Electric Parking Brake, Function - GF42.20-P-0001RR
Model 190
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 "Circuit 15R ON" status which is required for releasing the parking brake is made available on chassis CAN 1 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) then goes out.
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.
- 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.
- 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 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), drive train and engine running status, from the ME-SFI control unit via the engine CAN, powertrain control unit and chassis CAN 1
- Current selector lever position, from AMG gateway control unit over the chassis CAN 1
- Road gradient (vehicle at a standstill), from turn rate, lateral acceleration and longitudinal acceleration sensor, via the vehicle dynamics CAN, Electronic Stability Program control unit and chassis CAN 1
- Total transmission ratio, from dual clutch transmission control unit over the drive train CAN and chassis CAN 1 using powertrain control unit interface
The driver door status message (from the left door control unit) is routed to the electronic ignition lock control unit via the interior CAN. The liftgate status message is read in directly by the rear SAM control unit with fuse and relay module. The rear SAM control unit with fuse and relay module sends 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 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 actuated and held down while driving, the electric parking brake control unit transmits a brake request on chassis CAN 1.
The request is received by the following control units:
- From the Electronic Stability Program control unit
- From the instrument cluster over the AMG gateway control unit and the user interface CAN
- From the rear SAM control unit with fuse and relay module 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 with fuse and relay module actuates 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. communication interrupted, parking brake warning lamp (yellow) lights up - a corresponding request is sent by the electric parking brake control unit over the chassis CAN 1), the brake is applied solely over the electric parking brake control unit by actuating the left and right electric parking brake actuator motors on 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 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 brake test stand rollers 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 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"
- 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)
- 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 | Instrument cluster message |
|---|---|---|---|---|
| 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 release of electric parking brake in child safety lock 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-2050-97HBA | ||
| Overview of system components for electric parking brake | GF42.20-P-9998RR |