Electric Parking Brake, Function - GF42.20-P-0001FG

Model 213 

up to model year 2021 

Block diagram 

Courtesy of MERCEDES-BENZ USA

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.

• Total transmission ratio, from the fully integrated transmission control unit (with code 421 (9G-TRONIC)) over the drive train CAN, powertrain control unit and the chassis FlexRay

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/liftgate status 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 chassis FlexRay.

The Electronic Stability Program control unit is woken by the corresponding FlexRay or CAN messages and releases the parking brake by actuating the integrated actuator motors on both rear brake calipers. If a fault prevents the electric parking brake from being

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 electronic ignition lock control unit makes the "circuit 15R ON" status, which is required for releasing the parking brake, available on the chassis FlexRay for the Electronic Stability Program control unit.

The parking brake release is requested by actuating the electric parking brake switch. The Electronic Stability Program control unit directly reads in the status of the electric parking brake switch.

As soon as the Electronic Stability Program control unit has actuated the left and right electric parking brake actuator motors at the same time and the EFB is fully released, the control unit generates a signal for the parking brake indicator (red). The parking brake indicator (red) 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 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 press the accelerator pedal.
• If reverse gear R is engaged, the trunk lid must be closed.

The electric parking brake is released 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 press the accelerator pedal or you shift on the flat from transmission position P into D or R.
• If 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 drove beforehand faster than 3 km/h.
• If the transmission is in position R, the trunk lid must be closed.

Function sequence for automatically release electric parking brake (Comfort-Go) 

The release automatic parking brake function (Comfort Go) is performed, as soon as a sufficient level of drive torque is applied to the wheels on the rear axle, to enable the vehicle to drive off.

The following signals from the individual control units are sent via CAN bus or FlexRay to the Electronic Stability Program 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 control unit (with gasoline engine) via the engine CAN, powertrain control unit and chassis FlexRay
• Current selector lever position, from the DIRECT SELECT selector lever to the steering column tube module control unit, via the chassis FlexRay
• Road gradient (with vehicle stationary), from the Supplemental Restraint System control unit, via the vehicle dynamics CAN, to the Electronic Stability Program control unit 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 stability program control unit detects a brake request.

The request is received by the following control units:

• From the instrument cluster
• From the rear SAM control unit

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.

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.
• The selector lever is in 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 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"
• With the key removed

Function sequence - child safety lock 

With the 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 (red)
• Parking brake warning indicator (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.