Transfer Case (VG), Function - GF28.10-P-0002VGG
Model 463 with transmission 725.0
as of model year 2019
Block diagram
Transfer case, general
The transfer case distributes the drive torque translated by the automatic transmission via the differential with a fixed allocation of 40/60. For this, 40 % of the drive torque is distributed to the front axle and 60 % to the rear axle. The transfer case fulfills the interaxle differential lock and off-road ratio functions and the hold function.
The separate transfer case is fitted to the transmission housing. Due to the design, there is the advantage that there is no reciprocal damage due to contamination (e. g. due to metal abrasion) through the separate oil circuits (of the automatic transmission and of the transfer case).
The lubrication of the transfer case is ensured by an integrated mechanical oil pump.
In addition to the transfer case, the vehicle is equipped with differential locks in the front axle as well as in the rear axle.
Interaxle differential lock, function
The switchable interaxle differential lock function is implemented in the transfer case using a multiplate clutch. In automatic mode, the lock value is continually, electronically set by the all-wheel drive control unit between 0 and 100 % depending on the steering angle or accelerator pedal position, for example.
With a completely closed interaxle differential lock (lock value 100 %), a frictional connection is produced between the front and rear axle.
The axles are rigidly connected with each other and turn with the same speed. With a completely open lock (lock value 0 %), the axles are connected by the differential, can turn at different speeds depending on substrate quality and therefore distribute a maximum of 40 % or 60 % of the drive torque respectively.
The interaxle differential lock is regulated in the automatic mode of the transfer case. The all-wheel drive control unit calculates the required locking torque of the interaxle differential lock. The locking ratio is set depending on the detected wheel slip and the steering angle. The ESP® control unit transmits the signals "Wheel speed" and "Wheel rotation direction" to the all-wheel drive control unit via the suspension FlexRay. The electrical power steering control unit transmits the wheel angle to the all-wheel drive control unit via the suspension FlexRay. The steering wheel angle sensor transmits the position of the steering wheel to the steering column tube module control unit, which sends the information on the steering wheel angle to the all-wheel drive control unit via the suspension FlexRay. If, during comparison with the signals of the Electronic Stability Program control unit, understeer or oversteer is detected, the all-wheel drive control unit regulates the drive torque distribution via the integrated all-wheel drive actuator and the multidisk clutch.
The linear increase in the locking effect of the interaxle differential lock from 0 to 100 % usually takes place within approx. 180 ms and the reduction within approx. 130 ms. In addition, the interaxle differential lock can be manually engaged and disengaged again by touching the lock switch in the vehicle on the electronic differential control unit (Full lock mode). If the interaxle differential lock is requested by the driver, ABS and ESP® are deactivated. After deactivation of the interaxle differential lock, the automatic mode is activated again immediately. The brake control system can open the interaxle differential lock in different control situations. The ESP® control unit transmits information on the braking status and braking torque to the all-wheel drive control unit via the suspension FlexRay. The SRS control unit detects the yaw rate as well as lateral and longitudinal acceleration of the vehicle using the integrated sensor unit and transmits this to the all-wheel drive control unit via the vehicle dynamics CAN, the ESP® control unit and the suspension FlexRay. The fully integrated transmission control unit transmits the signals "Current output speed" and "Transfer case shift operation by the automatic transmission permissible" to the all-wheel drive control unit via the drive CAN, the powertrain control unit and the suspension FlexRay. The ME-SFI control unit or the CDI control unit transmits information on the engine speed and the accelerator pedal position to the all-wheel drive control unit via the drive CAN, powertrain control unit and the suspension FlexRay. The Drive Unit control unit (on models 463.276/277) transmits information on the chosen drive program and the torque request to the all-wheel drive control unit via the suspension FlexRay. In addition, the instrument cluster transmits the displayed vehicle speed to the all-wheel drive control unit via the user interface CAN, the electronic ignition lock control unit and the suspension FlexRay.
The all-wheel drive control unit transmits the information "All-wheel drive operating mode" and "All-wheel drive error status" to the Drive Unit control unit (on models 463.276/277), the ESP® control unit, the powertrain control unit and the electronic ignition lock control unit via the suspension FlexRay. The powertrain control unit transmits the signals to the fully integrated transmission control unit via the drive CAN. The electronic ignition lock control unit transmits these signals to the instrument cluster via the user interface CAN, which shows a corresponding message.
The temperature of the transfer case and the transmission oil is calculated based on a temperature model. Specifically, the temperature of the disks, transmission oil, housing of the multiplate clutch (drum) and housing of the transfer case are calculated. The temperature of the steel disks is regarded as the critical temperature of the multiplate clutch. If the critical temperature of the multiplate clutch of 220 ° C or the critical temperature of the all-wheel drive actuator is reached, the multiplate clutch is deactivated. The system is activated again when the temperature has dropped and no high power loss is expected. The calculated temperature of the transmission oil is transmitted to the electronic ignition lock control unit and the powertrain control unit via the suspension FlexRay. The electronic ignition lock control unit transmits the signals for outputting warning messages to the instrument cluster via the user interface CAN.
Front axle/rear axle differential lock function
The differential locks improve the traction potential of the vehicle off-road and through water.
The all-wheel drive control unit transmits information on the status of the multiplate clutch and the current gear of the transfer case to the electronic differential control unit via the suspension FlexRay, the electronic ignition lock control unit and the user interface CAN. The electronic ignition lock control unit transmits further information on the wheel speed and wheel rotation direction, the braking status and the request for deactivating the differential locks by the ESP®, the direction of travel and the speed of the vehicle to the electronic differential control unit via the user interface CAN. The electronic differential control unit checks via the signals whether the differential locks can be engaged. The differential locks can be engaged under the following conditions:
- Engine running.
- An active ESP® regulation is not taking place.
- The all-wheel drive control unit transmits no error status.
- The front axle/rear axle differential lock actuators send no error status.
The electronic differential control unit transmits information on the current status of the differential lock of the front and rear axle as well as the interaxle differential lock to the all-wheel drive control unit via the user interface CAN, the electronic ignition lock control unit and the suspension FlexRay and to the instrument cluster via the user interface CAN for outputting the corresponding messages.
Off-road ratio function
The transfer case has two shift ranges:
- HIGH RANGE (on-road drive mode for normal driving situations)
- LOW RANGE (off-road gear for off-road driving, driving through water and on steep uphill or downhill gradients)
The off-road ratio in the LOW RANGE shift range is performed with a factor of 2.93. For activation into the off-road gear, the following conditions must be fulfilled:
- The vehicle speed is a maximum of 40 km/h forward or 10 km/h backwards.
- The automatic transmission is in position "N".
- The LOW RANGE button on the right lower control panel is operated.
In the LOW RANGE off-road gear, the maximum design speed is 85 km/h. The automatic transmission can switch in its own gears. A lower speed per gear range is reached than in HIGH RANGE due to the step-down ratio in the transfer case. The drive torque and the tractive force increase.
Switching from the off-road gear into HIGH RANGE is possible under the following conditions:
- The vehicle speed is a maximum of 70 km/h forward or 20 km/h backwards.
- The automatic transmission is in position "N".
- The LOW RANGE button on the right lower control panel is operated.
The engaged gear of the transfer case is recorded by the all-wheel drive actuator and verified by the transfer case absolute sensor. The all-wheel drive control unit transmits the signal "Transfer case current gear" to the instrument cluster via the suspension FlexRay, the electronic ignition lock control unit and the user interface CAN. If the off-road ratio is active, the display "G" appears in the instrument cluster.
When the LOW RANGE button on the right lower control panel is operated, the signal "Low range switch operated" is transmitted to the all-wheel drive control unit via the LCP-LIN, the Audio/COMAND control panel, the telematics CAN, the head unit, the user interface CAN, the electronic ignition lock control unit and the suspension FlexRay. The all-wheel drive control unit then activates the off-road ratio and transmits information on the current gear of the transfer case to the ESP® control unit, the electronic ignition lock control unit and the powertrain control unit via the suspension FlexRay.
Switching between HIGH RANGE and LOW RANGE affects the vehicle dynamics, similar to down-shifting the automatic transmission into a lower or higher gear.
Hold function
The transfer case is equipped with a SSHD (shift system hold device) hold function. This can cause a static lock mode and holds the interaxle differential lock mechanically closed if the ignition is switched off. While driving, the interaxle differential lock is ensured by the all-wheel drive actuator. At vehicle stillstand, the hold function is automatically activated as soon as the lock switch for the interaxle differential lock is operated and the automatic transmission is in position "P". The hold function is deactivated again as soon as the driver shifts from "P" to "N", "D" or "R".
The following table provides an overview of the vehicle model and major assembly overviews:
| Sales designation | Model | Engine | Automatic transmission | Additional information |
|---|---|---|---|---|
| Model series 463 | ||||
| G 500 | 463.260 | 176.980 | 725.035 | |
| G 550 | 463.261 | 176.980 | 725.035 | |
| G 63 AMG | 463.276 | 177.980 | 725.085 | |
| G 63 AMG | 463.277 | 177.980 | 725.085 | |
| G 400 d | 463.349 | 656.929 | 725.035 |
Further subfunctions are described in a separate document:
- Transfer case oil supply, function
Transfer case
| Transfer case oil supply, function | GF28.55-P-3000VGG | ||
| Overview of system components, transfer case (VG) | GF28.10-P-9998VGG |