Part 2

A precondition for general activation of SBC Hold is a corresponding signal which contains the status "Battery control active". The vehicle power supply control unit (N82/1) transmits this signal on the interior CAN after initialization. The central gateway control unit (N93) receives the signal and transmits it to the ESP control unit via the engine compartment CAN. The status signal is withdrawn in the event of general faults of the vehicle power supply control unit or in the event of faults which prevent power supply via the starter battery.

Plausibility checking of the status of the left door contact switch is used to determine faults in the signal path from the left door contact switch to the instrument cluster.

The status of the left door contact switch is read in by the rear SAM control unit and placed on the interior CAN.

If as a result of an electrical or mechanical fault "driver door closed" or "driver door open" is detected erroneously although this is not the case, this is detected by the plausibility logic in the instrument cluster and a corresponding fault is entered in the instrument cluster.

The driver can no longer activate the SBC Hold function or a warning message is issued in instrument cluster if SBC Hold is already active.

In the event of an attempt to activate SBC Hold, a fault is entered in the ESP control unit.

Precharging function sequence
The recharging function is constantly active and improves the braking power by applying pressure to the brake linings (recharging), when the driver switches quickly from the accelerator pedal to the brake pedal.

As a result, the response time of the brake is reduced and the stopping distance reduced during emergency stops.

To reduce the response time during emergency braking, the air gap of the wheel brake system is closed as soon as the accelerator pedal is released and the brake linings are applied with a pressure of p ≤ 3 bar.

If the driver subsequently depresses the brake pedal, a spontaneous braking effect is achieved and thus the stopping distance is reduced.

The accelerator pedal release gradient is used in the process to evaluate the requirement for rapid braking. The recharging process is terminated after a short time if the driver does not apply the brake.

The recharging function is implemented by the ESP control unit and is subdivided into two functions:

- Continuous detection of triggering threshold for brake recharging
- recharging of brake system

Continuous detection of triggering threshold:

If the boundary conditions are met, the associated release gradient is determined each time the accelerator pedal is released completely.

The triggering threshold does not change up to a defined value.

If the release gradient exceeds this range, the triggering threshold is also adjusted (it becomes less sensitive).

The aim of this procedure is to obtain a driver-specific triggering threshold.

Precharging of brake system:

The actual recharging function starts once the triggering threshold is exceeded. The strength of the function depends on the vehicle speed and the release gradient.

The following applies within specified thresholds:

The higher the vehicle speed or the quicker the accelerator pedal is released, the stronger the recharging function is performed. However, this is limited to a maximum value so that the driver is not irritated.

Precharging is not active in the case of:

- ESP faults (system passive)
- Speed v > 200 km/h
- Initialization of ESP control unit not yet complete
- Control interventions (ABS, ESP, ASR)
- ME-SFI [ME] control unit (N3/10) faults
- CAN fault
- Selector lever position N
- Standstill or filling pulse performed for t = 800 ms

Dry braking function sequence
This function is continuously active and improves braking performance in wet conditions. The response time of the brake is reduced by cyclical dry braking of the brake disks.

To control the dry braking function, the ESP control unit reads in the position of the windshield wipers via the "Windshield wiper outside park position" contact (circuit 31b). The dry braking function is triggered after 600 wipe cycles i.e. every t = 7 to 14 min depending on the wiper stage.

However, if the driver brakes before 600 wipe cycles are reached, the counter is set back by 200 cycles per second.

Dry braking is performed for t = 3 s with a brake pressure of p 1.5 bar only on the front axle with the pressure built up and reduced in a ramp-shaped pattern.

When 600 wipe cycles are reached, dry braking is only performed if the lateral acceleration is a 1 m/s, the vehicle speed is between v = 30 and 200 km/h and the pedal value of the accelerator pedal sensor is 5 %.

ESP function
ESP prevents breakaway when the vehicle oversteers or understeers. Within physical limits it ensures that the vehicle does not deviate from the course specified by the driver. Brake forces are produced selectively at the individual wheels to correct any deviations. Furthermore, reduction of the drive torque takes place in order to increase directional stability and road adhesion.

The ESP control unit immediately carries out a self-test upon circuit 15 ON.

If there are no system errors, the ESP becomes operational and starts carrying out its functions.

During operation, the ESP control unit continuously performs system tests.

Any faults are stored in the fault memory.

The ESP control unit processes the following measured quantities to determine the vehicle behavior:

- Yaw rate and lateral acceleration recorded by yaw rate and lateral acceleration sensor
- Steering wheel angle recorded by steering angle sensor, made available by steering column tube module
- Brake pressure, made available by SBC control unit
- Engine torque
- Transmission shift stage, made available by ETC control unit (N15/3) (with transmission 722.6) or by fully integrated transmission control unit (VGS) (Y3/8n4) (with transmission 722.9)

The side-slip angle (angle between vehicle longitudinal axis and direction of movement of the vehicle's center of gravity) is calculated using the yaw rate (speed of vehicle rotation about vertical axis of vehicle).

The yaw rate, the lateral acceleration and the turning angle of the front wheels (calculated from the steering wheel angle) can be used to determine the lateral forces on the wheels. The longitudinal forces on the wheels are calculated using the engine torque, transmission stage and brake pressure at each wheel. If the yaw angle velocity measured does not match the specified value or if the determined side-slip angle is too large, the ESP control unit generates a signal for brake force build-up or reduction for the relevant wheel. The resulting forces stabilize the vehicle. A distinction is made between under steering and over steering vehicle behavior.

Intervention in the case of oversteer:

If the vehicle begins to oversteer, brake pressure is built up at the outer front wheel. The resulting reduction in lateral force at the outer front wheel generates a yawing moment which counteracts the tendency of the vehicle to rotate inward. The vehicle speed decreases as a result of the brake force at the front wheel, which also enhances stability.

Intervention in the case of understeer:

If the vehicle understeers, the maximum possible lateral force at the front axle has been exceeded. This means that the vehicle pushes itself over the front axle and toward the outer edge of the corner. If at this point the driver depresses the accelerator pedal, the drive torque is first of all reduced.

If this is not enough or the accelerator pedal is not depressed, the incipient instability is prevented by brake application at up to three wheels:

Stage 1: Brake pressure reduction at inner rear wheel

Stage 2: Stage 1 plus brake pressure buildup at outer rear wheel

Stage 3: Stage 2 plus brake pressure buildup at inner front wheel

Depending on the brake force, a torque is generated which causes the vehicle to rotate inward with a simultaneous reduction in speed. This has a stabilizing effect.

The ESP control unit transmits via engine compartment CAN a signal for drive torque reduction to the ME-SFI [ME] control unit, which reduces the engine output accordingly.

A pending shift operation is suppressed for the duration of control intervention.

To achieve this, the ESP control unit transmits a signal via engine compartment CAN to the ETC control unit (with transmission 722.6) or to the fully integrated transmission control unit (VGS) (with transmission 722.9). The ETC control unit or fully integrated transmission control unit (VGS) then suppresses any upcoming shift operations.

ESP is made up of the following sub functions:

^ Anti lock brake system (ABS) function sequence
^ Acceleration Slip Regulation (ASR) and Electronic Traction System (ETS) function sequence
^ Brake Assist (BAS) function sequence
^ Hill Start Assist function sequence
^ Passive ASR and ESP switching function sequence
^ Exhaust test/roller dynamometer mode function sequence
^ System fault display function sequence

ABS function sequence
ABS prevents the wheels from locking up when braking and as a result maintains the steerability and directional stability and road adhesion during vehicle deceleration. If a locking wheel is detected by the ESP control unit on the basis of the signals from the rpm sensors, the brake pressure is reduced at the appropriate brake cylinder until the wheel begins to turn again.

ASR/ETS function sequence
ASR/ETS prevents the driven wheels from spinning when the vehicle is being driven. In addition it causes an improved directional stability and road adhesion with an increased traction potential over the entire vehicle speed range.The ESP control unit records the spinning of the drive wheels via the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque.

The ESP control unit transmits via engine compartment CAN a signal for drive torque reduction to the ME-SFI [ME] control unit, which then reduces the engine output accordingly.

The system constantly checks whether the drive torque specified by the driver via the accelerator pedal sensor can be allowed again e.g. due to improved road adhesion. The drive torque is transmitted to the opposite, stable drive wheel by means of intervention by the brake system on the spinning wheel.

BAS function sequence
BAS detects emergency braking situations from rapid actuation of the brake pedal and, if necessary, increases the brake pressure in order to achieve maximum possible deceleration. The ESP control unit evaluates the increase in pressure in the brake system and initiates an emergency stop if a certain triggering threshold is exceeded.

Hill Start Assist function sequence
Hill Start Assist prevents the vehicle from rolling back contrary to the direction of the gear range engaged when starting off during the time it takes for the driver to move his/her foot from the brake pedal to the accelerator pedal.

The function is automatically triggered when the yaw rate and lateral acceleration sensor detects an incline of a > 5 degrees which would cause the vehicle to roll back contrary to the direction of the gear range engaged with the vehicle at stand still and the brake pedal actuated. The signals from the yaw rate and lateral acceleration sensor are read in by the ESP control unit over the vehicle dynamics CAN. The SBC control unit reads in the status of the brake light switch and transmits this to the ESP control unit via the electro hydraulic brake CAN.

The status of the parking brake indicator switch is read in by the driver-side SAM control unit and placed on the interior CAN. The central gateway control unit forwards this over the engine compartment CAN to the ESP control unit.

The ESP control unit transmits it to the SBC control unit via the electro hydraulic brake CAN.

The brake pressure applied by the driver is maintained in the brake calipers by the SBC hydraulic unit. Once the brake pedal is released, the brake pressure is modulated based on the balance of torques (downward torque due to slope, braking torque and drive torque).

When the torque is sufficient to move off, the Hill Start Assist function is deactivated and the vehicle starts off.

However, if the driver does not actuate the accelerator pedal, the pressure in the brake calipers is reduced to zero after t = 0.7 s.

Passive ASR and ESP switching function sequence
The ASR and ESP functions can be switched passive via the ESP OFF button (N72s14) in the lower control panel control unit (N72). The status of the ESP OFF button is read in by the lower control panel control unit and transmitted on the interior CAN.

The central gateway control unit forwards this over the engine compartment CAN to the ESP control unit.

If the functions are switched passive, the ESP and ABS warning lamp (A1e41) lights up in the instrument cluster. The control thresholds are raised when the functions are switched passive. ABS cannot be deactivated. ESP is always active during a brake application.

Exhaust test/roller dynamometer mode function sequence
For vehicle test purposes, ESP can be set to roller dynamometer mode if the workshop menu is activated via the instrument cluster and the engine is then started. ESP, ABS and ASR are then switched passive. The ESP and ABS warning lamp and the ABS indicator lamp (A1e17) light up in the instrument cluster.

In addition, a corresponding message is displayed in the multifunction display of the instrument cluster.

Roller dynamometer mode can also be activated via diagnosis.

System fault display function sequence
The driver is informed of the system status and any system faults by means of the following:

- ABS indicator lamp
- ESP and ABS warning lamp
- Message in instrument cluster











SBC Hold display concept:

The display concept and plausibility checking of the signal path of the left door contact switch are implemented in the instrument cluster

Function-related messages (e.g. SBC H OFF) are only actuated by the ESP control unit.

Warning messages are actuated independently by the ESP control unit or instrument cluster depending on the event.

The driver is informed about the system status or faults in accordance with the following table.