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Adaptive brake (ABR), function - GF42.47-P-0001CA

MODEL 207.3/4 up to model year 2014 

Function requirements, general 

ABR, general 

ABR assists the driver in dangerous situations which occur suddenly and thus serves active safety.

To this end, the Electronic Stability Program control unit (N30/4) except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit (N30/7) with CODE 233 (DISTRONIC PLUS) evaluates the data for the following components to monitor the driving situation at any given time:

There are 2 ABR variants in use:

IMPORTANT There are no differences between the variants with respect to the ABR subfunctions described below.

The system is made up of the following subfunctions:

Function sequence for ESP® 

ESP® prevents the vehicle from breaking away when oversteering or understeering. It ensures that the vehicle does not deviate from the course specified by the driver (within physical limits). 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 driving stability.

The Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) processes the following performance indicators for determining the vehicle behavior:

Differentiation is made between the following intervention types:

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. In other words, the vehicle pushes via the front axle to the outer edge of the curve.

The resulting instability is counteracted by means of a reduction in drive torque and the following brake interventions at up to 3 wheels:

Depending on the brake force a torque arises which leads to the vehicle turning in with a simultaneous speed reduction, leading to a considerable stabilizing effect. The Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) sends a drive torque reduction signal to the CDI control unit (N3/9) (with diesel engine) or ME-SFI control unit (N3/10) (with gasoline engine) via chassis CAN (CAN E), which reduces the engine power accordingly. A pending shift operation is suppressed for the duration of control intervention.

For this, the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) sends a signal the CDI control unit (with diesel engine) or ME-SFI control unit (with gasoline engine) via chassis CAN, which then sends the information to the electronic transmission control control unit (N15/3) (with transmission 722.6) or to the fully integrated transmission control unit (Y3/8n4) (with transmission 722.9) via drive train CAN (CAN C). The electronic transmission-control control unit (with transmission 722.6) or fully integrated transmission control unit (with transmission 722.9) suppresses the shift operation.

EBD function sequence 

EBD [EBV] provides assistance when the driver applies medium force to the brake pedal. EBD [EBV] prevents overbraking of the rear axle and increases vehicle stability when braking in a curve by reducing the pressure at the rear wheel on the inside of the curve or increasing it at the front wheel on the outside of the curve as required.

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). The yaw rate, the lateral acceleration and the steer angle of the front wheels (calculated from the steering wheel angle) are 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 measured yaw velocity does not match the specified value, or if the determined side-slip angle is too large, the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) generates a signal to either increase or reduce brake force at the required wheel. The resulting forces stabilize the vehicle.

Function sequence of exhaust test, dynamometer test mode 

For vehicle test purposes, ABR can be set to roller dynamometer mode if the workshop menu is activated via the instrument cluster (A1) and the engine is then started. ESP®, ABS and ASR are then switched to passive. Additionally, a corresponding message is shown in the instrument cluster. The dynamometer mode can also be activated using the Diagnosis Assistance System (DAS).

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 the signals from the rpm sensors indicate to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) that a wheel has locked up, the brake pressure at the respective brake cylinder is reduced until the wheel starts turning again.

Function sequence for ASR, ETS 

ASR and ETS prevent the drive wheels from spinning when driving. ASR and ETS also serve to provide improved directional stability and road adhesion for increased traction potential over the entire vehicle speed range. Any spinning of the drive wheels is picked up by the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) which reads the signals from the rpm sensors. Wheel spinning is countered by reduction of the drive torque.

For this purpose, the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) sends a drive torque reduction signal to the CDI control unit (with diesel engine) or ME-SFI [ME] control unit (with gasoline engine) via the chassis CAN, which reduces engine power accordingly. A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor (B37) can be permitted again, e.g. due to an improvement in road surface 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 the maximum possible deceleration. The Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) does this by interpreting the degree of pressure rise in the brake system and initiates emergency braking if a certain triggering threshold is exceeded.

Function sequence for BAS PLUS, with CODE 233 (DISTRONIC PLUS) 

BAS Plus increases the brake pressure depending on the speed of the brake pedal operation and the distance of the vehicle driving in front.

Function sequence for ESP® trailer stabilization with CODE 550 (Trailer hitch) 

The ESP® trailer stabilization detects rocking motion in the tractor/trailer combination based on the yawing vibrations caused in the tractor vehicle by the trailer. The function is activated when a trailer is detected by the trailer recognition control unit (N28/1) and a signal sent via the interior CAN (CAN B) to the front SAM control unit with fuse and relay module (N10/1).

The front SAM control unit with fuse and relay module routes the signal on the chassis CAN, via which the signal from the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) is received. If pendulum motion of the vehicle/trailer occurs, this is detected by the yaw rate sensor for lateral and longitudinal acceleration. Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) receives this information via the vehicle dynamics CAN (CAN H).

IMPORTANT No additional sensors are used on the trailer or trailer hitch.

The ESP® trailer stabilization stabilizes the tractor/trailer combination by means of brake pressure requests on alternating sides of the front axle and, if necessary, slows down the tractor/trailer combination by reducing torque and by building up pressure at all wheels. The vehicle is stabilized only via the front axle. If heavy negative damping is detected, the system intervenes to decelerate the vehicle and stabilize the trailer.

If several ESP® interventions occur in quick succession, the simple stabilization intervention is no longer allowed by the system, regardless of the damping detected.

This prevents the tractor/trailer combination from constantly driving at a critical speed and prevents the brake system from being subjected to too much stress due to interventions in quick succession.

Active vehicle/trailer stabilization does not change the critical speed.

Additional function requirements for function sequence for Hill-Start Assist 

Hill Start Assist function sequence 

When starting off, the Hill-Start Assist prevents the vehicle from rolling back opposite the travel direction of the engaged gear range during the time it takes for the driver to change from the brake pedal to the accelerator pedal.

The Hill-Start Assist is triggered automatically if, when the vehicle is at a standstill, an upward or downward incline is detected via the yaw rate sensor for lateral and longitudinal acceleration, which would cause rolling in the opposite direction to the gear range engaged.

This is sent to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) via vehicle dynamics CAN.

The Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) reads in the brake lights switch directly and thus detects the status of the brake pedal.

The status of the parking brake is detected via the parking brake indicator switch (S12). The switch status of the parking brake indicator switch is read directly by the front SAM control unit with fuse and relay module and sent via the chassis CAN to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS).

The brake pressure applied by the driver is maintained in the brake cylinders by the traction system hydraulic unit (A7/3).

Once the brake pedal is released, the brake pressure is modulated based on the downward torque due to slope, braking torque and drive torque. As soon as the starting off torque is sufficient, the function is ended and the vehicle starts off.

However, if the driver does not actuate the accelerator pedal, the pressure in the brake cylinders is released after t = 1 s.

Additional function requirements for function sequence for dry braking 

IMPORTANT The status of the wiper system is sent by the front SAM control unit with fuse and relay module via chassis CAN to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS).

Dry braking function sequence 

The dry braking function enhances the performance capability of the front axle brake under wet conditions. The brake pads are applied cyclically for t = 0.5 s with a brake pressure p = 1 bar to remove the film of water from the brake disk. This improves the response time of the brake.

Function sequence for precharging dependent on accelerator pedal operation 

Upon detection of a possible emergency braking situation, the brake pads are brought into contact with the brake disks. The clearance between the brake pad and brake disk is thus compensated before the driver operates the brake. The response time of the brake is improved and a shorter braking distance can be achieved.

Emergency braking is detected from the release gradient of the accelerator pedal, which is formed from the accelerator pedal position.

If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. The function is activated and a brake pressure of p = 2 to 3 bar is requested.

IMPORTANT In order to adapt the calculated triggering threshold of the release gradient to the driver behavior, a learning algorithm is implemented which places the driver in the category "normal driver" or "hectic driver" on the basis of the average rate at which the accelerator pedal is released.

Function sequence for precharging depending on lateral acceleration 

During dynamic cornering, the brake pads are brought into contact with the front axle brake disks. This closes the air gap between the brake pad and brake disk in order to achieve the required braking power at the corresponding wheel as quickly as possible in the event of ESP® intervention by the brake system.

Function sequence for HOLD function 

The HOLD function assists the driver when starting off on a hill or during waiting times in traffic. The HOLD function is activated by briskly depressing the brake pedal at a standstill. The adjusted brake pressure is maintained until the driver depresses the brake pedal again rapidly or starts off.

Warning strategy 

If the HOLD function is activated and the driver wishes to exit the vehicle, the key cannot be taken out from the electronic ignition lock (N73) control unit.

An acoustic and optical warning will be issued through the instrument cluster.

Function sequence for PRE-SAFE® system 

The PRE-SAFE® system is activated in critical driving conditions, i.e. in a potential accident situation. The goal is to create the most favorable conditions possible for the occupants during the expected accident by activating the functions before the actual impact.

The PRE-SAFE® system is integrated in the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS). If the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) detects one of the following situations, the PRE-SAFE® system is activated:

IMPORTANT The PRE-SAFE® system is described in a separate function description.

Function sequence for passive activation of ASR and ESP (up to 31.5.10) 

The ASR and ESP® functions can be switched to passive using the ESP button (S6/1s5) in the instrument panel switch group (S6/1). The instrument panel switch group is read in via the instrument panel LIN (LIN 1) by the front SAM control unit with fuse and relay module and sent via the chassis CAN to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS).

If the system is activated passively, the Electronic Stability Program warning lamp (A1e41) lights up in the instrument cluster. The control thresholds are raised in the case of passive switching.

Function sequence for passive activation of ASR and ESP® (as of 1.6.10) 

In driving mode the ASR and ESP® functions can be switched to passive with the left multifunction steering wheel button group (S110) and right multifunction steering wheel button group (S111) via the "Assist" menu and "ESP" submenu in the IC.

The instrument cluster receives control signals from the multifunction steering wheel (MFL) in the following way:

The instrument cluster then sends the "ESP_OFF" request via chassis CAN to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or the Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS). If the functions are switched passive, the warning lamp for the Electronic Stability Program lights up in the IC. The control thresholds are raised when the ASR and ESP® functions are switched to passive. ABS cannot be deactivated. ESP® is always active during a brake application.

Function sequence - standstill coordinator (SSK) 

When Assist systems, such as the HOLD function for example, are activated and the driver status is unclear, the standstill coordinator requests that the vehicle be transferred to a secured condition.

The driver status is deemed to be unclear, if the driver door is open and the belt buckle of the drive seat belt is not inserted into the belt lock. Plugging the seat belt tongue into the seat belt buckle is detected by the driver seat belt buckle restraint system switch (S68/3). The Supplemental Restraint System control unit (N2/10) reads in the status of the driver seat belt buckle restraint system switch directly and sends it to the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) via chassis CAN.

The support systems cannot be operated.

System fault display function sequence 

The driver is informed about the system status and about faults by the following displays:

In case the Electronic Stability Program control unit except CODE 233 (DISTRONIC PLUS) or Premium Electronic Stability Program control unit with CODE 233 (DISTRONIC PLUS) fails, the basic braking function without ABS is always available.

IMPORTANT The status of the brake fluid level switch (S11) and parking brake indicator switch is read in by the front SAM control unit with fuse and relay module and transmitted via the interior CAN to the instrument cluster.

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