Transfer Case (VG), Function - GF28.10-D-0002TSB
Model 907
with code ZG3 (Engageable all-wheel drive with gear reduction)
General
On vehicles with code ZG3 (Engageable all-wheel drive with transmission ratio), the vehicle's gradeability is increased by the engageable transmission ratio (i = 1: 1.432). The transmission ratio is achieved by using a planetary gear set. The engageable all-wheel drive has an additional planetary gear set (in the transfer case) and 2 open differentials (in the rear and front axle). When the all-wheel drive is engaged the torque distribution between the front and rear axle is permanently set by the planetary gear set at 33 % to the front axle and 67 % to the rear axle.
What makes this so special is that there are no mechanical longitudinal and transverse differential locks. Instead, the all-wheel drive operates together with the Electronic Stability Program (ESP®) that is specifically tuned to the all-wheel drive. The ESP® is automatically switched on when one or more wheels start to spin on slippery ground. When this happens, it selectively brakes the spinning wheel or the spinning wheels or it influences the engine torque. This simultaneously increases the drive torque at those wheels that still have sufficient road grip. Automatic brake application by the ESP® can simulate the effect provided by up to 3 differential locks.
The engageable all-wheel drive enables the number of driven wheels and therefore the degree of road adhesion to be improved. As a result, slip is reduced and the driving power is distributed to all of the wheels. The distributed driving power results in higher traction and higher lateral stability at the wheels.
Function requirements for successful engagement of all-wheel drive
The following requirements must be satisfied:
- Circuit 15 ON
- Vehicle speed below 14 km/h; as of 14 km/h, the activation must be completed.
- Switch-on signal of all-wheel drive activation button (S76/11)
- ESP® not operating.
- Engine torque below defined limit value.
Engagement of all-wheel drive, function
During activation, the light-emitting diode (LED) in the all-wheel drive activation button flashes and the indicator lamps for the ESP® as well as acceleration skid control (ASR) light up in the display.
If the all-wheel drive is switched on successfully, the LED in the all-wheel drive activation button lights up permanently and the indicator lamps in the display for the ESP® as well as ASR go out.
If all-wheel drive cannot be engaged, the LED in the all-wheel drive activation button flashes 3 times.
Function requirements for successful activation of the transmission ratio
The following requirements must be satisfied:
- All-wheel drive engaged
- Vehicle stationary
- Switch-on signal of all-wheel drive step-down ratio switch (S76/12)
- Service brakes operated
- Transmission neutral position
- Clutch pedal actuated with manual transmission.
Activation of transmission ratio, function
The LOW RANGE indicator lamp (A1e37) in the instrument cluster flashes when the transmission ratio is activated. If the transmission ratio is successfully engaged, the LOW RANGE indicator lamp lights up permanently. If the transmission ratio cannot be activated, the LOW RANGE indicator lamp flashes 3 times.
General notes
ASR and ESP® are not available during all-wheel drive activation. ABS is available.
At "circuit 15 OFF", the all-wheel drive is switched off. The transmission ratio that may be activated is also switched off.
If, owing to the gear position in the transfer case, it is not possible to immediately activate the all-wheel drive or the transmission ratio, the respective sliding sleeve is pre-compressed by the torsion spring in the worm gear of the transfer case actuator motor (M46/2), by the selector drum and the corresponding shift fork. This means that the all-wheel drive or the transmission ratio can be activated at the next gear wheel rotation.
Possible faults in the all-wheel drive transmission ratio system are stored in the ESP® control unit (N30/4) and they can be read out there using XENTRY Diagnosis.
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