 2 | E | Negative lead from the incremental sensor and classification resistor |
| 3 | A | Positive lead for the incremental sensor |
| 4 | - | |
| 5 | E | Classification resistor |
| 6 | E | From the incremental sensor, indicates the frequency of the electric servomotor |
| 7-14 | - | |
| 15 | A | Positive lead to the electric servomotor |
| 16-17 | - | |
| 18 | E | Negative lead from the electric servomotor |
Key: A = Output, E = Input. For current specifications, refer to the BMW diagnosis system.
System Functions
xDrive comprises the following functions:
- Control Of The Locking Torque For The Multi-Plate Clutch
- Emergency Operation
Control Of The Locking Torque For The Multi-Plate Clutch
Control of the locking torque for the multi-plate clutch allows the front axle to be coupled infinitely variably to the drive train. Input torque at the front axle can be increased or reduced based on road situation and conditions.
The DSC control unit calculates the locking torque for the multi-plate clutch based on:
- Pre-activation (driver's command)
- Driving dynamic control
- Detection of different tyre rolling circumferences
Pre-Activation
Pre-activation reflects the driver's command and is used to calculate the required locking torque. Evaluation criteria include:
- Accelerator-Pedal Value
- Engine Torque
- Engine Speed
- Car Road Speed
- Gear Engaged
- Steering Angle
Driving Dynamic Control
Driving dynamic control monitors slip behavior on the front and rear axles, aiming to achieve optimum traction and maintain or stabilize the car's stability. Evaluation criteria include:
- Wheel speeds
- Yaw rate
- Lateral acceleration
- Steering angle
In normal all-wheel drive driving, input torque is distributed as follows:
- 40% to the front axle
- 60% to the rear axle
Torque distribution is oriented towards the torque each axle can support. For instance, during full acceleration from a standstill, higher axle load on the rear axle allows it to convey more torque. Conversely, if front wheels have high friction and rear wheels are on ice, nearly 100% of torque goes to the front axle.
In curves, lateral acceleration creates centrifugal force pushing the car outward. "Understeer" occurs when the car presses outward over the front wheels, while "Oversteer" happens when rear wheel adhesion decreases, causing the rear to slide outward. xDrive minimizes these tendencies by optimally distributing power between the front and rear axles.
Input Torque Distribution During A Tendency To Understeer
The multi-plate clutch separates fully, relocating input torque entirely to the rear axle. The front axle is relieved of driving forces, allowing for higher lateral cornering force to the front wheels, thus reducing understeer.
Input Torque Distribution During A Tendency To Oversteer
The multi-plate clutch closes, shifting input torque more to the front axle. The rear axle is relieved of driving forces, allowing for higher lateral cornering force to the rear wheels, thus reducing oversteer.
Detection Of Different Tyre Rolling Circumferences
Discrepancies in tyre rolling circumference (due to different makes/types or wear) cause drivetrain twisting when the multi-plate clutch is closed, leading to faster tyre wear. Slip in the multi-plate clutch can compensate for these differences by reducing locking torque when driving dynamic control is not critical.
Emergency Operation
In emergency operation, driving dynamic control and ADB-X functions are unavailable. The transfer case control unit has an integral regulator for emergency control, providing redundancy to maintain all-wheel drive as long as possible during DSC control unit malfunctions or sensor signal loss. Substitute values are calculated for missing sensor signals and used until effective control is no longer possible.
Dynamic Stability Control (DSC) Functions with xDrive:
In addition to standard DSC 8 features, xDrive incorporates:
- All-Wheel Control
- Automatic Differential Brake (ADB-X)
- Hill Descent Control (HDC)
All-Wheel Control: DSC provides the nominal value for all-wheel control, responding to tendencies of oversteer, understeer, and wheelspin. The transfer case/control unit adjusts torque distribution between front and rear axles based on this value.
Automatic Differential Brake (ADB-X): ADB-X simulates limited-slip differentials by applying brakes to individual wheels. If a wheel tends to spin, ADB-X brakes it to a preset slip level, increasing torque to wheels with better traction.
Hill Descent Control (HDC): HDC is a cruise control for downhill driving. It can be engaged/disengaged via a separate button. When activated, it reduces road speed to a low level (approx. 8 kph) by applying brakes on all wheels, maintaining this speed while DSC functions remain active. Road speed can be adjusted using the accelerator, brake pedal, or cruise control buttons.
Digital Engine Electronics (DME) Or Digital Diesel Electronics (DDE): The DME or DDE modifies engine behavior as required by the DSC control unit, such as reducing power to prevent wheelspin.
Accelerator Pedal Position Transmitter: This sensor monitors the driving condition.
Instrument Cluster Display: System states are displayed via telltales and warning lights:
- 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, control unit in transfer case OK; OR DSC OK, control unit in transfer case defective.
- All-wheel drive in emergency operation: DSC/xDrive telltale and warning lights, ABS telltale and warning lights, and general brake warning lamp light up with an acoustic signal, indicating total failure or malfunction of the DSC and/or transfer case control unit.
Notes For Service Staff
Service staff should note the following:
- General information: Refer to E83, E53 - XDRIVE, GENERAL INFORMATION FOR SERVICE STAFF.
- Diagnostics: (Details not provided here)
- Encoding/programming: (Details not provided here)
- Car and Key Memory: (Details not provided here)
Subject to change.
E83, E53 - xDrive, General Information For Service Staff
The following general information is provided for service staff:
- Towing cars with xDrive
- Using brake test stands
Towing Cars With xDrive
IMPORTANT: Always raise cars with xDrive at both axles during towing. Do not tow with only one axle lifted. Ensure no wheels contact the road surface. Even if the electric servomotor is de-energised, the clutch may not be fully separated, potentially causing the car to move. Damage can occur if wheels on the raised axle are blocked. Comply with warning notices and Owner's Handbook notes.
IMPORTANT: Pulling cars with xDrive is permissible with restrictions: Towing speed maximum 70 kph, towing distance maximum 150 km. Comply with Owner's Handbook information.
Using Brake Test Stands
IMPORTANT: Switch off the Hill Descent Control (HDC) before operating the brake test stand and do not switch it on again. The HDC indicator lamp must not be illuminated. HDC may be temporarily unavailable due to high brake temperatures.
Cars With xDrive And Automatic Transmission
IMPORTANT: Only check brakes when the selector is in "N" (Neutral) and do not accelerate when rollers start. This keeps the multi-plate clutch separated, preventing the car from moving off the brake test stand.
Cars With xDrive And Manual Transmission
IMPORTANT: Do not engage a gear and do not accelerate on the brake test stand. Applying throttle causes the multi-plate clutch to close, activating the stationary axle and pushing the car off the test stand, even if no gear is engaged.
