Differential Lock: Description and Operation
Regulated Differential Lock
Regulated differential lock (GHAS)
The regulated differential lock (GHAS) is a demand-regulated rear-axle differential lock. The regulated differential lock provides optimal traction appropriate to the driving situation. The regulated differential lock (GHAS) allows the slip between right and left rear wheel to reduce in which both wheels are connected via a multidisc clutch. The clutch package of the multidisc clutch can be connected via an electronically controlled servo motor, if needed and acts between the rear axle differential and the right output. The maximum locking torque is 2500 Newton metre.
The advantages of the regulated differential lock (GHAS) are:
- Improved handling
- Optimal traction
- Increased driving stability
Brief component description
The following components are described for the regulated differential lock (GHAS):
- Regulated differential lock
- Regulated differential lock control unit
- Rotor position sensor
- Servomotor with reduction gear
- Multidisc clutch and ball runway mechanism
- Transmission oil temperature sensor of the rear axle
Regulated differential lock
The regulated differential lock (GHAS) can apply a locking torque (depending on the driving condition) between the wheels of the rear axle. The locking torque is generated by a multidisc clutch. The necessary pressure in the axial direction is applied by a position-controlled servomotor with reduction gear. In the process the rotational movement of the servomotor is converted by a ball runway mechanism into an axial movement and the clutch package of the multidisc clutch is closed. The clutch package acts between the housing of the rear axle differential and the right output.
The regulated differential lock is active in the following situations:
- Driving off
- Stabilization under lateral acceleration in coasting/overrun mode
- Different wheel speeds for different road surface coefficients of friction right and left
- Dynamic cornering
- Drifting
The regulated differential lock in the rear axle differential primarily consists of the following components:
- Regulated differential lock control unit
- Servomotor with reduction gear
- Clutch package with ball runway mechanism
Regulated differential lock control unit
The regulated differential lock control unit is used for the control of the regulated differential lock (GHAS) in vehicles of the M GmbH. The regulated differential lock control unit is located in the luggage compartment floor in the area in front of the battery.
The regulated differential lock evaluates the dynamic handling characteristics signals made available from other control units. The regulated differential lock control unit uses these to determine the respective locking torque. The regulated differential lock control unit activates the servomotor which operates the multidisc clutch via a ball runway mechanism. For controlling the torque, the motor position angle of the servomotor is detected via 2 hall effect sensors. The communication of the electronic control with the vehicle occurs via FlexRay. There is a temperature sensor installed on the circuit board of the regulated differential lock control unit. For excessive temperature in the regulated differential lock control unit (above 105 °Celsius), the regulated differential lock (GHAS) is switched off.
The following actuators and sensors are connected directly to the regulated differential lock control unit:
- Servomotor
- Temperature sensor in the servomotor
- Rotor position sensor (2 hall effect sensors)
- Transmission oil temperature sensor of the rear axle differential
Rotor position sensor
The rotor position sensors detect the motor position angle of the servomotor. The motor position angle of the servomotor is determined via the evaluation of 2 hall effect sensors offset by 90°. The signals of the rotor position sensors are read and evaluated by the regulated differential lock directly.
Servomotor with reduction gear
The position-regulated servomotor with reduction gear is activated directly by the power electronics of the regulated differential lock control unit with pulse-width-modulated vehicle voltage. For determining the respective position and direction of rotation of the servomotor, this is equipped 2 hall effect sensors. There is a temperature sensor installed on the circuit board of the servomotor. For excessive temperatures on the servomotor (above 160° Celsius), the regulated differential lock (GHAS) is switched off.
NOTICE: Oil leakage possible when removing the servomotor.
When replacing the servomotor, a small amount of oil can escape. In relation to the entire amount of oil in the rear axle final drive, this amount of oil is so small that topping up final drive oil is not required.
Multidisc clutch and ball runway mechanism
The multidisc clutch with the ball runway mechanism is designed so that the regulated differential lock (GHAS) behaves like a conventional, open rear axle differential if a fault is recognised. In the event of a fault recognition, the servomotor is no longer activated. The ball runway mechanism releases the clutch package of the multidisc clutch and the lock opens.
Gearbox temperature sensor in the rear axle final drive
The signal of the transmission oil temperature sensor in the rear axle differential is directly transmitted to the regulated differential lock control unit. The regulated differential lock control unit monitors the signal of the temperature sensor. For excessive transmission oil temperatures (above 190°Celsius), the regulated differential lock (GHAS) is switched off.
System functions
The following system functions are described:
- Functional networking of the suspension control systems
- Driving dynamics control
Functional networking of the suspension control systems
The signals of the Integrated Chassis Management (ICM) and the Dynamic Stability Control (DSC) interact functionally for the distribution of the locking torque at the rear axle.
The Dynamic Stability Control (DSC) remains the deciding control unit for decisions regarding the stabilization of the drivability. In this case, the Dynamic Stability Control (DSC) request a defined locking torque for the stabilization of the vehicle. The opening of the lock is thus suppressed. In the process, the Integrated Chassis Management (ICM) makes signals available via the longitudinal acceleration, lateral acceleration and yaw rate.
The following information is obtained from the control units involved:
- Car access system (CAS)
Terminal control
- Digital engine electronics
Engine function, pedal sensor position and drive torque
- Dynamic stability control (DSC)
Wheel speed and torque distribution
- Integrated chassis management (ICM)
Lateral acceleration, driving speed, gradient and steering angle
- Junction Box Electronics (JBE)
Vehicle condition
- Instrument panel (KOMBI)
Date, time, ambient temperature and kilometer reading
- Central gateway module (ZGM)
Data exchange with the network components
Driving dynamics control
The driving dynamics control requests the locking torque in dependence on the current driving situation.
The driving dynamics control has the following functionalities:
- Driving off
Application of a constant locking torque.
- Traction (roadway with different coefficient of friction on the right and left)
For a difference in speed occurring at the rear axle, the drive torque will be transferred to the wheel that can transfer more drive power.
- Handling (accelerated cornering)
The drive torque is transferred to the outer cornering wheel via the wheel slip of the inner cornering wheel.
- Stabilization (load reversal for cornering or lane change)
A stabilizing torque is generated if oversteering is detected from the yaw-rate signal.
- Drifting
The lock is closed if provoked oversteering is determined from the yaw-rate signal and the signal of the pedal sensor position.
Notes for Service department
Diagnosis instructions
NOTICE: Observe the service function for commissioning.
The service function must be performed in the following cases:
- Renewal of the servomotor
- Renewal of the regulated differential lock control unit
- Renewal of the rear axle differential
- Programming
After the renewal of the regulated differential lock control unit, the regulated differential lock control unit must be re-encoded. The teaching-in of the regulated differential lock control unit at the rear axle is subsequently necessary. After the renewal of the complete rear axle differential, the regulated differential lock control unit must be taught-in at the new rear axle differential.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.