Transfer Case Control Unit, Basic Function - GF28.19-P-9890A

Model all with transmission 725.0/1 

with code M005 (Vehicles with 4MATIC/all-wheel drive) 

Overview 

This document contains information on:

• General 
• Function requirements 
• Function 
  • High/Low shift mechanism 
  • Interaxle lock 

General 

The electronically controlled transfer case combines the High/Low gearshift system and the interaxle differential lock (differential lock).

The transfer case control unit monitors and actuates the transfer case.

Function requirements 

• Transmitter key in "Ignition ON" position
• Drive range N engaged

Function 

High/Low gearshift system (On-road and off-road ratio) 

The transfer case is equipped with a planetary gear set which allows the driver to shift between the High on-road ratio and the Low step-down ratio by operating the Low Range button under certain shift conditions (High/Low < 40 km/h, Low/High < 70 km/h) during the journey. The planetary gear set is located in the central differential module. The High on-road ratio is 1:1, the Low step-down ratio is 2.93:1. The step-down ratio allows the driver to drive extremely slowly off-road or when pulling a trailer, which is detected by the trailer recognition control unit.

If all the shift conditions are satisfied, the transfer case control unit actuates the transfer case actuator motor and the ratio in the transfer case is changed.

The High/Low shift operation takes place irrespective of the transmission mode selection. It is, however, not possible to shift to the transfer case position Low with the Sport drive program engaged.

The transfer case absolute sensor registers the position of the High/Low shift fork in the transfer case and forwards the information to the transfer case control unit.

Interaxle lock 

The engine torque is distributed through the central differential, with opened interaxle differential lock clutch pack, in equal amounts between front and rear axle. To ensure optimum torque distribution is achieved between the front and rear axle in every driving situation, the transfer case is equipped with an electronically-regulated interaxle differential lock. The transfer case control unit detects wheel slip and locks the central differential by means of an interaxle differential lock (multidisk clutch) according to the driving situation. The transfer case control unit calculates an optimum locking power (pre-control) depending on the driving situation and the selected transmission mode. In order to avoid oversteer and understeer under load, a suitable slip range for the multidisk clutch (interaxle differential lock) is calculated in addition to the optimum locking torque. The interaxle differential lock is activated by the transfer case control unit via the transfer case switchover magnet and the transfer case actuator motor. The transfer case control unit also takes into account the requirements of the Anti-lock Brake System (ABS) and Electronic Stability Program (ESP®) systems to prevent negative influences on braking stability. To guarantee functionality, the interaxle differential lock is opened when required by the ABS and ESP® systems.

A lock overload protection guards the interaxle differential lock against damage and guarantees the maximum possible availability of the lock function.

The interaxle differential lock is activated by means of actuation of the transfer case switchover magnet and transfer case actuator motor by the transfer case control unit.

The status of the ESP® and ASR systems are displayed via the ESP®/ASR OFF warning lamps.