Electric Parking Brake, Function - GF42.20-P-0001GN
Model 166
as of model year 2016
Model 292
Block diagram
Function requirements - electric parking brake (EFB), general
- Status "Circuit 15R ON" (release parking brake)
Electronic parking brake (EFB), general
The electronic parking brake makes it possible to hold the vehicle at a standstill on gradients even in the absence of the driver, the braking parts being held de-energized in the brake position by a self-locking mechanical effect. The electric parking brake control unit actuates the left electric parking brake actuator motor and the right electric parking brake actuator motor.
Activation takes place manually via the electric parking brake switch or automatically depending on the driving condition.
Function sequence for emergency braking
If the electric parking brake switch is pressed and held down while driving, the electric parking brake control unit sends a brake request on chassis CAN 1.
The request is received by the following control units:
- Electronic Stability Program control unit
- Instrument cluster
- Electronic ignition switch control unit
- SAM control unit via the interior CAN from electronic ignition lock control unit
Depending on the operating condition of the electric parking brake (EFB) the driver's wish is converted into a corresponding tension force build up or reduction in the actuator motors.
The electronic parking brake (EFB) consists of the following subfunctions:
- Function sequence for release electric parking brake (manually, via electric parking brake switch)
- Function sequence for automatically release electric parking brake (Comfort-Go)
- Function sequence for emergency braking
- Function sequence for dynamometer test mode
- Function sequence - child safety lock
- Function sequence - display concept/acoustic signal
Function sequence for release electric parking brake (manually, via electric parking brake switch)
The "Circuit 15R ON" status which is required for releasing the electric parking brake is made available on chassis CAN 1 by the electronic ignition lock control unit. The electric parking brake release is requested by actuating the electric parking brake switch. The electric parking brake control unit reads in the status of the electric parking brake switch directly.
Once the electric parking brake control unit has actuated the left and right electric parking brake actuator motors at the same time and the EFB is fully released, it generates a signal for the parking brake indicator lamp (red). The parking brake indicator lamp (red) located in the instrument cluster then goes out.
The electric parking brake switch must be actuated for a sufficient length of time for release to be detected.
Additional function requirements for automatically release parking brake (Comfort-Go)
The electric parking brake is released if the following conditions are satisfied:
- Engine running.
- Selector lever position "D" or "R" is engaged.
- Liftgate is closed (with selector lever position "R").
- Driver seat belt inserted.
- The driver actuates accelerator pedal
When the driver seat belt is not fastened, the electric parking brake is released if the following conditions are satisfied:
- Driver door is closed.
- Change of gear from selector lever position "P" or if vehicle speed of 4 km/h previously exceeded
Function sequence for automatically release electric parking brake (Comfort-Go)
The automatically release electric parking brake function is performed once there is a sufficiently high drive torque on the rear axle wheels to overcome the prevailing grade resistance corresponding to the road gradient.
The following signals from the individual control units are sent via the respective CAN buses to the electric parking brake control unit:
- Accelerator pedal position (via the accelerator pedal sensor), drive train and "engine running" status, from the CDI control unit (with diesel engine) or ME-SFI [ME] control unit (with gasoline engine) via chassis CAN 1
- Current selector lever position provided by DIRECT SELECT-lever to steering column tube module control unit via the chassis CAN 1
- Road gradient (vehicle at a standstill) provided by yaw rate sensor for lateral and longitudinal acceleration over the vehicle dynamics CAN, Electronic Stability Program control unit and chassis-CAN 1
- Total transmission ratio provided by fully integrated transmission control unit via the drive train CAN and chassis CAN 1 with the CDI (with diesel engine) or ME-SFI [ME] (with gasoline engine) control unit interface
The driver door status messages (from left front door control unit) and the liftgate status messages (from rear control unit) are sent via interior CAN to the electronic ignition lock control unit. The electronic ignition lock control unit forwards all messages to the Electronic Stability Program control unit for evaluation. Together with the required wheel speed signals, the Electronic Stability Program control unit puts this information on chassis CAN 1.
The electric parking brake control unit is woken by the corresponding CAN message and releases the parking brake by actuating the integrated electric motors on both rear brake calipers. If the electronic parking brake is not released as a result of a fault, the instrument cluster outputs the message "Release parking brake" via the multifunction display and in addition an acoustic warning sounds.
Additional function requirement for emergency braking
- Vehicle speed v > 4 km/h
The vehicle is now braked via the service brake. The SAM control unit activates the brake light actuation. The instrument cluster issues the message "Release parking brake" on the multifunction display and a warning tone is sounded.
If the ESP® malfunctions (e.g. CAN communication interrupted, the parking brake warning lamp lights up (yellow) - the electric parking brake control unit sends a corresponding request to chassis CAN 1), brake application occurs only via the electric parking brake control unit by means of actuation of the left and right electric parking brake actuator motors at the rear axle service brake. The electric parking brake control unit evaluates the rear wheel speeds for this purpose in order to prevent the rear wheels from locking up. The request for brake light actuation is likewise issued by the electric parking brake control unit.
Additional function requirements for roller brake test stand
- Front wheels "stationary" and rear wheels turn slowly (brake test stand).
- A test must be performed after bedding-in the service brake.
- Engine running.
- Selector lever position "N"
Function sequence for dynamometer test mode
A roller brake dynamometer is integrated into the electric parking brake control unit for the purpose of testing the EFB on a brake test stand. If the vehicle is on the roller brake test stand and the rollers of the roller brake test stand are activated, the electric parking brake control unit automatically recognizes this from the rpm difference between the rear wheels and the front wheels and switches to the roller test stand mode. When roller test stand mode is activated, the inspector can gradually raise the tension force of the electric parking brake by actuating the electric parking brake switch multiple times until the rear axle wheels lift out of the rollers of the roller brake test stand at a correspondingly high tension force.
The following variables/values can be determined by checking a vehicle on the roller brake test stand:
- Rolling resistance
- Braking power
- Deviation in brake force between right and left rear wheel
- Start of locking tendency
Function requirements for child safety lock
- Status "Circuit 15R OFF"
- Ignition key removed
Function sequence - child safety lock
Pressing the electric parking brake switch with the ignition key removed or "Circuit 15R OFF", will not cause the electric parking brake control unit to actuate release of the left and right electric parking brake actuator motors.
This should prevent unintended deactivation of the electric parking brake and, as such, serves as an additional protection against the vehicle rolling away on slopes or inclines.
Function requirements for display concept/acoustic signal
- Circuit 30
Function sequence - display concept/acoustic signal
The following display tools are available for the display of function and condition of the electric parking brake:
- Parking brake indicator lamp (red)
- Parking brake warning lamp (yellow)
- Various status-/warning messages over the multifunction display in instrument cluster
- Acoustic warnings
The different displays and acoustic signals are actuated depending on the current operating condition of the electric parking brake.
| Error condition | Display actuation | Display | Acoustic signal | Message in multifunction display |
|---|---|---|---|---|
| Electric parking brake actuated | Permanently | Parking brake indicator lamp (red) | - | - |
| Faulty electric parking brake (permanent fault, service required) | Continuously/flashing | Parking brake warning lamp (yellow)/parking brake indicator lamp (red) | - | Parking brake "P" See owner"s manual! |
| Electric parking brake malfunction, electric parking brake inoperative | Flashing | Parking brake warning lamp (yellow) | - | Parking brake inoperative! "P" |
| Attempted electric parking brake release in child safety mode, ignition must be switched on ("circuit 15R ON") | Permanently | Parking brake indicator lamp (red) | - | Parking brake! "P" To release, switch on ignition! |
| Requirements for automatic release not fulfilled OR emergency braking with the electric parking brake | Flashing | Parking brake indicator lamp (red) | Bleep-interval | Parking brake! "P" Release brake! |
| Electrical function schematic for electric parking brake | PE42.20-P-2050-97NBA | ||
| Overview of system components for electric parking brake | GF42.20-P-9998GN |