The parking brake is an automated electromechanical parking brake system (EMF). The holding force of the parking brake is generated by external electromechanical or hydraulic power.
The parking brake (EMF) control unit is connected to the control units of the Dynamic Stability Control (DSC), the Electronic Gearbox Control (EGS) and the Digital Engine Electronics (DME) with the instrument cluster via the PT-CAN bus.
The Car Access System (CAS) control unit provides the parking brake control unit (EMF) with the signal terminal 15. Activation of terminal 15 initiates the transition from idle state to operating state. The parking brake control unit (EMF) can be coded and programmed.
The parking brake control unit (EMF) is an add-on control unit of the electromechanical actuating unit. Both parts are fitted in the luggage compartment floor plate between the spare-wheel well and the rear seat.
The Car Access System consists of the CAS control unit and the corresponding electronic ignition/starting switch with key recess and START/STOP button.
The Car Access System (CAS) incorporates functions such as terminal control, electronic vehicle immobilisation unit, remote control signal evaluation, central locking system and power window control. The Car Access System (CAS) communicates with the door modules (TM). The door modules (TM) and the Car Access System (CAS) are connected to the instrument cluster by the K-CAN PERIPHERIE data bus.
The instrument cluster connects as gateway module the data buses PT-CAN and K-CAN PERIPHERIE as well as the diagnostic bus.
The braking force is generated by the regulated servomotor.
The gearing transfers the holding force generated by the servomotor via the balance arm to the bowden cables. The entire holding brake force is supported purely mechanically by a wrap spring on the casing.
The balance arm ensures the left-right balance of the bowden cables. In accordance with direction of rotation of the shafts, the bowden cables are tightened or slackened via the balance arm.
The cable in the electromechanical actuating unit runs across swinging cable pulleys. The bowden cable reaction forces are braced directly on the casing of the parking brake.
The control button is located in the dashboard to the left of the steering wheel. The control button can be used to apply and release the parking brake and enables dynamic emergency braking.
The indicator lamp shows the current system mode of the parking brake. If appropriate, additional message texts are displayed on the instrument cluster.
The function of the parking brake consists of the generation of the holding force by the DSC hydraulic components and/or by an electromechanical actuating unit.
With the exception of a few special functions, when the vehicle engine is running all static and dynamic parking brake processes are implemented using the DSC hydraulic components. Here, the DSC hydraulic components intervenes in the service brake of all 4 wheels.
When the vehicle engine is switched off, the electromechanical actuating unit holds the vehicle stationary. The electromechanical actuating unit exerts its influence on the duo-servo parking brake of the rear axle via conventional brake bowden cables.
The parking brake function makes it possible to hold the vehicle securely in a stationary position on uphill or downhill gradients when the vehicle engine is off. This function is performed by the electromechanical actuating unit. The braking force generated by the mechanical actuator is evenly distributed across brake bowden cables to the duo-servo parking brake that operates on the rear axle.
Holding refers to keeping the vehicle at a halt when the vehicle engine is running. This prevents unintentional movement of the vehicle from a standstill. The holding force is generated by means of the DSC hydraulic components and is applied to all four service brakes. When the vehicle engine is running, all static and dynamic brake processes of the parking brake are implemented using the hydraulic components.
For all starting and stopping operations, there are automatic function transitions between the electromechanical actuating unit and the DSC hydraulic components.
The coach door lock (CoSi) is a necessary safety precaution in the case of vehicles with rear doors that open to the front in the direction of travel.
In order to prevent personal injury and vehicle damage, the parking brake (EMF) must be activated in certain driving situations. The activation is triggered by the interaction of several control units and functions (rear door modules, Car Access System (CAS), coach door lock (CoSi) (see functional description: coach door lock CoSi).
If the vehicle is in motion (v > 0 km/h), pressing the button for the parking brake signals a defined deceleration instruction to the parking brake interface of the DSC system. This results in the DSC pump setting up hydraulic brake pressure on all four service brakes. During braking, all the slip control systems are active.
For reasons related to traffic safety, emergency braking is indicated to the traffic behind by the brake lights.
The base setting of the duo servo brake (air clearance) is to be made in accordance with the repair instructions.
After replacement of the brake linings of the duo servo brake, the brakes must be braked in. The braking-in program (workshop braking-in) can be found in the DIS tester among the service functions. Program standby is indicated by the flashing red parking brake lamp. The brakes can be braked in on the roller test rig or on the road.
The system returns to its normal function when
- the program has run
- the program is not run within 30 minutes
- the signal ignition off came
- a chassis control operation was triggered during braking in
In order to increase the availability of the optimum duo servo braking action, the duo servo brake is braked in during driving at specified intervals. Braking in removes any corrosion on the duo servo brake shoes or on the brake drums. The braking-in procedure is activated automatically at a standstill (e.g. stop at traffic lights). On driving off, the duo servo brake shoes are braked in by light friction against the drums. When a certain speed or time threshold is reached, the parking brake is released again.
For safety reasons, the braking-in procedure is cancelled immediately if chassis control operations (ABS, DSC, ...) are acivated. Pressing the button again or ignition OFF also leads to cancellation of the braking-in procedure.
Wear and settling lead to an increase of the required travel. The Hall sensors of the servomotor are used to measure the travel. The reference point used to determine the absolute travel position is the release stop in the actuating unit. If a given limit value is exceeded, a warning is issued to the driver and a fault is stored in the fault code memory.
The operational test of the parking brake can be performed on the usual roller test rigs.
The precise procedure can be found in the repair instructions for the parking brake.
In the event of a fault in or failure of the power supply, there is a manual mechanical emergency release process. Direct intervention in the gearing mechanism of the actuating unit enables the driver to release the parking brake manually.
Unintentional operation of the parking brake button before the bowden cables have engaged in the wheel carrier can lead to assembly problems. In order to suppress the activation of the actuating unit by button operation, the software of the parking brake contains an assembly mode. This mode is activated on delivery of parts.
Before initial operation of the parking brake in the vehicle, the assembly mode must be deactivate via the diagnosis interface. Activation via button operation is then re-enabled.