Electric Parking Brake, Function - GF42.20-P-0001RD
Model 172.4
up to Model year 2017
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.
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 electric parking Brake automatic release/application on vehicles with defective electric parking Brake switch (for vehicles with manual transmission)
- 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 the chassis CAN by the electronic ignition lock control unit.
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). 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 disengaged, when the following conditions are fulfilled on vehicles with manual transmission:
- The driver door is closed.
- The engine is running.
- A gear is engaged and you are pressing the accelerator pedal.
- When reverse gear R is engaged, the trunk lid must be closed.
The electric parking Brake is disengaged, when the following conditions are fulfilled on vehicles with automatic transmission:
- The engine is running.
- The belt lock tongue is inserted into the seat belt buckle.
- The transmission is in position D or R and you are pressing the accelerator pedal or you shift on the flat from transmission position P into D or R.
- When reverse gear R is engaged, the trunk lid must be closed.
If the belt lock tongue is not inserted into the seat belt buckle, the following conditions must be fulfilled:
- The driver door is closed.
- You exit transmission position P or you were driving beforehand faster than 3 km/h.
- When the transmission is in position R, the trunk lid must be closed.
Function sequence for automatically release electric parking Brake (Comfort-Go)
The Release parking Brake automatically function (Comfort Go) is executed, as soon as a sufficient level of drive torque is available at the wheels on the rear axle to enable the vehicle to drive off.
The following signals for the individual control units are sent over each CAN bus to the electric parking Brake control unit:
- Accelerator pedal position (via the accelerator pedal sensor) and "Circuit 61 ON" status, from the CDI control unit (with diesel engine) or the ME-SFI control unit (with gasoline engine) via the chassis CAN
- Current gear range, from the electronic selector lever module control unit (with code 427 (7G-TRONIC)) or from the DIRECT SELECT INTERFACE (with code 421 (9G-TRONIC)) via the drive train CAN, the fully integrated transmission control unit, the drive train CAN and the chassis CAN with interface for CDI control unit (with diesel engine) or ME-SFI control unit (with gasoline engine)
- Road gradient (with vehicle stationary), from the turn rate, lateral acceleration and longitudinal acceleration sensor (with code 233 (DISTRONIC PLUS)) via the vehicle dynamics CAN and the chassis CAN or directly from the Electronic Stability Program control unit (except code 233 (DISTRONIC PLUS)) via the chassis CAN
- Total transmission ratio, from the fully integrated transmission control unit via the drive train CAN and the chassis CAN with the interface for CDI control unit (with diesel engine) or ME-SFI control unit (with gasoline engine)
Status messages from the driver door (from left door control unit) and the trunk lid (from rear SAM control unit with fuse and relay module) are routed over the interior CAN to the front SAM control unit with fuse and relay module. The front SAM control unit with fuse and relay module sends 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 chassis CAN.
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.
Function sequence for electric parking Brake automatic release/application on vehicles with defective electric parking Brake switch (for vehicles with manual transmission)
To ensure that it remains possible to start off even when the electric parking Brake switch is faulty, automatic release of the electric parking Brake occurs (as with the Comfort-Go function) after the release signal from the Electronic Stability Program control unit is received. As soon as sufficient drive torque is present at the wheels of the rear axle, the vehicle is accelerated from stationary in the direction of travel. The signal flow from the control units involved is the same as that for the Comfort-Go function; however, a reduction in comfort must be expected during the start-off procedure (emergency function). To prevent the vehicle from rolling away on uphill and downhill gradients, the electric parking Brake is applied automatically after the vehicle is switched off (circuit 15 OFF).
Additional function requirement for emergency braking
- Vehicle speed v > 4 km/h
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 chassis CAN.
The request is received by the following control units:
- Electronic Stability Program control unit
- Instrument cluster
- Front SAM control unit with fuse and relay module (gateway)
- Rear SAM control unit with fuse and relay module via the interior CAN from the front SAM control unit with fuse and relay module
The vehicle is now Braked via the service Brake. The rear SAM control unit with fuse and relay module actuates 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 Electronic Stability Program malfunctions, Brake application occurs exclusively through the electric parking Brake control unit by means of actuation of the left and right electric parking Brake actuator motors on the rear axle. 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
- The front wheels are "stationary" and the rear wheels turn slowly (roller Brake test stand).
- The test must be performed after bedding-in the service Brake.
- Engine running ("Circuit 61 ON" status).
- 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 rollers of the test stand are activated, the electric parking Brake control unit automatically recognizes this based on the RPM difference between the rear wheels and the front wheels and switches to the roller dynamometer test mode.
When roller dynamometer test 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 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"
Function sequence - child safety lock
Pressing the electric parking Brake switch with the 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 operator'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! |
| Conditions for automatic release not met OR Emergency stop with 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-2050-97TAA | ||
| Overview of system components for electric parking Brake | GF42.20-P-9998RD |