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Adaptive Brake (ABR), Function - GF42.47-P-0001RD

Model 172.4 

up to Model year 2017 

G13845874Courtesy of MERCEDES-BENZ USA

Block diagram 

Function requirements, general 

ADAPTIVE Brake, general 

ADAPTIVE Brake assists the driver in dangerous situations which occur suddenly, and thus enhances active safety.

To do so, the Electronic Stability Program control unit evaluates the data from the following components in order to detect the current driving situation:

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

IMPORTANT With the ADAPTIVE Brake premium system, the data from the turn rate, lateral acceleration and longitudinal acceleration sensor is read in by the Electronic Stability Program control unit via the vehicle dynamics CAN.

IMPORTANT With the ADAPTIVE Brake basic system, the turn rate, lateral acceleration and longitudinal acceleration sensor is integrated in the Electronic Stability Program control unit, thereby making a CAN connection to an external turn rate, lateral acceleration and longitudinal acceleration sensor unnecessary. The data for turn rate, lateral acceleration and longitudinal acceleration is therefore collected and evaluated directly by the Electronic Stability Program control unit.

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 processes the following measured quantities to determine 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. If the driver presses the accelerator pedal at this time, the drive torque will be reduced first.

If this is not enough or the accelerator pedal is not depressed, the incipient instability is prevented by Brake application at up to 3 wheels as follows:

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 considerable stabilizing effect. The Electronic Stability Program control unit sends a signal requesting a reduction in drive torque via the chassis CAN to the CDI control unit (on diesel engine) or the ME-SFI control unit (on gasoline engine) which reduces the engine power accordingly. A pending shift operation is suppressed for the duration of control intervention.

For this purpose, the Electronic Stability Program control unit sends a signal via the chassis CAN to the CDI control unit (with diesel engine) or the ME-SFI control unit (with gasoline engine) which then sends the information via the drive train CAN to the fully integrated transmission control unit (with automatic transmission). The fully integrated transmission control unit (with automatic transmission) 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 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 shift stage and Brake pressure at each wheel.

If the measured yaw rate does not match the specified value or if the determined side-slip angle is too large, the Electronic Stability Program control unit generates a signal for Brake force buildup or reduction for the relevant wheel. The resulting forces stabilize the vehicle.

Function sequence of exhaust test, dynamometer test mode 

For vehicle test purposes, the ADAPTIVE Brake can be set to dynamometer test mode if the workshop menu is activated via the instrument cluster and the engine is then started. ESP®, ABS and ASR are then switched to passive. The Electronic Stability Program warning lamp and the antilock Brake system indicator lamp in the instrument cluster light up. In addition, a message is displayed in the multifunction display of the instrument cluster. The roller dynamometer mode can also be activated via Xentry Diagnostics.

ABS function sequence 

ADAPTIVE Brake prevents the wheels from locking up when braking, thereby maintaining steerability and directional stability during vehicle deceleration. If a locking wheel is detected by the Electronic Stability Program control unit based on signals from the RPM sensors, the Brake pressure on the relevant wheel is reduced until the wheel begins to turn 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. Spinning of the drive wheels is detected by the Electronic Stability Program control unit using 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 sends a signal requesting a reduction in drive torque via the chassis CAN to the CDI control unit (on diesel engine) or the ME-SFI control unit (on gasoline engine) which reduces the engine power accordingly. A check is continuously performed to establish whether the drive torque specified by the driver via the accelerator pedal sensor 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.

For this purpose, the Electronic Stability Program control unit evaluates the increase in pressure in the Brake system and initiates an emergency stop 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.

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 function is triggered automatically if, when the vehicle is at a standstill, the internal turn rate, lateral acceleration and longitudinal acceleration sensor (except code 233 (DISTRONIC PLUS)) or the external turn rate, lateral acceleration and longitudinal acceleration sensor (with code 233 (DISTRONIC PLUS)) detects an uphill or downhill gradient which would cause the vehicle to roll opposite the direction of the selected gear range.

The Electronic Stability Program control unit reads in the status of the Brake light switch directly and in this way detects the status of the Brake pedal.

The status of the electric parking Brake switch is captured by the electric parking Brake control unit and sent via the chassis CAN to the Electronic Stability Program control unit.

The Brake pressure applied by the driver is maintained in the Brake system by the traction system hydraulic unit.

Once the Brake pedal is released, the Brake pressure is modulated based on the downward torque due to slope, the braking torque and the drive torque. When the torque is sufficient to move off, the function is deactivated and the vehicle starts off.

If, however, the driver does not depress the Brake pedal, the pressure in the Brake system is reduced after t = 1 s.

Additional function requirements - dry braking 

IMPORTANT The status of the windshield wiper system is sent by the front SAM control unit with fuse and relay module via the chassis CAN to the Electronic Stability Program control unit.

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 disks. 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 quickly pressing down hard on the Brake pedal after coming to 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 wants to leave the vehicle, removal of the key from the electronic ignition lock control unit is prevented.

An acoustic and visual warning is output by 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. If the Electronic Stability Program control unit detects one of the following situations, the PRE-SAFE® system is activated:

Function sequence for ASR and ESP® passive switching 

The ASR and ESP® functions can be switched to passive mode via the ESP OFF button in the upper control panel control unit. The signals from the upper control panel control unit are read in via the instrument panel LIN from the front SAM control unit with fuse and relay module and sent via the chassis CAN to the Electronic Stability Program control unit.

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

Function sequence for standstill control 

The standstill control function requests the changeover of the vehicle into a safe condition when driver assistance systems are active (e.g. the HOLD function) and the driver status is not clear.

The driver status is deemed to be unclear if the driver door is open and the driver seat belt is not inserted into the belt lock. The supplemental restraint system control unit reads in the status of the driver seat belt buckle restraint system switch directly and transmits the status via the chassis CAN to the Electronic Stability Program control unit.

The driver assistance systems cannot be operated.

System fault display function sequence 

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

If the Electronic Stability Program control unit fails, basic braking without ABS is always available.

IMPORTANT The status of the Brake fluid level switch is read in by the front SAM control unit with fuse and relay module and sent via the interior CAN to the instrument cluster.

  Electrical function schematic for Adaptive Brake (ABR)   PE42.47-P-2050-97TAA
  Overview of system components, Adaptive Brake (ABR)   GF42.47-P-9997RD 
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