Suspension: Testing and Inspection

Rear Axle Lateral Torque Distribution


IMPORTANT: Follow diagnosis instruction on Integrated Chassis Management (ICM).

Troubleshooting in the ICM is only error recognition with various observations (faults) after the vehicle has assumed the sleep mode. Therefore, a fault code that currently exists after a repair can only be deleted after the vehicle has assumed sleep mode.
If problems arise after a repair when deleting the fault memory, proceed as follows:
1. Rectify fault with ignition off.
2. Turn on ignition, clear fault memory.
3. Turn off ignition, allow vehicle to assume sleep mode. The vehicle has assumed sleep mode when the Start-Stop button light goes out.
4. Wait 10 seconds after the Start-Stop button light goes out. Then switch the ignition back on again.
5. Clear fault memory again.
6. Switch off ignition. The indicator and warning light goes out.

NOTE: Bear in mind the designation used by marketing.

The rear axle lateral torque distribution is standard equipment and is offered by marketing under the designation "Dynamic Performance Control".
The rear axle lateral torque distribution distributes the input torque specifically between both wheels of the rear axle.
In this development, the conventional rear axle differential has been extended to include 2 overriding gears. If required, this planetary gear train can be included in the power flow via a multi-plate brake.
Independently of the requested input torque of the engine, a difference with regard to the input torque between the left-hand and right-hand back wheel can be set actively up to 1800 Nm at any time. This difference has the effect of yaw moment on the complete vehicle.
Benefits:
- optimized directional stability as well as reduced steering effort
- improved traction
- Increased driving safety

Brief description of components
The following components are described for the rear axle lateral torque distribution:

Rear axle differential with 2 overriding gears
The rear axle differential has 3 oil chambers with shared ventilation. The assignment of the oil chambers is as follows:
- one oil chamber for the left-hand overriding gear
- one oil chamber for the right-hand overriding gear
- one oil chamber for the differential gear

The 2 overriding gears in the rear axle differential consist essentially of the following components:
- Electric motor
- Multi-plate brake
- Pressure disk with ball ramp
- Planetary gears (special feature: planetary gear train without ring gear)







QMVH control unit
The additional actuators on the rear axle differential are activated by the QMVH control unit ("QMVH" stands for "rear axle lateral torque distribution"). There are 2 processors built into the QMVH control unit. One processor activates the left-hand electric motor, the other processor activates the right-hand electric motor. In addition, each processor monitors the output signals of the other processor.
The power semiconductors fitted in the QMVH control unit function as final stages. These final stages are used to generate the phase voltages, as the electric motors are three-phase motors (asynchronous motors). When the phase voltages are switched off (e.g. fault detection), the electric motors are no longer activated. However, the shaft of the electric motor can be turned freely.







The following actuators and sensors are connected directly to the QMVH control unit:
- Electric motors
- Temperature sensors in the electric motor
- Motor position sensors
- Gearbox temperature sensors







Electric motor
The 2 electric motors are three-phase motors (asynchronous motors). The left-hand electric motor and right-hand electric motor are not identical components. The connections of the electric motors are coded differently. This means the electric motors cannot be confused or incorrectly connected.
The installation location means the electric motor makes contact with the differential oil. The complete rotor of the electric motor rotates in oil.

NOTE: Oil spillage is possible on removal of the electric motor.

On replacement of an electric motor, a small amount of oil can escape. In relation to the entire amount of oil in the rear axle differential, this amount of oil is so small that topping up differential oil is not required.
A temperature sensor is integrated in the electric motor. The temperature sensor is fitted in a sealing compound that seals the housing of the electric motor towards the connector. The coefficient of thermal conduction of the sealing compound means that the temperature sensor is exposed to virtually the same temperature as the remaining components of the electric motor.







Motor position sensor
Operating on what is known as the magneto resistive principle, the motor position sensor registers the rotor position of the electric motor. The motor position sensor is located on the side of the electric motor facing away from the drive. The motor position sensor sends its digital data across a two-wire connection to the QMVH control unit.

Multi-plate brake and ball ramp
The multi-plate brake and ball ramp are designed in such a way that the rear axle differential with lateral torque distribution behaves in the same way as a conventional rear axle differential if a fault is detected.
If a fault is detected in the system, the electric motor is no longer activated. However, the shaft of the electric motor can be turned freely. The compression springs and ball ramps open and hold open the multi-plate brake.







Gearbox temperature sensor in the rear axle differential
The signals from the 2 gearbox temperature sensors in the rear axle differential are transferred directly to the QMVH control unit. A gearbox temperature sensor is installed in each of the two oil chambers for the overriding gear. The QMVH control unit monitors the signals of the temperature sensors. Here, both the signals of the gearbox temperature sensors and those of the temperature sensors at the electric motors are taken into account. If the temperatures are too high, the torque request is slowly reduced.
If a very sports-oriented driving style is detected, there is a temporary rise in the temperature threshold by approx. 25 °C.

System functions
The following system functions are described:
- Functional networking of the chassis control system through the Integrated Chassis Management
- Lateral torque distribution
- Function display in the instrument cluster

Functional networking of the chassis control system through the Integrated Chassis Management
Even today, the chassis control systems work as a composite unit, but for the most part independently of one another. The new architecture of the Integrated Chassis Management (ICM) is handled by a higher-level control unit. The Integrated Chassis Management coordinates dynamic longitudinal and lateral control functions. This exploits new dynamic driving potentials. The Integrated Chassis Management is the central instance that links all of the individual items of information available in the vehicle and distributes these in coordinated form to the chassis control systems. The comprehensive analysis of the driving status on the basis of a great deal of information enables the Integrated Chassis Management to decide which interventions are most suitable in each driving situation in order to match the driver request and vehicle reaction.
The networking via Integrated Chassis Management also gives rise to new functions that only become possible the through the interaction of several control systems. For example, the rear axle lateral torque distribution has strong functional interaction with the dynamic stability control (DSC).
The DSC control unit remains the decisive control unit where stabilization of the driving characteristics in the limit range are concerned. In this case, the DSC control unit issues a message to the Integrated Chassis Management to cancel the dynamic driving interventions of the rear axle lateral torque distribution.







Lateral torque distribution
The system further enhances the very good longitudinal dynamics of xDrive, simultaneously optimizing the lateral dynamics with maximum stabilization. This makes the vehicle significantly more agile on cornering, even at low driving speeds. The torque distribution on the rear axle has 3 target fields:
- Driving dynamics
- Traction
- Road safety

The lateral torque distribution does not brake individual wheels to ensure stability, rather is specifically applies torque. If the driver accelerates strongly in the bend, the force is optimally distributed to both distributed depending on dynamic driving parameters (e.g. steering angle, lateral acceleration, yaw rate). The wheel on the inside of the bend is prevented from slipping from the outset, which would mean a loss of traction. The wheel on the outside of the bend is also assigned greater torque. Related to the complete vehicle, specific asymmetric torque distribution enables the lateral torque distribution to initiate yaw moments that lead to more direct and stable steering. This applies equally to accelerating and decelerating.
The rear axle lateral torque distribution is activated automatically as of the 'engine running' signal. Switching on or off or configuring manually are not possible.
The Integrated Chassis Management (ICM) uses the following information to calculate a nominal value for the QMVH control unit:
- Accelerator pedal position
- Engine speed
- Drive-dynamic state

This nominal value indicates how much input torque is to be applied to each back wheel. This message is output by the Integrated Chassis Management on the ICM CAN and read by the QMVH control unit.
The QMVH control unit has the task of implementing this nominal value by activating the electric motors. The signals of the motor position sensors provide the actual torque transferred for each back wheel. This information is output as a message on the ICM CAN and read by the by the Integrated Chassis Management.

Instrument cluster display
The display for the torque distribution can be switched on or off via the onboard computer display. The LC display shows a top view of the drive train and the wheels. Each wheel is assigned a bar gauge. This enables a visual display (bar gauges) of the distribution of the input torque by xDrive and the rear axle lateral torque distribution. If xDrive and/or the rear axle lateral torque distribution are unavailable, the LC display is in grey. Whether there is a fault in the xDrive system or rear axle lateral torque distribution is indicated by the corresponding symbol of the Check Control message. If the rear axle lateral torque distribution fails, the DSC / xDrive indicator and warning lamp lights up.

Notes for Service department

Diagnosis instructions

NOTE: For commissioning, comply with the service function.

After various actions, the service function must be run. Here, the following cases must be distinguished:
- Replacement of servomotor
- Replacement of control unit
- Replacement of control unit and servomotor
- Replacement of rear axle differential
- Replacement of rear axle differential and control unit
- Action after vehicle programming

No liability can be accepted for printing or other errors. Subject to changes of a technical nature