Parking Brake System: Description and Operation
Parking Brake
Parking brake (electromechanical parking brake = EMF)
The electromechanical parking brake is the same as a handbrake. 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 the following advantages:
- Operation via the ergonomic button in the centre console
- Safe engagement and disengagement of the electromechanical parking brake (EMF) under all preconditions
- 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 parking brake retaining force is applied electromechanically to the rear wheels via both EMF actuators.
Brief component description
The following components are described for the parking brake:
EMF actuator
The EMF actuator is attached to the brake caliper and has a direct effect on the brake pistons. The EMF actuator consists of a servomotor and gearbox.
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 nut in the brake piston ensures 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 retaining force. The vehicle is also securely held in position when in a de-energized state.
[more ...]
Parking brake control unit
The parking brake control unit is located in the spare wheel well.
The EMF control unit receives the driver request to engage the brake via the parking brake button on the centre console. The vehicle condition is detected via the bus system. The EMF control unit decides whether all the preconditions for engaging have been complied with. If this is the case, the two EMF actuators on the rear brake calipers are activated.
The parking brake control unit is powered three times with terminal 30 from the power distribution box on the battery.
[more ...]
Parking brake button
The parking brake button is located in the centre console behind the gear selector switch (GWS). The design of the parking brake button is based on the operating 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.
[more ...]
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 parking brake (determined from the speeds of the rear wheels). The signal is transferred across a separate line. The signal is primarily used for roll monitoring.
The DSC also delivers the information via the pressed 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 via various engine conditions. The signal is transferred across the PT-CAN.
EGS: electric transmission control unit
The electrical 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.
- Gearbox output speed as fall-back with 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).
ZGM: Central gateway module
The central gateway module (ZGM) is the gateway between the powertrain CAN and FlexRay.
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 downwards. However, to actually open the parking brake, terminal 15 must be on and at least one of the following preconditions must be met:
- Brake pedal operated
- Parking lock of the automatic transmission engaged
- Clutch pedal operated (only vehicles with manual gearbox)
This prevents the vehicle rolling away, for example if another occupant presses the parking brake button instead of the driver. If the parking brake is released, the red indicator lights in the instrument panel and the red LED in the parking brake button no longer illuminate.
Activating the EMF actuator sets the spindle in motion. The spindle rotation moves the spindle nut away from the brake pistons.
Dynamic emergency braking
The law requires there to be two operating facilities for the brake system. Alongside the brake pedal, the second operating facility is the parking brake button in the centre console. If the parking brake button is pulled upwards whilst driving, dynamic emergency braking defined during the procedure is carried out by means of the Dynamic Stability Control (DSC). This function is intended only for emergencies when the driver can no longer decelerate using the brake pedal.
Other occupants are also able to bring the vehicle to a standstill in this manner, for example if the driver suddenly loses consciousness. During dynamic emergency braking, hydraulic brake pressure is built up at all four wheel brakes. The DSC functions are fully active and the brake lights are activated. This represents a major advantage when compared against manual parking brakes.
Dynamic emergency braking is only carried out as long as the parking brake button is pulled. The deceleration set by the DSC is increased gradually. During dynamic emergency braking, the parking brake indicator lights are activated in the instrument panel. In addition, a Check Control message is output together with an acoustic signal to inform the driver of this special situation.
If the driver decelerates by simultaneously pressing the brake pedal and pulling the parking brake button, the DSC control unit priorities the requests. The greater deceleration request is converted. If dynamic emergency braking is continued until the vehicle is at a standstill, the vehicle will remain stationary even after the parking brake button has been released. The parking brake indicator lights in the instrument panel remain activated. The driver can then release the parking brake again (see Releasing the parking brake).
Temperature monitoring
The temperature monitoring ensures that the reduction in power arising from cooling hot brake discs is compensated. Temperature monitoring is activated if the temperature exceeds a defined value when changing the state of the parking brake from released to engaged.
The temperature of the brake discs is calculated individually for each wheel by the DSC control unit and is transmitted to the parking brake control unit. During a state change, the higher of the two temperatures is used for temperature monitoring. The corresponding temperature ranges with the associated re-tensioning times are stored in a characteristic map.
Depending on the temperature during the state change, the corresponding re-tensioning times are activated from the characteristic map. If the first re-tensioning time is reached, re-tensioning is carried out for the first time.
Following the second re-tensioning time, re-tensioning is carried out again; following the third re-tensioning time re-tensioning is carried out once more. In the characteristic map, the value 0 can also be entered for one or more re-tensioning time points. The corresponding re-tensioning processes will then not be carried out. The function ends when the last re-tensioning process is complete.
Roll monitoring
The roll monitoring function is intended to prevent the vehicle rolling away when the parking brake is engaged. Roll monitoring is always activated if the state of the parking brake changes from released to engaged and ends following a defined time period after this state change.
A signal from the DSC is used to detect any rolling. As soon as this signal detects that the vehicle is rolling during roll monitoring, the EMF actuators are immediately re-tensioned. This process involves the EMF actuators being fully supplied with current for 100 ms to increase the tensioning power. A waiting period of 400 ms then follows. If the vehicle rolls again, the re-tensioning process is repeated (maximum three times). If the system detects that the vehicle is still rolling after the third re-tensioning process, the functions ends with a fault entry in the fault memory.
Safeguard at a standstill
This function activates mechanical clamping if the vehicle must additionally be secured for hydraulic hold functions (Automatic Hold, Active Cruise Control with Stop&Go function), for example if the driver leaves the vehicle. The DSC control units detects if the driver leaves the hydraulically secured vehicle (CAN messages, e.g. seat belt buckle contact, door contact). The DSC then requests that the parking brake engage additional mechanical braking.
Notes for Service department
General notes
The Integrated Chassis Management (ICM) identifies the brake test stand using a plausibility check (wheel-speed comparison). The parking brake control unit changes to the brake test stand mode. The identification process 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 service function "workshop mode" must be carried out. 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 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 automatically exited at a driving speed above 3 km/h.
- Start-up
Procedure:
- Ignition ON
- Apply strong pressure to the brake pedal
- Hold the brake pedal pressed down
- Pull the parking brake button for approx. 1 second (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.