Electric Parking Brake, Function - GF42.20-P-0001LF
Model 217
Model 222
up to model year 2018
Block diagram
Function requirements - electric parking brake (EFB), general
- "Circuit 15R ON" status
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. 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 parking brake (manual, via electric parking brake switch)
- Function sequence for automatically release 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 parking brake (manual, via electric parking brake switch)
The "terminal 15R ON" status required to release the parking brake is sent by the electronic ignition lock control unit over the peripherals CAN.
The 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) symbol. The parking brake indicator lamp (red) symbol in the instrument cluster then goes off.
The electric parking brake switch must be actuated for a sufficient length of time for release to be detected.
Additional function sequences - 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.
- Trunk lid 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.
- Gear range changed from selector lever position "P"
- Vehicle speed exceeds 3 km/h
Function sequence for automatically release parking brake (Comfort-Go)
The automatically release 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 over the CAN bus or FlexRay to the electric parking brake control unit:
- Accelerator pedal position (through accelerator pedal sensor), drivetrain and engine running status, from CDI control unit or the ME-SFI [ME] control unit over the engine CAN, powertrain control unit and the peripherals CAN
- Current selector lever position, from DIRECT SELECT lever to the steering column tube module control unit, over suspension FlexRay, to electronic ignition lock control unit and over the periphery CAN
- Road inclination (at vehicle standstill), from turn rate, lateral acceleration and longitudinal acceleration sensor, over the vehicle dynamics CAN, to Electronic Stability Program control unit, over the suspension FlexRay, to electronic ignition lock control unit and over the periphery CAN
- Total transmission ratio, from fully integrated transmission control unit over the drive train CAN and the periphery CAN with the powertrain control unit interface
The driver door status message (from left front door control unit) is sent to the electronic ignition lock control unit over the interior CAN. The trunk lid status message is read in directly by the rear SAM control unit. The rear SAM control unit forwards all messages to the Electronic Stability Program control unit for evaluation. The Electronic Stability Program control unit places this information together with the required signals for the wheel speeds, on the suspension FlexRay.
The electric parking brake control unit is activated by the corresponding CAN messages, whereupon it disengages the parking brake through actuating the electric parking brake actuator motors at both rear brake calipers. If a fault prevents the electric parking brake from being released, the instrument cluster displays the "Release parking brake" message and a warning tone is sounded.
Function sequence for emergency braking
If the electric parking brake switch is pressed and held down while driving, the electronic parking brake control unit sends a brake request on the periphery CAN.
The request is received by the following control units:
- From the Electronic Stability Program control unit over the electronic ignition lock control unit and the suspension FlexRay
- From the instrument cluster over the electronic ignition lock control unit and the user interface CAN
- From the rear SAM control unit over the electronic ignition lock control unit and the interior CAN
The vehicle is now braked via the service brake. The rear SAM control unit activates the brake light actuation. The instrument cluster displays the "Release parking brake" message and a warning tone is also sounded.
If the Electronic Stability Program control unit malfunctions (e.g. CAN communication interrupted, the parking brake warning lamp symbol (yellow) lights up - the electric parking brake control unit sends a corresponding request on the periphery CAN), 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 wheel "stationary" and rear wheels turn slowly (roller 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 dynamometer test mode 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 brake test stand and the test stand rollers are activated, the electric parking brake control unit automatically detects this through the rpm difference between the rear wheels and the front wheels and switches to the roller test 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 evaluated by checking a vehicle on the 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 symbol (red)
- Parking brake warning lamp symbol (yellow)
- Diverse status/warning messages via the 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 | Multifunction display message |
|---|---|---|---|---|
| Electric parking brake actuated | Permanently | Parking brake indicator lamp symbol (red) | - | - |
| Faulty electric parking brake (permanent fault, service required) | Continuously/flashing | Parking brake warning lamp symbol (yellow)/parking brake indicator lamp symbol (red) | - | Parking brake "P" See operator's manual! |
| Electric parking brake malfunction, electric parking brake inoperative | Flashing | Parking brake warning lamp symbol (yellow) | - | Parking brake inoperative! "P" |
| Attempted release of electric parking brake in child safety lock mode, ignition must be switched on (circuit 15R ON) | Permanently | Parking brake indicator lamp symbol (red) | - | Parking brake! "P" To release switch on ignition! |
| Conditions for automatic release not met OR Emergency stop with parking brake |
Flashing | Parking brake indicator lamp symbol (red) | Bleep-interval | Parking brake! "P" Release brake! |
| Electrical function schematic for electric parking brake | PE42.20-P-2050-97SEA | ||
| Overview of system components for electric parking brake | GF42.20-P-9998LF |