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

MODEL 216.3, 221.0 /1 (except, 221.095 /195) as of model year 2009 /YoM 08 

Function requirements, general 

• Status Circuit 61 ON

  The status "Circuit 61 ON" is sent to the chassis CAN (CAN E) by the CDI control unit (N3/9) (with diesel engine) or the ME-SFI [ME] control unit (N3/10) (with gasoline engine).

Model 221 as of 1.6.09, model 216 as of 1.9.10 

There are 2 ABR variants in use:

• Basic ABR system (except code 233 (DISTRONIC PLUS) and code 239 (Adaptive cruise control Plus Light))
• Premium ABR system (with code 233 (DISTRONIC PLUS) and code 239 (Adaptive cruise control Plus Light))

General 

The Adaptive Brake (ABR) system assists the driver in dangerous situations which suddenly occur and thus serves active safety. The ABR system is an electrohydraulic brake system which regulates the braking requirements for each wheel individually.

The following components are involved:

• Traction system hydraulic unit (A7/3)
• BAS brake booster (A7/7)
• Accelerator pedal sensor (B37)
• Yaw rate sensor for lateral and longitudinal acceleration (B24/15)
• Left front rpm sensor (L6/1)
• Right front rpm sensor (L6/2)
• Left rear rpm sensor (L6/3)
• Right rear rpm sensor (L6/4)
• Steering angle sensor (N49)
• ESP control unit (N47-5)
• Brake light switch (S9/1), (basic system, model 221 as of 1.6.09, model 216 as of 1.9.10)

For vehicle test purposes the ABR system can be set to dynamometer test mode, if the workshop menu is activated via the instrument cluster (A1) and afterwards the engine is started. The vehicle dynamics control systems are switched passive. The ESP warning lamp (A1e41) and ABS indicator lamp (A1e17) in the instrument cluster light up. In addition, a message is displayed in the multifunction display (A1p13) of the instrument cluster. Roller dynamometer mode can also be activated via diagnosis.

The ABR system is comprised of the following partial functions:

• Electronic Stability Program (ESP) function sequence 
• Electronic brake force distribution (EBD) function sequence 
• Antilock brake system (ABS) function sequence 
• Acceleration skid control (ASR) function sequence 
• Brake Assist function sequence (BAS) (model 221 up to 31.5.09, model 216 up to 31.8.10) 
• Brake Assist function sequence (BAS) (model 221 as of 1.6.09, model 216 as of 1.9.10 with basic and premium system) 
• Tire pressure loss warning (RDW) function sequence (except code (475) Tire pressure monitor (Premium)) 
• Speed-sensitive power steering (SPS) function sequence 
• HOLD function sequence (starting-off aid) 
• Precharging function sequence 
• Dry braking function sequence 
• Function sequence for switching ASR and ESP to passive mode (model 216 and model 221 up to 31.5.09) 
• Function sequence for switching ASR and ESP to passive mode (model 216 and model 221 as of 1.6.09) 
• PRE-SAFE® system, PRE-SENSE function sequence (model 221 as of 1.6.09, model 216 as of 2.9.10) 
• Hill Start Assist function sequence 
• Failed Boost function sequence (with code (Z07) Highest protection, model 216.3 and model 221 with code (B03) ECO start/stop function) 
• Over Boost function sequence (model 216.3 and model 221 with engine 273 and code (B03) ECO start/stop function) 
• Function sequence - Fading Brake Support 
• Air gap reduction function sequence 
• Function sequence - Standstill coordinator 
• System fault display function sequence 

Function sequence, ESP 

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.

Furthermore, the ESP control unit carries out system tests continuously during operation. Malfunctions and faults are stored in the fault memory.

To determine the vehicle handling the ESP control unit processes the following measured quantities:

• Yaw velocity and lateral acceleration recorded by the yaw rate sensor for lateral and longitudinal acceleration
• Steering wheel angle recorded by the steering angle sensor
• Brake pressure, recorded by left front pressure sensor (A7/3b3), right front pressure sensor (A7/3b4), left rear pressure sensor (A7/3b5), right rear pressure sensor (A7/3b6) (except model 221 as of YoM 09 model refinement package) and by the front axle brake pressure sensor (A7/3b1) in the traction system hydraulic unit
• Engine torque, provided by CDI control unit or ME-SFI [ME] control unit
• Transmission shift stage, provided by the ETC [EGS] control unit (N15/3) (with transmission 722.6) or fully integrated transmission control unit (VGS) (Y3/8n4) (with transmission 722.9) on the drive train CAN (CAN C) and passed on by the central gateway control unit (N93) to the chassis CAN

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 at the wheels are calculated from the engine torque, transmission shift stage and brake pressure. 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 the following interventions:

• Intervention in the case of oversteer 
• Intervention in the case of understeer 

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 sufficient, brake pressure is built up at the rear wheel on the inside of the curve. 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.

To reduce the drive torque the ESP control unit transmits a signal via the chassis CAN to the CDI control unit or to motor electronics control unit which thereupon reduces the engine power accordingly.

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

For this the ESP control unit transmits the "Downshift suppression" signal on the chassis CAN. The CDI control unit or the motor electronics control unit passes this on to the ETC [EGS] control unit or VGS control unit. The ETC [EGS] control unit or VGS control unit suppresses any imminent shift operation.

EBD function sequence 

EBD assists the driver in the partial braking area. Overbraking of the rear axle is prevented. Vehicle stability is increased while braking in the curve, whilst if necessary the pressure at the rear wheel on the inside of the curve is reduced or at the front wheel on the outside of the curve is increased.

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 function sequence 

ASR prevents the drive wheels from spinning while driving. In addition it causes improved driving stability for increased traction potential over the entire speed range.

ASR is comprised of the following partial functions:

• Function sequence - drive torque control 
• Function sequence - brake torque control 

Function sequence for drive torque control 

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. For this the ESP control unit transmits a signal for drive torque reduction via the chassis CAN to the CDI control unit or to the motor electronics control unit which reduces the engine power 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.

Function sequence - brake torque control 

Selective interventions by the brake system at the spinning drive wheel means that the drive torque is transmitted to the opposite drive wheel which is running normally.

Brake Assist (BAS) function sequence (model 221 up to 31.5.09 and model 216 up to 31.8.10) 

BAS detects emergency braking situations by means of a rapid brake pedal operation via the BAS membrane travel sensor (A7/7b1) and if necessary increases the brake pressure via the BAS solenoid valve (A7/7y1) 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.

Brake Assist (BAS) function sequence (model 221 as of 1.6.09, model 216 as of 1.9.10) BAS detects emergency braking situations by means of a rapid 

brake pedal operation via the front axle brake pressure sensor (A7/3b1) in the traction system hydraulic unit and if necessary increases the brake pressure via the high pressure and return flow pump (A7/3m1) of the traction system hydraulic unit 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.

On the basic system with brake light switch (S9/1), the following components on the BAS brake booster are not needed:

• BAS diaphragm travel sensor
• BAS solenoid valve
• BAS release switch (A7/7s1)

Tire pressure loss warning function sequence (except code (475) Tire pressure monitor (Premium)) 

The tire pressure loss warning (RDW) system detects a significant pressure loss at a tire and then displays a warning message in the instrument cluster.

Functional principle of tire pressure loss warning system: 

On tires, there is a link between the rolling circumference and the tire inflation pressure. This link is evaluated by performing a relative comparison of the wheel speeds.

Furthermore, the following ESP sensor signals recorded by the yaw rate sensor for lateral and longitudinal acceleration are included in the calculation:

• Lateral acceleration
• Turn rate
• Wheel moments

Properties of function: 

• Detection of significant pressure loss at a wheel.
• Active from a minimum speed (v > approx. 15 km/h).
• Active during straight-ahead travel, lateral acceleration less than 1.5 m/s2, during uniform driving, longitudinal acceleration less than 1 m/s2.
• Inactive during control interventions (ABS, ASR, ESP).
• Inactive with high drive torque and during braking.
• Partially active with a low coefficient of friction, on rough roads, snow chains.
• The tire pressure loss warning (without position indication) takes place after a few minutes of driving. In the subsequent ignition sequences, a reminder message is displayed, provided that the system has not been reactivated.
• The tire pressure loss warning system has to be reactivated in the instrument cluster's menu if tires or wheels are changed or the tire inflation pressures are changed. This erases any pending warning messages.
• After reactivation by the driver, the system automatically teaches in the new conditions. The teach-in process takes roughly 10 min per speed interval. There are 7 intervals. The system is able to issue warnings in the intervals which have been taught in (and the directly adjacent intervals).
• Uniform pressure loss at all 4 tires (e.g. due to diffusion) cannot be detected.
• Incorrect operation by the driver cannot be detected (e.g. reactivation despite significantly low pressure).

Speed-sensitive power steering (SPS) function sequence 

The task of speed-sensitive power steering is to adjust the steering assistance depending on the vehicle speed. This is achieved by a current-controlled SPS solenoid valve (Y10) on the steering gear which is actuated by the ESP control unit. Greater steering assistance is provided at low vehicle speeds and less steering assistance is provided at high vehicle speeds. The ESP control unit receives the vehicle speed signal from the instrument cluster via the chassis CAN.

Additional function requirements for HOLD function (starting-off aid) 

• Transmission selector lever not in position "P"
• Vehicle not secured with parking brake
• Incline detected with vehicle at standstill

HOLD function sequence (starting-off aid) 

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 activation of the HOLD function is displayed to the driver by the message "HOLD" in the instrument cluster.

The adjusted brake pressure is held until the driver again presses with a certain pressure on the brake pedal or starts off.

The intelligent servo module for DIRECT SELECT (A80) transmits the transmission selector lever position on the drive train CAN. The CDI control unit or ME-SFI [ME] control unit passes this status on to the ESP control unit via the chassis CAN.

The electric parking brake controller unit (A13) transmits the status of the parking brake via the front end CAN (CAN G). The central gateway control unit passes this status on to the ESP control unit via the chassis CAN.

The yaw rate sensor for lateral and longitudinal acceleration transmits information on a detected incline to the ESP control unit via the vehicle dynamics CAN (CAN H).

Furthermore, the HOLD function will be deactivated automatically under the following conditions:

• Operation of parking brake
• Change of transmission selector lever position into position "P"

Under the following conditions the HOLD function is switched off automatically before the driver leaves the vehicle:

• Driver door opened with "Circuit 15 ON" and seat belt not fastened (message on multifunction display)

The status "Circuit 15 ON" is sent to the chassis CAN by the EZS control unit (N73).

On model 216 the status of the driver door is defined via the status of the left rotary tumbler switch (A85s1) which is read in by the left door control unit (N69/1).

On model 221 the status of the driver door is defined via the status of the left front rotary tumbler switch (A85s1) which is read in by the left door control unit (N69/1).

The status of the driver door is transmitted in each case on the interior CAN (CAN B). The central gateway control unit passes this status on to the ESP control unit via the chassis CAN.

Additional function requirements - precharging 

• No active control intervention function by ABS, ASR or ESP
• ESP not switched passive

Precharging function sequence 

The precharging function applies the brake pads to the brake disks in the event of a supposed emergency braking situation being detected

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 therefore contributes to the reduction in braking distance.

Emergency braking detection takes place via the release gradient of the accelerator pedal sensor, which is based on the (Controller Area Network (data bus/CAN Bus) (CAN) signal) "Accelerator pedal position".

If a rapid release of the accelerator pedal is detected, the system assumes that the driver intends to brake. The precharging function is activated and a brake pressure of p = 3 to 4 bar is requested.

Additional function requirements - dry braking 

• No active control intervention function by ABS, ASR or ESP
• Windshield wiper active

The front SAM control unit with fuse and relay module (N10/1) transmits the status "windshield wiper active" on the interior CAN. The central gateway control unit passes this status on to the ESP control unit via the chassis CAN.

Dry braking function sequence 

Dry braking assists the performance capability of the front axle brake under wet conditions. The water film is wiped off the brake disk by cyclically applying the brake pads for t = 0.5 s with a brake pressure p ≤ 1 bar.

This improves the response time of the brake.

Function sequence for switching ASR and ESP to passive mode (model 216 and model 221 up to 31.5.09) 

The ASR and ESP functions can be switched passive by operating the ESP OFF switch (S6/2s11) in the cockpit switch group (S6/2). The upper control panel control unit (N72/1) reads in the status of the ESP OFF switch via the instrument panel LIN (LIN 1) and transmits this status on the interior CAN. The central gateway control unit passes this status on to the ESP control unit via the chassis CAN. If the functions are switched passive, the ESP warning lamp lights up in the instrument cluster. The control thresholds are raised when the ASR and ESP functions are switched passive.

ABS cannot be deactivated. ESP is always active during a brake application.

Function sequence for switching ASR and ESP to passive mode (model 216 and model 221 as of 1.6.09) 

In driving mode the ASR and ESP functions can be switched 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 instrument cluster. The IC receives control signals from the multifunction steering wheel (MFL) in the following way:

• Steering LIN (LIN E1)
• Steering column tube module (N80)
• Chassis CAN
• Central gateway control unit
• Central CAN (CAN F)

The instrument cluster then transmits the request "ESP_OFF" to the central gateway control unit. The central gateway control unit passes this on to the ESP control unit via the chassis CAN.

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

PRE-SAFE® system, PRE-SENSE function sequence, (model 221 as of 1.6.09, model 216 as of 2.9.10) 

The PRE-SAFE® system, PRE-SENSE 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, PRE-SENSE is integrated in the ESP control unit. If the ESP control unit detects one of the following situations, the PRE-SAFE® system, PRE-SENSE is activated:

• Panic after-braking (the driver's braking wish is greater than physically possible)
• Pronounced oversteer (breaking away of vehicle rear in combination with severe ESP control intervention functions)
• Severe understeer (the vehicle leans heavily over the front wheels for an extensive period))
• Rapid steering movements at high speeds, which suggest a shock reaction from the driver and may lead to vehicle instability

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

Additional function requirements for the Hill Start Assist 

• Transmission not in position "N"
• Vehicle not secured with parking brake
• Incline detected with vehicle at standstill

Hill Start Assist function sequence 

When starting off Hill Start Assist prevents the vehicle rolling back opposite to the gear range engaged during the time it takes for the driver to move his/her foot from the brake pedal to the accelerator pedal.

The function is triggered automatically, if when the vehicle is at a standstill an 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.

The data from the yaw rate sensor for lateral and longitudinal acceleration is read in by the ESP control unit via the vehicle dynamics CAN. For this the brake pressure applied by the driver is maintained by the traction system hydraulic unit for a period of t = 0.8 to 1.5 s and switched off by a sufficiently large starting off torque.

The intelligent servo module for DIRECT SELECT () transmits the transmission selector lever position on the drive train CAN. The CDI control unit or ME-SFI [ME] control unit passes this status on to the ESP control unit via the chassis CAN.

The electric parking brake controller unit () transmits the status of the parking brake on the front end CAN.

The central gateway control unit passes this status on to the ESP control unit via the chassis CAN.

Failed Boost function sequence (with code (Z07) Highest protection, model 216.3 and model 221 with code (B03) ECO start/stop function) 

The Failed Boost function serves to safeguard a sufficiently high brake pressure level in the system in the event of a shortage of brake boosting by the BAS brake booster.

If, during brake application by the driver, the front axle brake pressure sensor (with engine 275 with code (Z07) Highest protection) or the brake vacuum sensor (B64/1) (model 216.3 and model 221 with code (B03) ECO start/stop function) detects that the BAS brake booster is not providing any driver assistance, the Failed Boost function actuates the high-pressure and return flow pump (A7/3m1) to ensure the driver receives support. On engine 275 with code (Z07) Highest protection, hydraulic amplification is provided with an appropriate amplification factor.

Over Boost function sequence (model 216.3 and model 221 with code (B03) ECO start/stop function) 

If the brake booster reaches the limits of its pneumatic boost, the brake pressure is increased through actuation of the high pressure and return flow pump, thereby ensuring that the usual level of brake boost is then provided.

The function sequence for Failed Boost and the function sequence for Over Boost differ in terms of the triggering thresholds and hydraulic boost.

Function sequence - Fading Brake Support 

The Fading Brake Support function assists the driver when the brakes overheat and thus the brake force decreases.

With extreme overheating a message is output via the multifunction display. The fading brake support function serves to provide additional driver assistance when the brake system is hot in order to achieve the best possible deceleration in this operating case as well.

During extreme overheating, a driver information message can also be provided via the multifunction display of the instrument cluster. Overheating (fading) is detected based on a brake disk temperature model. If a particular threshold is exceeded fading is assumed, "Fading Brake Support" becomes active. In addition this function can also be activated by comparison of the brake pressure level with the actual deceleration or by direct request through the ABS system.

Air gap reduction function sequence 

After dynamic cornering is detected (evaluation of yaw rate by yaw rate sensor for lateral and longitudinal acceleration), the air gap reduction function applies the brake pads to the front axle brake disks. Consequently the clearance between the brake pad and brake disk is compensated in order to receive the required braking power at the corresponding wheel as quickly as possible in the event of any ESP intervention by the brake system.

Function sequence - standstill coordinator 

The standstill coordinator requests the changeover of the vehicle into a safe condition when the support systems are activated, such as 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 belt buckle of the drive seat belt is not inserted into the belt lock. The plugging in of the seat belt tongue into the seat belt buckle is detected by the driver seat belt buckle switch and seat belt reminder warning switch (S68/1). The restraint systems control unit (N2/7) reads in the status of the driver seat belt buckle switch and seat belt reminder warning switch directly and transmits this to the ESP control unit via the chassis CAN.

The support systems cannot be operated.

System fault display function sequence 

The ESP control unit carries out system tests continuously while driving. If malfunctions or faults are detected, the ESP control unit transmits corresponding information on the chassis CAN. The central gateway control unit passes these on to the instrument cluster via the central CAN.

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

• ABS indicator lamp
• ESP warning lamp
• Messages in the multifunction display in the instrument cluster

In order to avoid malfunction due to too low fill level of brake fluid, the brake fluid control switch (S11) monitors the fill level in the brake fluid reservoir. The status of the brake fluid control switch is read in directly by the front SAM control unit and provided on the interior CAN. The central gateway control unit passes this on to the ESP control unit via the chassis CAN. A basic brake function is always available in the event of failure of the ESP control unit.

Adaptive Brake (ABR) location of components	Model 216 Up to model year 2010 /modification year 09 Model 221 Up to model year 2010 /YoM 09 model refinement package	GF42.47-P-0001-02LE
	Model 216 As of model year 2010 /modification year 09 Model 221 As of model year 2010 /YoM 09 model refinement package	GF42.47-P-0001-02LEA
Adaptive Brake (ABR) block diagram	Model 216 Up to model year 2010 /modification year 09 Model 221 Up to model year 2010 /YoM 09 model refinement package	GF42.47-P-0001-03LE
	Model 216 As of model year 2010 /modification year 09 Model 221 As of model year 2010 /YoM 09 model refinement package	GF42.47-P-0001-03LEA
Overview of system components, Adaptive Brake (ABR)		GF42.47-P-9997LE