Electric Parking Brake, Function - GF42.20-P-0001LFM
Model 222
as of 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 Electronic Stability Program 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 releasing or engaging parking brake (manually, through electric parking brake switch)
- Function sequence for automatic releasing of parking brake (Comfort-Go)
- Function sequence for automatic engaging of parking brake
- 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 releasing or engaging parking brake (manually, through electric parking brake switch)
The electronic parking brake (EFB) can be manually operated using the electronic parking brake (EFB) switch, as follows.
Press electronic parking brake (EFB) switch: The EFB is applied.
Pull electronic parking brake (EFB) switch: The EFB is completely released.
The Electronic Stability Program control unit directly reads in the status of the electric parking brake switch. The Electronic Stability Program control unit then actuates the electric parking brake actuator motors for this purpose. The electronic parking brake (EFB) is applied or completely released. If the electronic parking brake (EFB) is applied or released, the Electronic Stability Program control unit sends the request to switch the parking brake indicator lamp (red) on or off over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster. The parking brake indicator lamp (red) on the instrument cluster then lights up or goes off.
Additional function sequences - automatically release parking brake (Comfort-Go)
- Engine running
- Driver seat belt fitted or driver front door closed (driver presence check)
The CDI control unit or the ME-SFI [ME] control unit sends the "Engine running" signal over the engine CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.
The supplemental restraint system control unit sends the status of the driver seat belt buckle switch over the vehicle dynamics CAN to the Electronic Stability Program control unit.
The left front door control unit sends the status of the driver front door over the interior CAN, electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit.
Function sequence for automatic releasing of parking brake (Comfort-Go)
As soon as a sufficient level of drive torque is applied to the wheels on the rear axle, to enable it to drive off, the Electronic Stability Program control unit completely releases the electronic parking brake (EFB).
The following conditions are taken into consideration here:
- Driver seat belt fitted:
- Transmission in position "D" or "R" and accelerator pedal operated
- Transmission in position "R" and trunk lid closed
- Driver seat belt not fitted and driver front door closed:
- Transmission in position "R" and trunk lid closed
- Change of transmission position from "P" or vehicle speed v > 3 km/h
The Electronic Stability Program control unit evaluates the starting point by evaluating the following variables:
- Selector lever position
The steering column tube module control unit sends the current selector lever position over the chassis FlexRay to the Electronic Stability Program control unit.
- Drive position
The fully integrated transmission control unit sends information about the engaged gear range via drive train CAN, powertrain control unit and chassis FlexRay to the Electronic Stability Program control unit.
- Accelerator pedal position
The powertrain control unit sends information on the accelerator pedal position over the chassis FlexRay to the Electronic Stability Program control unit.
- Status of trunk lid
The rear SAM control unit sends the status of the trunk lid over the interior CAN, electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit.
- Yaw rate lateral and longitudinal acceleration road inclination (at vehicle standstill)
Information on the yaw rate, lateral and longitudinal acceleration as well as the road inclination is recorded by the Supplemental Restraint System control unit. The supplemental restraint system control unit sends this information via the vehicle dynamics CAN to the Electronic Stability Program control unit.
- Engine torque
The CDI control unit or the ME-SFI [ME] control unit sends information about the engine torque over the drive train CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.
On uphill gradients over 5 % the engine torque is also evaluated.
The electronic parking brake (EFB) is only released at a sufficiently high level of engine torque.
- Vehicle speed
The Electronic Stability Program control unit calculates the vehicle speed from the wheel speeds. The wheel speeds are detected by the rpm sensors. The Electronic Stability Program control unit directly reads in the signals from the rpm sensors.
The Electronic Stability Program control unit releases the electronic parking brake (EFB) through actuation of the electronic parking brake actuator motors on the rear brake calipers.
If a fault prevents the electronic parking brake from being released, the instrument cluster displays the "Release parking brake" message and a warning tone is sounded.
Function sequence for automatic engaging of parking brake
If the vehicle has been brought to a standstill, the engine switched off and the driver wishes to get out of the vehicle, the Electronic Stability Program control unit automatically actuates the electronic parking brake actuator motors. The electronic parking brake (EFB) is activated.
The Electronic Stability Program control unit evaluates the following variables for this:
- Engine running status
The CDI control unit or the ME-SFI [ME] control unit sends the
"Engine running" signal over the engine CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.
- Status of driver seat belt
The supplemental restraint system control unit sends the status of the driver seat belt over the vehicle dynamics CAN to the Electronic Stability Program control unit.
- Status of driver front door
The left front door control unit sends the status of the driver front door over the interior CAN, electronic ignition lock control unit and the chassis FlexRay to the Electronic Stability Program control unit.
- Request to activate electronic parking brake (EFB) through driver assistance system
- Request through HOLD function:
The HOLD function is activated by quickly pressing down hard on the actuated brake pedal after coming to a standstill. The Electronic Stability Program control unit activates the electronic parking brake (EFB).
- Request from Distance Pilot (with code 23P (Driving Assistance Package)):
The Mercedes-Benz Intelligent Drive control unit sends the "Activate electronic parking brake (EFB)" request over the suspension FlexRay to the Electronic Stability Program control unit.
- Request through Parking Assist (with code 235 (Active Parking Assist with PARKTRONIC) or with code P44 (Parking Package)):
The parking system control unit sends the "Activate electronic parking brake (EFB)" request over the chassis FlexRay to the Electronic Stability Program control unit.
- Request through HOLD function:
The Electronic Stability Program control unit activates the electronic parking brake (EFB), when a system malfunction occurs or the power supply breaks down.
If the electronic parking brake (EFB) is activated, the parking brake indicator lamp (red) is shown on the instrument cluster.
The Electronic Stability Program control unit sends the status of the electronic parking brake (EFB) over the chassis FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster.
If the engine is switched off by the ECO start/stop function, the electronic parking brake (EFB) is not automatically activated.
The CDI control unit or the ME-SFI [ME] control unit sends drivetrain operational signal over the engine CAN, powertrain control unit and the chassis FlexRay to the Electronic Stability Program control unit.
Operating the electronic parking brake (EFB) switch can prevent the electronic parking brake (EFB) from being activated.
Function sequence for emergency braking
If the electronic parking brake (EFB) switch is operated and held while driving, the Electronic Stability Program control unit interprets this as a brake request and brakes the vehicle through the service brake. At the same time, the Electronic Stability Program control unit sends the status of the service brake over the chassis FlexRay, electronic ignition lock control unit and the interior CAN to the rear SAM control unit. The rear SAM control unit then activates the brake light actuation.
The Electronic Stability Program control unit then sends the request to issue a message over the chassis FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster. The instrument cluster displays the message "Release parking brake" and it emits a warning tone.
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 in the Electronic Stability Program control unit to allow the EFB to be tested on a brake test stand.
If the vehicle is on the brake test stand and the rollers of the brake test stand are activated, the Electronic Stability Program control unit detects this automatically from the rpm difference between the rear wheels and front wheels and then switches to roller dynamometer mode.
When roller dynamometer test mode is activated, the inspector can gradually increase the tension force of the EFB 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 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
With the ignition key removed or circuit 15R OFF, pressing the electric parking brake switch will not cause the Electronic Stability Program 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-2050-97SEB | ||
| Overview of system components for electric parking brake | GF42.20-P-9998LF |