brake BMW X5 1999 E53 DSC System Workshop Manual

TABLE OF CONTENTS
Subject Page
Dynamic Stability Control (DSC III) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Corner Braking Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Electronic Brake Proportioning (EBV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Dynamic Braking System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
DSC New Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
DSC New Feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Control Module Hydraulic Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Rotation Rate/Transverse Acceleration Sensor . . . . . . . . . . . . . . . . . . . . . . . .19
Brake Pressure Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
DSC III Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Steering Angle Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Brake Fluid Level Warning Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Diagnosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

2
DYNAMIC STABILITY CONTROL (DSC III)
DSC FOR THE X5
INTRODUCTION
The X5 is equipped with DSC III for its stability control system. The components are the
same as the DSC III system (version 5.7) currently installed in the E38/E39 vehicles.
However the software and programming are changed to allow for new functions associat-
ed with all wheel drive and off-road use.
New/changed functions of the DSC III include:
• New Hill Decent Control (HDC) function
• New Differential Brake Control (ABD) function
• Changed traction control function
• Changed ABS function
DSC - X5 OVERVIEW
The DSC system for the X5 provides all of the functions and features of the DSC III system
currently in the E38/E39 vehicles. these functions/features include:
• Maintaining traction under all driving conditions
• Maintaining the directional Control (Longitudinal Stability)
• Providing Lateral Stability (Correction for oversteer/understeer)
• Providing Corner Braking Control (CBC)
• Providing Dynamic Braking System features of - Dynamic Braking Control and
Maximum Braking Control (DBC and MBC)
• Providing Electronic Brake Proportioning (EBV)

3
TRACTION CONTROL SYSTEM OVERVIEW (ASC)
THEORY OF TRACTION CONTROL
Traction Control Systems are referred to as Automatic Stability Control (ASC), Automatic
Stability + Traction Control (ASC+T), Dynamic Stability Control (DSC) or All Season Traction
(marketing term). The traction control system has gone through several changes since its
introduction in Model Year 1988. While each new version offered new innovations, the prin-
ciple of traction control remains the same for any of these versions.
The link between the vehicle and its driving environment is established by the frictional con-
tact between the tires and the road surface. The ability to control the vehicle while driving,
is lost as a result of excessive wheel spin or slip if either rear tire exceeds its ability to:
• Maintain traction
• Accept side forces to maintain directional control (lateral locating forces)
The primary function of the traction control system is to maintain traction and stability of the
vehicle regardless of the road surface condition. This is achieved by reducing the drive
torque applied to the rear wheels or pulsing the rear wheel brakes to eliminate wheel slip
depending on the version of traction control installed.
The electronics for traction control operation are incorporated in the ABS control module
and share many of the same components and sensors. The module determines the speed
of the vehicle through a calculated average of the four wheel speed sensor inputs. Wheel
slip/spin is then recognized by comparing the speed of the driven wheels to the front
wheels.
A critical slip ratio of greater than 5% between the wheels will cause the traction control regu-
lation to begin. This slip ratio is established when the control module detects a wheel speed
difference of 2 MPH or higher.

CORNER BRAKING CONTROL (CBC)
Corner Braking Control (CBC) was an additional feature first added to the ASC/5 system of
the E39 at introduction. CBC improves stability control while braking through curves.
As the vehicle enters a curve, the weight of the car shifts to the outside of the curve. With
non CBC equipped vehicles if the driver brakes while driving through a curve an equal
hydraulic force is applied to each wheel. Though the pressure is equal, the braking effort
is unequal at the tire footprint due to the increased weight of the vehicle on the outside
curve.
With a CBC equipped sys-
tem, the feature regulates
the apply pressure to the
wheels based on the turn-
ing angle (curve recogni-
tion). When the driver
brakes in the same situa-
tion, an unequal hydraulic
pressure is applied to each
side of the vehicle.
Though the apply pressure
is unequal, the dynamic of
the vehicle’s weigh transfer
compensates for the
unequal apply pressure, as
a result braking effort is
equal at the tire footprint.
The ASC control module
monitors the speed signals
from all four wheels to
determine when this regu-
lation is required.
This is a feature found on
vehicles equipped with the
Dynamic Stability Control
System (DSC), however
this ASC/5 system does
not incorporate full DSC
control.
9

10
ELECTRONIC BRAKE PROPORTIONING (EBV)
Electronic brake proportioning allows the DSC system to adjust the braking force to the rear
wheels based on the vehicle’s loading to maximize the braking force at all wheels.
The DSC control module monitors the wheel speed sensors inputs when the brakes are
applied, for activation of this feature. The control module compares the speed at which the
front and rear axles are slowing down to determine vehicle loading.
If the rear axle is slowing at a similar rate to the front, it indicates that the vehicle is loaded
and more braking force can be applied to the rear calipers to slow and stop the vehicle.
If the decel rate of the rear axle is much less than the front, the vehicle is lightly loaded and
less force can be applied to the rear. If the same force were applied, then the vehicle could
become unstable.
OPERATION
The values for the decel rate are programmed in the DSC control module. Equal force will
be applied to the front and rear axles until the threshold values are exceeded. At this point,
the DSC control module will cycle the inlet valves at the rear calipers to reduce the braking
force on the rear axle.

11
DYNAMIC BRAKING SYSTEM
The dynamic braking system is designed to enhance the braking control of the DSC for the
driver of the vehicle. The dynamic braking control and maximum braking control are func-
tions that are programmed into the control electronics of the DSC with no additional hard-
ware changes.
DYNAMIC BRAKING CONTROL (DBC)
The DBC function is designed to provide the maximum braking force available during rapid
(panic) braking situations. The DSC control module looks at the inputs from the brake pedal
switch and the signal from the brake pressure sensor on the master cylinder. The criteria
for activation of DBC is how rapidly is the brake pressure built up with the brake pedal
depressed. The total criteria required for DBC activation includes:
• Brake switch ON
• Brake pressure build up > threshold value
• Vehicle road speed > 5MPH
• Vehicle not in reverse
• Not all wheels in ABS regulation
If the threshold for DBC activation is achieved, the DSC control module will activate a pres-
sure build up regulation phase through the hydraulic unit. The pressure at all wheels is
increased up to the ABS regulation point. This occurs even if the driver does not achieve
the ABS regulation point with the pedal.
The rear axle brakes are controlled with select-low regulation and the front axle brakes are
controlled individually. ABS regulation will continue until the driver releases the pedal and
the pressure in the master cylinder drops below the threshold value stored in the DSC con-
trol module.
MAXIMUM BRAKE CONTROL (MBC)
The MBC function is also designed to enhance a driver initiated braking procedure. The
MBC will build up the pressure in the rear brake circuit when the front brakes are already in
an ABS regulation cycle. The additional braking pressure at the rear wheels will shorten the
stopping distance. The following criteria must be met before the DSC control module will
activate MBC:
• Both front wheel brakes in ABS regulation
• Vehicle speed > 5 MPH
• Vehicle not in reverse
• DBC and pressure sensor initialization test OK
• Rear wheels not in ABS regulation

12
DSC NEW FUNCTIONS
HILL DECENT CONTROL (HDC)
Hill Decent Control (HDC) is designed for off road use to automatically slow the vehicle and
maintain a steady speed on steep gradients. This function allows the driver to focus on
steering and controlling the vehicle without having to use the brakes to slow the vehicle.
HDC is activated manually through the push button switch located in the center switch
panel located in the console. When activated, the vehicle’s speed is held to approximately
5 MPH by the DSC system pulsing the brakes to maintain the speed.
The following conditions must be met before the HDC will activate:
• Push button pressed - LED “ON”
• Vehicle speed: < 25 MPH
• Accelerator pedal pressed <15%
• Downhill driving recognized
Downhill is recognized from the vehicle’s speed and engine load from the engine control
module. The accelerator pedal and engine load signals are passed over the CAN bus to the
DSC control module.
The HDC switch can be pressed at speeds < 37MPH and the LED will come on to indicate
standby mode. However the system will not activate until the vehicle’s speed is below 25.
The driver can accelerate with HDC active up to approximately 20% engine load. The HDC
will stop regulating as long as the driver is requesting a speed increase. If the vehicle’s
speed is > 37 MPH, the HDC function is automatically switched OFF.

13
DSC - NEW FUNCTIONS
AUTOMATIC DIFFERENTIAL BRAKE (ADB)
On dry pavement with no wheel slip, the vehicle’s driving torque is split 68% rear and 32%
front through the planetary gear differential transfer case.
If one or more wheels spin or slip, the DSC will activate a regulation process and the brakes
on the effected wheel(s) will be pulsed. The driving torque will be distributed through the dif-
ferential to the remaining wheels with a good coefficient of friction
The driving torque will be applied through the differential as follows:
• Three wheels - with one wheel spinning.
• On two diagonal wheels with two wheels spinning on different axles.
• One axle only with two wheels spinning on the same axle.
Once the spin or slip is under control, the drive torque will again be applied to all wheels.

15
DSC - NEW FEATURE
TRACTION CONTROL
The traction control feature of the DSC III on the X5 cannot be switched OFF with the DSC
switch. Pressing the button will only switch off the dynamic stability control function of the
DSC.
The traction control feature is needed at all times to provide anti-spin control especially
when driving the vehicle off road. The DSC warning lamp in the instrument cluster will still
come on when the system is switched off manually or there is a fault in the system.
Off road traction control or HDC regulation places an additional load on the brakes. To pre-
vent overheating of the brakes and rotors, the temperature of the rotors is calculated over
time from the road speed and amount of brake application. If the temperature of one or
more rotors goes above 600o
C, the traction control or HDC function for that wheel will be
cancelled temporarily. Once the temperature of the rotor goes below 500o
C, the traction
application will be re-activated.
The normal braking function (including ABS) is always active at any temperatures.
DSC - NEW FEATURE
OFF-ROAD ABS Control Module Programming
The programming for ABS regulation have changed to allow a higher threshold for ABS reg-
ulation when driving off-road in loose gravel or dirt. A locked wheel is more effective in slow-
ing or stopping the vehicle because of the wedge that is built up in front of the tire.
The programming in the DSC control module allows one or both front wheel(s) to lock up
to a speed of 12 MPH as long as the vehicle is traveling straight ahead. If the DSC control
module receives a signal from the steering angle sensor indicating a turn, ABS regulation
will activate to prevent the lock and allow the vehicle to be steered.

20
BRAKE PRESSURE SENSOR
A brake pressure sensor is installed in the hydraulic unit to determine the degree of pres-
sure build-up for Dynamic Brake Control. It is installed in the front axle brake circuit.
The sensor receives its power supply from the DSC control module and produces a linear
voltage input signal ranging from 0.5 to 4.5 volts depending on how hard the brakes are
applied.