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Transfer case control, function - GF28.19-P-3000G

MODEL 463.306/336/346 with TRANSFER CASE 750.654 as of Model Year 2011/YoM 10 

MODEL 463.202/222/236/237/270/271/303/340/341 with TRANSFER CASE 750.654 as of Model Year 2009/YoM 08 

Fig 1: Transfer Case Control Function Diagram
G07889577Courtesy of MERCEDES-BENZ USA

Transfer case control, general 

The transfer case actuation function consists of the following subfunctions:

Function sequence for receiving, evaluate and forwarding of shift requests 

The fully integrated transmission control (VGS) control unit sends an analog signal over a direct line to match the three possible shifting states ("LOW", "NEUTRAL", "HIGH").

After the fully integrated transmission control (VGS) control unit has recorded and evaluated the transfer case operating condition, the electromotive shift operation is initiated through actuation of the transfer case actuator motor using an analog shift signal.

IMPORTANT A shift to HIGH is only enabled when a differential lock was not preselected or is not active.

Function sequence for recording and evaluation of the operating condition of the transfer case 

The fully integrated transmission control (VGS) control unit evaluates the instantaneous transfer case operating condition and thereby checks whether the preconditions for a shift operation are given.

The data required for this are sent as digital input signals over the drive train CAN from the control units listed below:

A Hall sensor integrated transfer case actuator motor sends pulses over a rotation angle and a directional output to the fully integrated transmission control (VGS) control unit, which correspond the rotation angle and the rotor's direction of rotation. Together with the default position ("LOW", "NEUTRAL", "HIGH") stored in the fully integrated transmission control (VGS) control unit, this calculates the instantaneous position (rotation angle and running direction) of the transfer case actuator motor.

The transfer case engagement indicator shows the shifting state of the transfer case:

The fully integrated transmission control (VGS) control unit releases the shift request (preselection) for the differential locks over a discrete line. A release only occurs however, when the transfer case is in LOW. Otherwise the release is canceled and actuation of the differential locks is deactivated.

Function sequence for storing data 

The instantaneous shift position is stored cyclically during each shift operation in the fully integrated transmission control (VGS) control unit memory. This shift position is defined as the default position for the next shift operation.

The force travel optimized characteristics stored in the memory define the actuation signal for the transfer case actuator motor.

In the event of malfunctions, operating errors or impermissible shift commands, the shift operation is dealt with by stored logical shift programs. Fault codes that can be read out by the data link connector (X11/4) can also be stored in the fully integrated transmission control (VGS) control unit.

Function sequence for actuation of special cases 

After an aborted shifting attempt the shifting logic of the fully integrated transmission control (VGS) control unit starts a second shifting attempt in the selected gear. If this shift attempt results in yet another incomplete shift operation, a downshift is made to the last engaged gear. If this downshift results in another incomplete shift operation, the gearshift sleeve moves to the neutral position and an acoustic and optical signal informs the driver that a gear has not been engaged in the transfer case.

If another problem prevents a gear from being shifted or if the gearshift sleeve moves to the neutral position and no other shift operation is permitted by the fully integrated transmission control (VGS) control unit, (exception: Towing) the driver is notified of this by means of an acoustic and an optical signal that no gear has been engaged in the transfer case.

If during the shift operation the ignition is switched off, then the shift operation is continued until it has been completed If however, in this instance, the shift operation results in an incomplete shift, then the fully integrated transmission control (VGS) control unit authorizes a downshift into the last engaged gear. If this operation results in another incomplete shift operation, the gearshift sleeve moves to the neutral position and an acoustic and optical signal informs the driver that a gear has not been engaged in the transfer case.

When at a standstill and - at the same time - in selector lever position "P" the software ensures that either the on-road or the off-road gearwheel in the transfer case is engaged. If this however, is not possible and a gear has not been engaged, then the driver is notified of this by mean of an acoustic and an optical signal.

If the shift position is lost, the insofar as no mechanical and electrical damage is present, another shift operation can be performed. If for further shift operations, a comparison of the shift travel before and after the shift position was lost it is determined that these travels are of a uniform value (within a specific tolerance limit), the fault stored in the fully integrated transmission control (VGS) control unit is marked as inactive.

After every start of the engine the position stored in the fully integrated transmission control (VGS) control unit is checked when neutral is engaged for the first time and then readjusted.

Function sequence for voltage supply 

The fully integrated transmission control (VGS) control unit receives its supply voltage

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