Parking Brake

Parking Brake

Parking brake (electromechanical parking brake = EMF)
Standard equipment includes a parking brake. The parking brake is the same as a handbrake. For the first time, the EMF actuator is integrated in the brake caliper. The parking brake is operated using the parking brake button in the centre console.
Deployment of the electromechanical parking brake has to the following advantages:
- Operation via the ergonomic button in the centre console
- Safe engagement and disengagement of the electromechanical parking brake (EMF) under all conditions
- Dynamic emergency braking function is ensured even with low coefficient of friction via the control systems (ABS)
- Space for additional storage options in the area of the centre console in that no parking brake lever is required.







The parking brake serves to secure the stationary vehicle to prevent it rolling away. The retaining force of the EMF is electromechanically applied to the rear wheels via the two EMF actuators.

Brief component description
The following components are described for the parking brake:

EMF actuator
The EMF actuator is attached to the brake calliper and has a direct effect on the brake pistons. The EMF actuator consists of a servomotor and gearing.
The two EMF actuators are activated by the EMF control unit.







The power is transferred to a two-stage planetary gear train via the servomotor and a belt drive. A spindle with a nut in the brake piston provides the self-locking facility. The power is transferred to the brake piston via the spindle and nut. The brake piston exerts force on the brake pads in the same way as hydraulic operation, and the pads are pressed onto the brake disc. The self-locking facility of the spindle maintains the tension. The vehicle is held in place even when it is de-energized.

Parking brake control unit
The EMF control unit is located in the rear right of the luggage compartment.
The EMF control unit receives the driver request to engage the brake via the parking brake button on the centre console. The vehicle status is detected via the bus system. The EMF control unit decides whether all the conditions for engaging have been complied with. If this is the case, the two EMF actuators on the rear brake calipers are activated.







The EMF control unit is supplied with terminal 30: by the front power distribution box and twice by the power distribution box on the battery.

Parking brake button
The parking brake button is located in the centre console behind the gear lever switch (GWS). The design of the parking brake button is based on the operations logic of a parking brake:
- Pull parking brake button upwards: the parking brake is activated
- Pull parking brake button downwards: the parking brake is deactivated
The status engaged is indicated by a red LED in the parking brake button as well as by the parking brake indicator light in the instrument panel. If the parking brake of the vehicle has already been engaged, pulling the parking brake button again has no effect.







System network







The following additional control units are involved in the electromechanical parking brake (EMF):

DSC: Dynamic Stability Control
The DSC delivers the conditioned wheel-speed in the form of a frequency-modulated signal to the EMF (determined from the speeds of the rear wheels). The signal is transferred across a separate line. The signal is mainly used for roll monitoring. The DSC also delivers the information on the actuated service brake.
Via DSC, dynamic emergency braking can also be initiated with the parking brake button.

DME: Digital Engine Electronics
The engine control delivers information on various engine conditions. The signal is transferred across the PT-CAN.

EGS: electric transmission control unit
The electric transmission control unit delivers the following signals:
- Selector lever position (e.g. D): This detects driving off against the parking brake (Check Control message). The selector lever position is also used as a release condition.
- Transmission output speed as the fall-back level for Automatic Hold

INSTRUMENT PANEL: Instrument panel control unit
In the event of failure of or a fault in the EMF, the warning and indicator light for the EMF lights up. In addition, a Check Control symbol is displayed in the instrument panel.







If there is a Check Control message, additional supplementary instructions can be displayed on the central information display (CID).

JBE: Junction Box electronics
The junction box electronics (JBE) form the gateway between the PT-CAN and K-CAN.

System functions
The following system functions are described for the parking brake:
- Releasing the parking brake
- Dynamic emergency braking
- Temperature monitoring
- Roll monitoring
- Safeguard at a standstill

Releasing the parking brake
To release the parking brake, the parking brake button is pressed down. For the parking brake to actually disengage, terminal 15 on and at least one of the following preconditions must be fulfilled:
- Brake pedal operated
- The parking lock of the automatic transmission is engaged
- The clutch pedal is actuated (only in vehicles with a manual transmission)

This prevents the vehicle from rolling away inadvertently if, for example, an occupant other than the driver presses the parking brake button. When the parking brake is released, the red indicator light in the instrument panel and the red LED in the parking brake button go out.
Activation of the EMF actuator sets the spindle into motion. The spindle rotation causes the spindle nut of the brake piston to move away.

Dynamic emergency braking
The law requires two operating facilities for brake control. The second operating facility in addition to the brake pedal is the parking brake button in the centre console. If the parking brake button is pulled up while the vehicle is moving, a specifically defined dynamic emergency braking intervention takes place via the Dynamic Stability Control (DSC). This function is intended for emergencies in which the driver is unable brake the car using the brake pedal.
It can also be used by other occupants to brake the vehicle to a standstill, such as if the driver should suddenly lose consciousness. During dynamic emergency braking, hydraulic brake pressure is built up at all four wheel brakes. The DSC functions are all fully active and the brake lights are activated. This is a significant advantage over manual parking brakes.
Dynamic emergency braking only takes place while the parking brake button is being pulled. The braking action created by the DSC is increased on the basis of a ramp-shaped characteristic curve. During dynamic emergency braking, the parking brake indicator light in the instrument panel is activated. In addition, a Check Control message is issued together with an acoustic signal to alert the driver to this special situation.
If the driver operates the brake pedal while also pulling the parking brake button, the DSC control unit is given priority. The larger braking action is put into force. If the dynamic emergency braking is continued until the vehicle comes to a standstill, the vehicle is fixed in a stationary position even after the parking brake button is released. The parking brake indicator light in the instrument panel remains active. The driver can then release the parking brake again (see Releasing the parking brake).

Temperature monitoring
Temperature monitoring ensures that the braking force reduction that occurs when hot brake discs cool is compensated for.
Temperature monitoring is activated if the temperature exceeds a defined value when the parking brake status is changed from released to engaged.
The temperature of the brake discs is individually calculated for each wheel by the DSC control unit and transmitted to the parking brake control unit. During a state change, the higher of the two temperature is used for temperature monitoring. The corresponding temperature ranges with the associated retensioning times are stored in a characteristic map.
Depending on the temperature during the state change, the corresponding retensioning times are activated from the characteristic map. When the first retensioning time is reached, retensioning occurs for the first time. After expiry of the second retensioning time, retensioning occurs again; this is repeated once more after a third retensioning time. In the characteristic map, the value 0 can be stored for one or more retensioning times. In this case, the associated retensioning processes are omitted. The function ends when the last retensioning process is finished.

Roll monitoring
The roll monitoring function prevents the vehicle from rolling with the parking brake engaged. Roll monitoring is always activated when the parking brake status changes from released to engaged and ends after a defined time elapses after this status change.
The input parameter used to detect rolling is a signal from the DSC. When roll monitoring is active, the EMF actuators are retensioned as soon as this signal indicates that the vehicle is beginning to roll. The EMF actuators are supplied with full current for 100 ms to increase their tension. Then the system waits for 400 ms. If the vehicle begins rolling again, the retensioning procedure is repeated (a maximum of three times). If, after the third retensioning, the monitor still detects the vehicle as rolling, the function ends with a fault entry in the fault memory.

Safeguard at a standstill
This function activates the mechanical brake engagement if the vehicle needs to be additionally secured when hydraulic holding functions are active (Automatic Hold, Active cruise control with Stop&Go function), such as if the driver leaves the vehicle. The DSC control unit detects when the driver leaves the hydraulically fixed vehicle (CAN messages, e.g. seat belt buckle contact, door contact). The DSC then requests that the parking brake provide additional mechanical braking.

Notes for Service department

General notes
The DSC control unit detects the brake test stand on the basis of a plausibility check (wheel speed comparison). The parking brake control unit changes to the brake test stand mode. This detection takes approx. 6 seconds.

Diagnosis instructions
The following service functions are available in the diagnosis system:
- Workshop mode
Before replacement of the brake pads, the 'Workshop mode' service function must be run. The parking brake is moved into the opened position and temporarily disabled. Subsequently, the brake pad can be replaced.
As long as the parking brake is in the workshop mode, for safety reasons it is not possible to close the parking brake. If the parking brake button is activated despite this, the indicator light in the instrument panel flashes yellow. The workshop mode is exited automatically at a driving speed above 3 km/h.

- Startup
Procedure:
- Ignition ON
- Apply strong pressure to the brake pedal
- Hold the brake pedal pressed down
- Pull the parking brake button for approx. 1 seconds (engage)
- Push the parking brake button (disengage)

Notes on encoding/programming
After replacement, the EMF control unit must be coded.
No liability can be accepted for printing or other faults. Subject to changes of a technical nature