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

WARNING: This page is about a different car, the 2011 Mercedes-Benz C300, 2010 Mercedes-Benz C300, 2009 Mercedes-Benz C300, and 2008 Mercedes-Benz C300. However, it is still accessible from the selected car via links, so may be relevant.

MODEL 204.0/2 up to Model Year 08/Modification Year 07 

Function requirements 

ABR function 

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

To do so, the electronic Stability Program control unit (N30/4) evaluates the data from the following components in order to detect the current driving situation:

ABR enables the following subfunctions: 

ESP function 

ESP prevents breakaway when the vehicle oversteers or understeers. 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 directional stability and road adhesion.

A distinction is made between understeering and oversteering vehicle behavior. The ESP control unit detects the vehicle behavior using the data from the steering angle sensor and yaw rate, lateral and longitudinal acceleration sensor.

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 considerable stabilizing effect.

Via the chassis CAN, the ESP control unit sends a signal to the CDI control unit (N3/9) (with diesel engine) or ME-SFI [ME] control unit (N3/10) (with gasoline engine) requesting a reduction in drive moment. The engine output is then reduced accordingly.

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

For this purpose, the ESP control unit sends a signal via the chassis CAN to the CDI control unit or ME-SFI [ME] control unit. The CDI control unit or ME-SFI [ME] control unit then sends the information via the drive train CAN to the electronic transmission control unit (N15/3) (with transmission 722.6) or to the fully integrated transmission control controller unit (Y3/8) (with transmission 722.9). The electronic transmission control unit or fully integrated transmission control controller unit suppress the shift operation.

ABS function 

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 

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. Spinning of the drive wheels is detected by the ESP control unit using the signals from the RPM sensors. Wheel spinning is countered by reduction of the drive torque.

For this purpose, the ESP control unit sends a signal requesting a reduction in drive torque via the chassis CAN to the CDI control unit or ME-SFI [ME] control unit, which then reduces the engine output. 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 

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.

Trailer stabilization function 

The trailer recognition control unit (N28/1) detects a trailer and sends the status via the interior CAN to the front SAM control unit with fuse and relay module (N10/1), and the front SAM control unit then routes the information to the chassis CAN. The ESP control unit receives the information. If vehicle/trailer instability occurs, this is detected by the yaw rate, lateral and longitudinal acceleration sensor.

The ESP control unit receives its information via the vehicle dynamics CAN. The trailer is stabilized by means of selective intervention by the brake system.

Hill start assist function 

Additional function requirements 

The hill start assist function prevents the vehicle from rolling back (when driving off on uphill gradients) when the driver switches from the brake pedal to the accelerator pedal. The ESP control unit reads in the brake light switch discretely and thus detects the status of the brake pedal.

The yaw rate, lateral and longitudinal acceleration sensor detects inclination of the vehicle along its longitudinal axis and sends the information to the ESP control unit via the vehicle dynamics CAN. The control unit then delays the release of pressure in the brake system.

The status of the parking brake is detected via the parking brake indicator switch (S12). This switch is read in discretely via the front SAM control unit and its status is sent to the ESP control unit via the chassis CAN.

The position of the selector lever (with transmission 722) is read in by the electronic selector lever module control unit (N15/5). The status is sent via the drive train CAN to the CDI control unit or ME-SFI [ME] control unit. The CDI control unit or ME-SFI [ME] control unit sends the status to the ESP control unit via the chassis CAN.

Precharging function 

If the driver suddenly takes his foot off the accelerator pedal, brake pressure is built up immediately in order to reduce the response time for a braking operation which may follow (e.g. by the BAS).

The signals from the accelerator pedal sensor are sent from the CDI control unit or ME-SFI [ME] control unit via the chassis CAN to the ESP control unit. On the basis of the information for pedal travel and release speed of the accelerator pedal, the ESP control unit decides whether or not to trigger the precharging function.

Dry braking function 

Additional function requirements 

IMPORTANT The status of the wiper system (WSA) is sent from the front SAM control unit via the chassis CAN to the ESP control unit.

Dry braking enhances the performance of the brakes on the front axle in wet driving conditions. The brake pads are applied cyclically 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.

Air gap reduction function 

The function reduces the air gap between the brake disk and brake lining in order to accelerate pressure buildup for ASR or ESP operations which may follow.

Passive ASR and ESP activation function 

The ASR and ESP functions can be activated passively via the ESP button (S6/1s5) in the dashboard switch group (S6/1). The dashboard switch group is read in via the dashboard LIN by the front SAM control unit. The status of the switch group is sent to the ESP control unit via the chassis CAN.

If the system is activated passively, the Electronic Stability Program warning lamp (A1e41) lights up in the instrument cluster (A1). The control thresholds are raised in the case of passive activation. ABS cannot be deactivated. ESP is always active during a brake application.

Fault handling function 

The range of functions can vary depending on the received signals.

The following table represents the function shutdown due to component failure or defect.

Function deactivation Component failure
Steering angle sensor or Yaw rate, lateral and longitudinal acceleration sensor Controller Area Network (bus/CAN bus) (CAN) Stop lamp switch Pressure sensors of ESP control unit RPM sensor
ABS   X     X
ESP X X X X X
ASR/BAS/ETS X X X X X
Hill Start Assist X X X X X

System fault display function 

The driver is informed of the system status and any faults by means of the following:

In the event of a failure of the ESP control unit a basic brake 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 and sent via the interior CAN to the instrument cluster.

REFER TO SYSTEM WIRING DIAGRAMS Electrical function schematic for Adaptive Brake (ABR)   PE42.47-P-2050-99FAA 
  Adaptive Brake (ABR) location of components   GF42.47-P-0001-02CW 
  Adaptive Brake (ABR) block diagram   GF42.47-P-0001-03CW 
  Yaw rate, lateral and longitudinal acceleration sensor, component description B24/15 GF42.45-P-4810CW 
  Component description for the RPM sensor L6/1, L6/2, L6/3, L6/4 GF42.30-P-4552CW 
  Front SAM control unit with fuse and relay module, component description N10/1 GF54.21-P-6070CW
  Electronic Stability Program control unit component description N30/4 GF42.45-P-5119CW 
  Steering angle sensor, component description N49 GF42.45-P-5107CW 
  ESP button, component description S6/1s5 GF42.45-P-5122CW 
  Brake light switch, component description S9/1 GF42.10-P-4003CW 
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