The parking brake is an automated electromechanical parking brake system. The holding force is generated by external electromechanical or hydraulic power. The driver also has the possibility to activate an Automatic Hold setting via the controller (in the centre console). The parking brake will be fitted as standard in the 7 Series as of model year 2001.
The parking brake control unit is connected to the control unit of the DSC and the vehicle peripherals (instrument cluster, DME, EGS) via PT-CAN. The Car Access control unit provides the parking brake control unit with the signal terminal 15. Activation of terminal 15 initiates the transition from idle state to operating state. The parking brake control unit can be coded and programmed.
The parking brake control unit is located as an add-on control unit of the electromechanical actuating unit in the luggage compartment floor plate between the spare-wheel well and the rear seat cushion.
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, as before, directly on the casing of the parking brake.
The control button is located on the E65 in the dashboard to the left of the steering wheel. This control button can be used to brake and release the vehicle and enables dynamic emergency braking.
The indicator lamp shows the current system mode of the parking brake. If appropriate, supplementary information appears in the display field of 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 Automatic Hold functions are available to the driver by activating the Automatic Hold mode with the controller. Holding is automatic when the standstill signal (v = 0 km/h) is detected. In the case of stop-and-go traffic, foot pressure on the brake is no longer required to hold the automatic creep force. In addition, the brake creaking that occurs in the transition from static to sliding friction is significantly reduced. Automation of the release action enables comfortable driving off on gradients without backward roll.
Here, the vehicle is initially braked to a standstill using the brake pedal. The braking force this requires is locked in the hydraulic system. If the vehicle condition changes, e.g. change to the load status or road gradient, the DSC hydraulic pump actively supplies extra pressure. Thus extra supply takes place on detection of vehicle movement by the wheel sensors at the latest.
The brake is released automatically when the gearbox drive position is engaged by pressing the accelerator pedal.
If the vehicle is in motion (v > 0 km/h), pressing the button 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 as before (see 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.