GF42.20-P-0001LE Electric Parking Brake, Function
GF42.20-P-0001LE Electric parking brake, function
Function requirements - electric parking brake (EF6), general
^ C. 30 ON
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 EFB operates the parking brake at the rear axle via cables by an electric motor integrated in the electric parking brake controller unit (A13).
Activation takes place manually via the electric parking brake switch (S76/15) or automatically depending on the driving condition.
Depending on the operating condition of the parking brake the driver's wish is converted into a corresponding tension force build up or reduction at the actuators.
Mechanical release makes it possible to release the electronic parking brake in the event of a defect and in the de-energized condition. The release cable is located at the fuel filler flap and can be operated using the vehicle tool kit.
The electronic parking brake (EFB) function is comprised of the following partial functions:
^ Function sequence - apply and release parking brake
^ Function sequence - release parking brake automatically
^ Function sequence - emergency braking
^ Function sequence - dynamometer test mode
Additional function requirements - applying and releasing parking brake
^ Vehicle speed v < 3.5 km/h
^ Terminal 15R On or with code (889) Keyless Go terminal 15 On (only for releasing the parking brake)
Function sequence - applying and releasing the parking brake
The electric parking brake controller unit calculates the vehicle speed from the wheel speeds which the ESP control unit (N47-5) provides on the chassis CAN.
The EIS [EZS] control unit makes available the circuit status circuit 15 on the chassis CAN N73
The central gateway control unit (N93) passes on all the relevant information to the electric parking brake controller unit via the front end CAN
By operating the electric parking brake switch, applying or releasing the parking brake is requested corresponding to the operating condition.
The electric parking brake controller unit reads in the status of the electric parking brake switch directly, transmits this on the front end CAN and actuates the integrated electric motor accordingly. The parking brake is applied or released via the brake cables. The operating period of the electric parking brake switch does not have any influence on the function.
The central gateway control unit passes on the status of the electric parking brake switch to the instrument cluster (A1) via the central CAN. The instrument cluster receives this and actuates the electric parking brake warning light (red).
Additional function requirements - release parking brake automatically
^ Engine running (circuit 61 ON)
^ Selector lever position "D" or "R"
^ Engine hood closed
^ Trunk lid closed (only for selector lever position "R")
^ Driver door closed (only if driver is not wearing the seat belt)
Function sequence - release parking brake automatically
The function of release parking brake automatically is carried out as soon as the accelerator pedal position exceeds a certain threshold value. The accelerator pedal position is recorded via the accelerator pedal sensor (B37), the signals of which are read in by the CDI control unit (N3/9) (with diesel engine) or by the motor electronics control unit (N3/10) (with gasoline engine).
The CDI control unit or motor electronics transmits circuit status circuit 61 and the accelerator pedal position on the chassis CAN. The intelligent servo module for DIRECT SELECT (A80) transmits the current transmission selector lever on the drive train CAN. The CDI control unit or motor electronics control unit passes this on on the chassis CAN.
The left door control unit (N69/1) (model 216) or left front door control unit (N69/1) (model 221) makes available the status of the driver door on the interior CAN.
The rear SAM control unit with fuse and relay module (N10/2) transmits the status of the trunk lid and engine hood on the interior CAN.
The central gateway control unit passes on all the relevant information to the electric parking brake controller unit via the front end CAN
The electric parking brake controller unit is woken up by the corresponding CAN messages and releases the parking brake by actuating the integrated electric motor.
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 (A1p13) and in addition an acoustic warning sounds.
If circuit 30 has been interrupted automatic release is not possible. The electronic parking brake must be woken up via the electric parking brake switch or circuit 87. The electric parking brake controller unit receives the signal circuit 87 directly from the front SAM control unit with fuse and relay module (N10/1).
Additional function requirements - emergency braking
^ Vehicle speed v > 3.5 km/h
(calculated by electric parking brake controller unit)
Function sequence - emergency braking
If the electric parking brake switch is operated and held while driving, the electric parking brake controller unit transmits a brake request on the front end CAN. The central gateway control unit passes this on to the following control units:
- ESP control unit via the chassis CAN
- Instrument cluster via the central CAN
- Rear SAM control unit with fuse and relay module via 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 gives the message "Release parking brake" via the multifunction display and in addition an acoustic warning sounds.
In the event of a service brake malfunction (e.g. CAN communication faulty) the brake application only takes place by the electric parking brake control unit via the parking brake. For this the electric parking brake controller unit evaluates the signals from the left front rpm sensor (L6/1), right front rpm sensor (L6/2), left rear rpm sensor (L6/3) and right rear rpm sensor (L6/4) in order to prevent the rear wheels locking. The signals from the rpm sensors are provided on the chassis CAN by the ESP control unit and passed on to the electric parking brake controller unit by the central gateway control unit via the front end CAN.
The request from the brake light actuation is likewise issued by the electric parking brake controller unit.
In addition the electric parking brake warning lamp (yellow) is actuated in the instrument cluster. The electric parking brake controller unit transmits corresponding requests on the front end CAN. The central gateway control unit passes these on to the instrument cluster via the central CAN.
Additional function requirements - test bench mode
^ Front wheel "stationary" and rear wheels turn slowly (roller brake test stand)
^ ESP Off switch (S6/2s11) operated
Function sequence - dynamometer test mode
Activation of the roller test mode prevents faults being stored in the ESP control unit.
The electric parking brake controller unit is actuated by operating the electric parking brake switch.
The force is built up at the brake cables in a constantly rising manner until the maximum force is applied.
As a result the deceleration values of the parking brake can be recorded and the brakes of the parking brake bedded in if necessary.
The process can be repeated up to 3-times, in which a cooling off time should be maintained between the brake applications.
To replace the brake cables the electronic parking brake is either brought into the assembly position or unlocked manually. Once the work is complete the electronic parking brake is again put into the operational condition by pressing the electric parking brake switch.
