Feature

ABS Update

The 1978 S-Class (model series 116) was the first production passenger car in the
world with ABS.

Mercedes-Benz
 September 2018

It’s been exactly 40 years since Mercedes-Benz

introduced the first commercially-viable antilock braking
system to the automotive world. Although no hard
statistics exist, presumably it’s saved many thousands
of lives, and many more serious injuries. On top of that,
it set the stage for ESP (Electronic Stability Program),
which has kept humankind from more grief. Of less
importance to life and limb, there’s the benefit of traction
control, in which the spinning wheel is automatically
braked so torque is sent through the differential to the
other side to help keep you from getting stuck in snow or
mud. And there are yet more advantages to having ABS
components aboard, as we’ll see.

Thanks to digital technology, the electronic components

of ABS have always been capable of recording,
comparing, evaluating, and transforming sensor data
into governor pulses for the brakes’ solenoid valves
within milliseconds. Since 1984, ABS has been standard
equipment on all Mercedes-Benz passenger cars.
Ten years after the introduction, as many as a million
Mercedes-Benz cars with ABS were being operated on
the roads throughout the world.

As you’d expect, ABS development hasn’t stood still, and

many improvements have been made, especially through the
This safety essential collaboration of Mercedes-Benz and Robert Bosch. Here,
we’ll explain what the latest advances in this technology
is changing all the mean to you, the ISP (Independent Service Provider).
time. Make sure you The innovations continue
understand newer The complete control system is becoming smaller, more
efficient, and more reliable. The initial, typical pulsating
systems and how of the brake pedal, indicating ABS activation, has largely
they’re integrated been eliminated today. However, the system not only
safely brakes the vehicle while maintaining perfect
with other features. steering, it also serves as the basis and pulse generator
for the acceleration skid control (ASR) system, the
Electronic Stability Program (ESP), the Brake Assist and
of course also for the electro-hydraulic brake system,
Sensotronic Brake Control (SBCTM).
While this 1978 ABS control unit doesn’t look all that different from
a modern one, it’s way less sophisticated.
ABS Update

In Mercedes-Benz passenger cars, the wheel sensor data individual wheels to correct any deviation. In addition,
also serves other functions such as being processed by reduction of the drive torque takes place in order to
the electronically-controlled automatic transmission that increase directional stability and road adhesion. The
adjusts to the driver’s wishes, the navigation computer, ESP control unit processes the following measurement
DISTRONIC proximity control, the engine and windshield quantities to determine the vehicle’s behavior:
wiper control, the active suspension control (ABC), and 1. Yaw velocity
4MATIC. In short, almost everything in the car that is 2. Steering wheel angle
controlled on the basis of speed is using the wheel speed 3. Brake pressure
sensor data. 4. Engine torque
5. Transmission stage
Adaptive braking 6. Lateral acceleration
When we talk about Mercedes-Benz ABS, we are really
talking about a group of integrated systems that all Two types of intervention can take place. In the case of
work together to carry out the function of safe, efficient oversteer (fishtailing), brake pressure is built up at the
braking. Adaptive braking (ABR) assists the driver in outer front wheel. The resulting reduction in lateral force at
dangerous situations that occur suddenly, so it’s an this wheel generates a yawing moment which counteracts
active safety component. The ESP control unit (N30/4) the tendency of the vehicle to rotate inward. Speed
evaluates data from the following components in order to decreases as a result of the brake force at the front wheel,
detect the current driving situation: which also enhances stability. In the case of understeer,
1. Yaw rate, lateral and longitudinal acceleration sensor the maximum possible lateral force at the front axle has
2. Wheel speed sensors or axle rpm sensors (all four) been exceeded. In other words, the vehicle pushes via the
3. Steering angle sensor front axle to the outer edge of the curve. The rear inner
4. Stop lamp switch wheel is braked to generate a torque to help counteract the
“plowing.” If the driver presses the accelerator pedal at this
The ABR system is made up of the following sub-functions: time, the drive torque will be reduced first.
• Electronic stability program (ESP)
• Electronic brake force distribution SBC brake control
• Exhaust test/roller dynamometer mode One of the most complex systems involving ABS that you
• Antilock brake system will encounter today is Sensotronic Brake Control (SBC).
• Acceleration skid control
(ASR), electronic traction
system (ETS)
• Brake Assist System
(BAS), or BAS Plus
• Trailer stabilization
• Hill start assist
• Various other functions
based on model numbers
and option codes.

ESP
ESP prevents the vehicle
from breaking away
when oversteering or
understeering. It helps keep
the vehicle from deviating
from the course intended by
the driver. Brake forces are
distributed selectively to the
The SBC hydraulic unit

6 Mercedes-Benz StarTuned
You will probably only see this on S-Class models, some (A7/3n1), ESP, SPS, and BAS control units (N47/5),
212 E-Class models, and a few others, but you should the steering angle sensor (N49), the steering column
be prepared. The SBC assists the driver in dangerous tube module (N80), the yaw rate and lateral acceleration
situations that occur suddenly, thus serving as an active sensor (B24/15), the SBC pedal value sensor (B37/1),
safety component. SBC is an electro-hydraulic brake and the wheel speed or rpm sensors.
system that controls the braking requests for each wheel
individually via “brake-by-wire.” The SBC hydraulic unit is located at the right front of the
engine compartment. The SBC control unit is installed
In normal operation, the hydraulic link between brake on the SBC hydraulic unit. The SBC control unit has
pedal and the wheel brakes is interrupted by separation these responsibilities:
valves. In contrast to conventional pre-ABS brakes, • Records the driver’s brake application via the SBC
electro-hydraulic control is actuated for each type of pedal value sensor (B37/1) and the front axle pre-
braking application. This provides some advantages such pressure sensor (A7/3b1)
as improved metering of the required brake pressure, • Supplies the ESP with data about how rapidly the driver
reduction in stopping distances, and improved vehicle wishes to brake, and the pre-pressures at the individual
safety dynamics. wheels via its own CAN (Controller Area Network)
• Supplies the ESP with the vehicle speed signal
The main components of the SBC system include directly via its own CAN
the SBC hydraulic unit (A7/3) with SBC control unit • Carries out all control functions during
normal braking
• Performs all pressure control functions for ABS,
ASR, ESP, and BAS control
• Passes on the brake lights signal to the rear SAM
control unit with fuse and relay module (N10/2) to
directly actuate the brake lights through the CAN
(ESP, SPS, and BAS control unit)

The SBC pedal value sensor’s main task is conveying

information on the position of the brake pedal to
the SBC control unit (at the right front of the engine
compartment) via a Hall sensor. The sensor has a
mechanism that converts the vertical movement of
the brake pedal into a rotary movement for the sensor.

The yaw rate and lateral acceleration sensor is

located under the rear SAM control unit. This
measures the yaw rate about the vertical axis of the
vehicle. The integrated lateral acceleration sensor
(AY sensor) measures the lateral acceleration. The
integrated longitudinal acceleration sensor reports
on the vehicle’s pitching motion. The respective
signals from these sensors are read by the ESP, SPS,
and BAS control units.

The wheel speed or rpm sensors are located in the

front at each steering knuckle, and the left and right
rear sensors are mounted on the rear axle wheel
carrier. These sensors have not changed much in
recent years. They are still the Hall Effect type , which
The brake pedal sensor with the master unit
sends data on current wheel speeds to the SBC

September 2018 7
ABS Update

left and right control units. The newer

sensors, however, are more sensitive
than they once were, and can detect
unsatisfactory installation positions and
relay this information to the SBC control
unit as well. They still produce the
square wave form you are used to seeing
when you are performing a diagnostic
test to see if you have a faulty sensor.
This is the familiar “known-good”
Here’s where it is imperative to make
wheel speed sensor waveform.
sure of a clean installation and proper
torqueing of new or replacement units
in order to avoid complications with
your repair. Use a little high-temperature lubricant in
the bore of the housing to ensure a proper seating of
the new sensor.

The steering column module is located on the steering

column, as you would expect. The module is connected
to the engine compartment and interior CANs.
Communication between the multi-function steering
wheel and the steering column tube module takes place
via the LIN network. It’s designed to read the sensor
switches, buttons, and signals so that the SBC sees
all possible driver requests and settings. The steering
angle sensor is also integrated into the steering column
tube module. The sensor records the current position The pinout view of the
of the steering wheel and provides the steering column SBC connector
tube module with a voltage signal proportional to the
steering lock angle.

Troubleshooting
A common symptom scenario: The
customer states that the ABS and
Sensotronic Brake Control (SBC)
warning lights are on. The instrument
cluster displays “Service Brakes Visit
Workshop.” First, find out if any other
brake work has been performed on
the car. If so, here are some possible
items to sleuth out:
1. If a brake flush was not performed
with scan tool direction, a battery
charger, and a pressure bleeder,
air may have been injected into
system. Perform another brake
flush correctly.
2. If a brake job was performed with- This view of an early Robert Bosch ABS hydraulic unit shows why you don’t want dirty old brake
out using a scan tool, you must fluid in your customers’ cars.

8 Mercedes-Benz StarTuned
 connect your scan tool and correctly deactivate and If you find yourself starting from scratch with your
reactivate the brakes. Perform a brake flush. troubleshooting, follow these steps:
3. Test the Sensotronic Brake Control (SBC) system for 1. Connect a scan tool and check for Diagnostic Trouble
leaks and air with the scan tool. Codes (DTCs) in the ESP and Sensotronic Brake
4. Check the operation of the pressure sensor and Control (SBC) units.
reservoir in SBC with the scan tool-guided tests. 2. With DTCs for hydraulic faults or insufficient pressure
5. You may find the scan tool cannot perform guided supply, inspect for brake fluid leaks or damaged
tests. If so, the SBC unit may have an internal hydraulic lines.
problem such as a damaged diaphragm. Servicing this 3. Perform a brake flush with a scan tool and pressure
unit will be a topic for a future StarTuned article, but bleeder. Follow all listed instructions on the scan tool.
we will include an important safety note here: Never 4. If the pump fails to build pressure, or hydraulic faults
remove, disassemble, or otherwise tamper with the continue, replacement of the control unit is needed.
SBC unit unless you are familiar with the system and Replacing this module will require a factory connector
the potentially dangerous pressures involved. Always and programmer used with a factory or compatible
deactivate and depressurize the system according to scan tool. Of course, if you have XENTRY, you’re all set.
WIS recommendations before service. 5. Test powers and grounds to the SBC unit before
6. If the fault will not clear with a good reservoir and no replacement. See the connector pinout view in
air in the system, the pressure sensor in the unit has the photo.
failed. Replace the Sensotronic Brake Control (SBC).

The system pressure sensor is part of the hydraulic unit Proper bleeding and equipment
and cannot be replaced separately, whereas the pressure XENTRY or a compatible scan tool with a bleeding system
accumulator can be replaced separately. Control units menu is needed for this operation. Also necessary is a
will require programming. If the fault code C249F is also battery-maintaining charger to keep the voltage constant,
set, do not perform testing. Replace the SBC as the and a power brake bleeder. Here is the procedure from WIS:
operating time of the unit has been exceeded. 1. Deactivate SBC brake system using STAR Diagnosis
2. Connect the battery charger
3. Connect the brake fluid changing
equipment (your power bleeder)
4. Connect the scan tool
5. Raise the vehicle and remove the wheels
6. Carry out bleeding operation using STAR
Diagnosis menu
7. Check the brake fluid level in the
reservoir and correct if necessary

As noted above, trying to service later-

model Mercedes-Benz vehicles without the
proper equipment and repair information will
be futile, and will result in many lost hours
of work and damage to your reputation. It
should be a given that only Mercedes-Benz
approved DOT 4 brake fluid should be used
from a clean, sealed container. Remember
that brake fluid is hygroscopic and should
be replaced on most vehicles every two
years. Informing your customers of this
need will greatly reduce their chances of
future problems in the system. |
Make sure you have the proper pressure bleeding hook up before you start.

September 2018 9