Part 1
GF42.46-P-0001RI Sensor Brake Control (SBC), function
MODEL 230.4
except CODE (P99) Special model "AMG Black Series"
as of Model Year 2009
/YoM 08 MOPF
General function requirements
^ Circuit 61 ON (Sensor Brake Control (SBC) and Electronic Stability Program (ESP) active)
When the interior CAN is active, the SBC functions are available regardless of the status of circuit 61.
SBC, general
SBC assists the driver in dangerous situations which occur suddenly and thus serves active safety.
SBC is an electro hydraulic brake system which controls the braking requests for each wheel individually.
The following components are involved:
- SBC hydraulic unit (A7/3) with SBC control unit (A7/3n1)
- ESP, SPS [PML] and BAS control unit ()N47-5
- Steering angle sensor (N49) and steering column tube module (N80)
- Yaw rate and lateral acceleration sensor (B24/15)
- SBC pedal value sensor (B37/1)
- Left front rpm sensor (/)L6/1
- Right front rpm sensor (/)L6/2
- Left rear rpm sensor (/)L6/3
- Right rear rpm sensor (/)L6/4
The system is made up of the following component functions:
^ SBC function
^ ESP function
SBC function
The SBC control unit evaluates the basic information (initial pressure and pedal travel) to determine the driver's braking request and transmits this information to the ESP control unit via the electro hydraulic brake CAN.
Based on these values, the ESP control unit calculates the specific target pressure in the SBC hydraulic unit.
The brake pressure, which is individually calculated for each wheel based on the control algorithms, is sent to the SBC control unit by the ESP control unit via the electro hydraulic brake CAN.
The pressure regulators in the SBC hydraulic unit adjust the pressure in the corresponding brake circuits to the required brake pressure individually for each wheel.
In the event of failure of the ESP control unit or of the electro hydraulic brake CAN or in the event of communication faults between the control units, the SBC hydraulic unit provides a basic braking function without the functionality of the ESP control unit.
The SBC function is comprised of the following sub functions:
^ SBC system activation function sequence
^ SBC system deactivation function sequence
^ SBC operational states
^ SBC function scope
^ SBC additional support functions function sequence
SBC system activation function sequence
The SBC function can be activated by various wake-up events:
- Circuit 15 On
- Brake light switch (S9/1) actuated
- Parking brake indicator switch (S12) actuated
- Opening a vehicle door
- Locking the vehicle via the central locking (CL [ZV])
The SBC control unit performs a self-test straight after it is activated.
During the initialization period which is required to initialize the system (e.g. for sensor adjustment), the system is not fully operational. When the system is woken up via the brake light switch, an increased pedal travel is therefore adjusted during the first pedal operation. The pedal can pulsate when it is subsequently released. The next time the brake pedal is actuated, the pedal travel is normal. If there are no system errors, the system becomes operational and starts carrying out its functions.
The ESP control unit and the SBC control unit perform further partial tests at regular intervals.
The corresponding fall back level is actuated depending on the faults detected and a fault message is displayed in the instrument cluster (A1).
SBC system deactivation function sequence
SBC is deactivated after:
- t = 20 s, after global vehicle locking
- t = 2 min, e.g. after circuit 15R OFF
- t = 4 min, e.g. after service brake is released with circuit 15R OFF
The system is not deactivated as long as the vehicle is rolling or the service brake is operated.
SBC operational states
- SBC system is fully active including ESP
- SBC system is capable of operating to a limited extent, with emergency ABS, without ESP and without ASR
- SBC system provides a basic braking function (without ESP, ASR, BAS, ABS and ETS) i.e. only the brake booster function is active (preliminary and run-on phase with shutoff of the above specified sub functions)
- SBC system is in a fall back level (reduced or no brake booster function) i.e. brake pressure is only built up via foot force at the front wheels without brake boosting
SBC function scope
- Brake boosting during driving
- Brake boosting during preliminary and run-on phase
- Individual pressure regulation for each wheel
- Safety software
The safety software monitors the following electrical components and their inputs and outputs:
- Rpm sensors, accelerator pedal sensor (B37) and SBC pedal value sensor
- Valve paths
- Pump motor path
- Supply path
- Hardware of ESP control unit and SBC control unit
In addition, the safety software monitors the plausibility of the signals of the following components:
- Accelerator pedal sensor and SBC pedal value sensor
- Reservoir monitoring
- Modulator monitoring
- Plausibility of pressure request relative to actual pressure at individual wheel = inference of valve fault
The safety software also performs active tests during initialization (Pre-Drive Check) and while driving (Drive Check) (check of pressure sensors and valves).
SBC additional support functions function sequence
The additional support functions of the SBC system consist of the following sub functions:
^ SBC Hold function sequence (not with code (494) USA version and code (460) Canada version)
^ recharging function sequence
^ recharging function sequence
^ Dry braking function sequence
SBC Hold function sequence
SBC Hold is a convenience function which further reduces the stress on the driver.
On vehicles with code (494) USA version and code (460) Canada version, the SBC Hold function is deactivated via the country coding.
SBC Hold makes life easier for the driver upon request:
- When starting off, particularly on mountain roads
- When maneuvering on mountain roads
- When waiting in traffic
When the vehicle is stationary, the driver can activate the SBC Hold function by pressing the brake pedal. The vehicle is then held in position without any further actuation of the brake pedal. When the vehicle starts off, the braking effect is canceled by actuation of the accelerator pedal.
SBC Hold can be activated when:
- The vehicle is stationary with the brake pedal actuated
- The driver door and the engine hood are closed
- The parking brake is disengaged
- The engine is running (circuit 61 ON)
- The electronic selector lever module control unit (N15/5) is not in the P position
The status of the driver door is recorded via the left door contact switch (S17/3) by the rear SAM control unit (N10/8). The status of the engine hood is recorded by the passenger-side SAM via the left ATA engine hood switch (S62) and the right ATA engine hood switch (S62/1).
The signals are transmitted over the interior CAN.
SBC Hold is activated when the brake pedal is depressed far enough that the "SBC H" indicator appears on the left multifunction display (A1p 13). The brake pedal can then be released. The vehicle is held in position automatically.
SBC Hold is deactivated automatically when:
- The accelerator pedal is actuated
- The electronic selector lever module control unit is shifted to position P
- The brake pedal is actuated again until the "SBC H" indicator disappears from the left multifunction display
The ESP control unit must be fully initialized for the SBC Hold function to be activated. When the vehicle is stationary and the SBC Hold function has been activated, a corresponding signal is placed on the electro hydraulic brake CAN. The passenger-side SAM control unit (N10/11) is informed of the successful activation via a separate line. Furthermore, all the specified pressures requested by the ESP control unit are monitored by the SBC control unit so that the pressure control function can be taken over by the SBC control unit at any time if necessary.
If no specified pressures can be transmitted by the ESP control unit to the SBC control unit in this state, the last specified pressures are used and maintained (electro hydraulic brake CAN inactive or faulty). The last specified pressures are also maintained in the event of a fault in the ESP control unit.
This is signaled to the SBC control unit via a fault bit. Faults in the ESP control unit whereby specified pressures can still be transmitted to the SBC control unit (e.g. yaw rate, lateral and longitudinal acceleration sensor defective) are handled by the additional support function module.
With circuit 15 OFF, holding of the brake pressure is finished when the driver secures the vehicle after circuit 15 OFF. The status of circuit 15 is placed on the engine compartment CAN by the EZS control unit (N73).
This either takes place when the electronic selector lever module control unit is moved to the P position or via the service brake when the brake pedal is actuated with sufficient pressure.
As soon as the P position is detected after SBC Hold was active, the SBC hydraulic unit is de energized after approx. t = 2 s. This causes a rapid pressure reduction.
The SBC hydraulic unit is then in hydraulic backup mode until it is woken up by a new wake-up event.
If there is a fault in the ESP control unit, the signal "Selector lever position P" is also transmitted via the electro hydraulic brake CAN to reduce the pressure.
If however it can no longer be transmitted, deactivation is then only possible via the service brake.
When the circuit status changes from circuit 15 OFF to circuit 15 ON during pressure holding, the pressures are communicated back to the ESP control unit as follows.
After initialization of the ESP control unit when the signal "SBC hydraulic unit holding pressure" is set, the SBC Hold function is automatically activated in the ESP control unit.
The SBC hydraulic unit recognizes the automatic activation based on the pressure specifications of the ESP control unit and the additional support function status bit on the electro hydraulic brake CAN. Monitoring of the specified pressures is then performed in the same way as described above in case of a further event resulting in pressure holding.
Faults in the SBC hydraulic unit that are relevant to the run-on period (faults whereby the SBC hydraulic unit cannot adjust pressures in the run-on period) result in immediate pressure reduction after circuit 15 OFF. To counteract this, the SBC control unit transmits a signal to the ESP control unit to prevent activation of SBC Hold.
In order to force deactivation of SBC Hold by the driver before he/she leaves the vehicle, visual and acoustic warning messages are output in various stages.
- Stage 1 (circuit 15 ON and door open)
The instruction "Move selector lever to position P" is displayed on the right multifunction display (A1p15).
- Stage 2 (circuit 15 OFF and door is or was open)
In addition to stage 1, the left fanfare horn (H2) and right fanfare horn (H2/1) (horns sounded to attract attention) are actuated by the passenger-side SAM control unit.
- Stage 3 and 4
The horns are actuated more vigorously to attract attention according to further events.
The horns stop always stop sounding as soon as the driver has deactivated SBC Hold.
The interior CAN is kept awake by the driver-side SAM control unit (N10/10) as long as SBC Hold is active.
SBC Hold on-board electrical system control:
If SBC Hold is active with circuit 15 OFF, the power supply for maintaining SBC Hold must be ensured. The starter battery (G1/4) may be used for this purpose under certain circumstances.
The driver-side SAM control unit transmits a corresponding signal defining the active status of SBC Hold on the interior CAN regardless of the status of circuit 15.