GF28.60-P-3100VG Interaxle Differential Lock, Function

GF28.60-P-3100VG Interaxle differential lock, function
- with engine 156, 272, 273, 642 [model 164.1]
- with engine 273, 642 [model 164.8]
- Transmission 722.9
- with CODE (430) Offroad package

Component Identification:

Function requirements for interaxle differential lock, general
^ Ignition ON
^ Transmission in position "N"

Interaxle differential lock, general
The interaxle differential lock distributes the engine torque (50:50) between front axle and rear axle via a differential.

Interaxle differential lock, function sequence
The function sequence is subdivided into the following steps:
^ Function sequence for differential stages
^ Function sequence for specified locking power
^ Function sequence for locking control
^ Function sequence for safety concept
^ Function sequence for slip-sensitive locking control
^ Function sequence for locking actuation on antilock brake system (ABS)/Electronic Stability Program (ESP) control
^ Function sequence for locking overload protection

Function sequence for differential stages
The driver can use a differential lock selector wheel (N72s34) located in the lower control panel control unit (N72) to select between three different differential stages.

The following differential stages are available in the "High" on-road ratio:
- 1st. stage: transfer-case differential (interaxle differential lock) is automatically locked
- 2nd stage: Transfer case differential (interaxle differential lock) is manually locked (100 %)
- 3rd stage: Transfer case differential (interaxle differential lock) and the rear axle differential (interwheel differential lock) are manually locked (100 %)

The following differential stages are available in the "Low" off-road ratio:
- 1st. stage: transfer-case differential (interaxle differential lock) is automatically locked
- 2nd stage: Transfer case differential (interaxle differential lock) is manually locked (100 %) and the rear axle differential (interwheel differential lock) is automatically locked
- 3rd stage: Transfer case differential (interaxle differential lock) and the rear axle differential (interwheel differential lock) are manually locked (100 %)

Each differential stage has a red LED to indicate the active state.

Function sequence for specified locking power
When the lock is actuated, the transfer case control unit (N15/7) calculates a specified locking torque according to the current operating state.

The information about the specified locking torque is sent by the transfer case control unit via the engine-compartment CAN (CAN C) to the rear axle differential lock control unit (N15/9). This pre-control of the locking torque has the following advantages:
- Ensure start-off traction
- Avoiding overbraking of the rear axle during manual downshifts
- Avoiding oversteer or understeer under load

To allow pre-control of the locking torque, the transfer case control unit requires the following information:
- Vehicle speed
- Engine torque
- Accelerator pedal position
- Cornering recognition
- Wheel speeds

Function sequence for locking control
The interaxle differential lock regulates the distribution of the engine torque between the front and rear axles.

The transfer case control unit actuates the transfer case switchover magnet (Y108). The transfer case switchover magnet switches between the "High/Low shift" function and the interaxle differential lock function. When the transfer case actuator motor is actuated by the transfer case control unit, the transfer case actuator motor turns the lock control disk (9).

The turning of the lock control disk (9)produces a contra rotation of the front and rear lock levers (10, 11). As the lock levers (10, 11) turn, the balls (2) in the ball ramp mechanism run up to the ramps (1), producing an axial movement of the rear lock lever (11). This causes an axial displacement of the lock piston (3). This axial displacement produces a defined friction torque in the disk pack (5). The differential housing (7) and the front differential side gear (8) are mutually locked.

Function sequence for safety concept
In the de-energized state (power failure), the transfer case switchover magnet interrupts the connection between the transfer case actuator motor and the front and rear lock levers (10, 11), and the disk pack (5) is pushed back to the starting position by the disk spring pack (4).

Function sequence for slip-sensitive locking control

While lock control is active, the transfer case control unit, in combination with the pre-control, calculates a suitable range for the slip threshold and the optimum locking torque in order to avoid oversteer and understeer under load. For this the transfer case control unit requires the following information:
- Vehicle speed
- Engine torque
- Cornering recognition
- Tire tolerances
- Wheel speeds

Function sequence for locking actuation on antilock brake system (ABS)/Electronic Stability Program (ESP) control
The transfer case control unit must take into account the requirements of the ABS and ESP systems in order to avoid negative influences on driving stability. For this purpose the transfer case control unit is capable of opening the interaxle differential lock.

To guarantee this function, the following information is required:
- Stop light signal
- Deceleration at the rear axle
- Vehicle speed
- ABS request
- ESP requirement

Function sequence for locking overload protection
The aim of the lock overload protection is to protect the interaxle differential lock against damage and guarantee the maximum possible availability of the lock function.

After ignition "OFF/ON" for greater than 10 s the 1st stage of the differential lock is automatically activated. The current differential stage selection remains active if the period is less than 10 s.