Brakes - Service Techniques BMW Z4 E85/E86

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Introduction

The E85 is equipped as standard with dynamic stability control Mk60. DSC Mk60 was first introduced on the E46 in 09/2000, but without DTC function (DTC = dynamic traction control). From 09/2000 to 09/2001 DSC in the E46 was equipped with the ADB function (automatic differential brake). From 09/2001, DSC Mk60 has been available in the E46 with DTC function.

System supplier for DSC Mk60 is Continental Teves.

DSC is a dynamic driving system that maintains driving stability.

DSC optimizes

1. driving stability when pulling away, accelerating and braking
2. traction

In addition, DSC recognizes unstable driving situations such as understeering or oversteering. Within the limitations of the laws of physics, DSC helps to keep the vehicle on a safe course.

To do this, DSC must know the following parameters of driving dynamics

1. yaw rate as a measure of rotary movement of the vehicle around the vertical axis
2. lateral acceleration as a measure of the coefficient of friction with the road surface
3. road speed

In addition, the driver's requirements are recognized from the angle of the steering wheel and the pressure on the brake pedal. Moreover, speed sensors deliver information about the speed of each individual wheel. The actual situation in which the vehicle is currently moving is computed from the available data. The actual situation is compared to reference data computed by the DSC control unit.

If the actual data differ from the reference data, DSC is activated and intervenes in the braking system or in the engine control.

The advantages

The active intervention of DSC in the engine control and brakes provides outstanding driving stability and traction in all driving situations. DSC counters all negative dynamic driving forces and so creates a maximum degree of active safety within the limitations of the laws of physics. DSC thus enhances driving comfort by allowing more relaxed driving.

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Brief Description Of Components

Dynamic stability control DSC is comprised of the following components

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1. DSC Hydraulic Unit The DSC hydraulic unit comprises the DSC control unit and hydraulic unit, and can be dismantled into these two components. The DSC control unit electronically actuates the hydraulic unit which in turn hydraulically controls the DSC.
2. Installation Location The DSC hydraulic unit is located at the front left, on the suspension strut dome.
3. Construction The DSC hydraulic unit consists of the following components: DSC control unit, Hydraulic unit with hydraulic valves and Integrated return pump. The separating line for the DSC hydraulic unit runs through the solenoid valve: The electrical components of the valves (valve coils) are assigned to the DSC control unit and directly connected to the circuit board. The mechanical components of the valve (basically the valve domes with the valve tappets) are peened in the hydraulic unit. The valve tappets are moved by the magnetic field generated by the valve coils. The valve tappets open/close the hydraulic connections by valve seat. The return pump is integrated into the hydraulic unit. The return pump has 2 pump elements (driven by eccentrics) that deliver fluid to the brake circuits in the front and rear axles. The return pump is driven by an electric motor. The DSC control unit is integrated into the vehicle electrical system via a 47-pin connector. How It Works When DSC regulation is active, the pressure is built up and reduced to control the force of the brakes on the wheels. When pressure is reduced in the wheel brakes, the brake fluid is fed back towards the tandem-brake master cylinder by the return pump (please refer to hydraulic system schematic). When traction control is active, the pressure is also built up by the return pump. The direction of activity is determined by the switchover valves. The return pump draws in fluid from the expansion tank via the tandem-brake master cylinder. The intake characteristics were optimized to make sure that the pressure can build up quickly for DSC regulation: Tandem-brake master cylinder, Hydraulic lines, Return pump and Brake fluid.
4. 4 Wheel Speed Sensors The wheel speed sensors measure the speed of the individual wheels. DSC Mk60 makes use of active speed sensors. The active wheel speed sensors work according tot he principle of magnetic resistance (same technology as E46). Magnetic resistance means: change in electrical resistance under the influence of a magnetic field.
5. DSC sensor The DSC sensor measures the rotation of the vehicle around the vertical axis (yaw rate) and the lateral acceleration of the vehicle. Installation Location The DSC sensor is secured to the floor of the vehicle under the front-passenger seat.
6. Steering-angle sensor The steering-angle sensor measures the angle of rotation of the steering wheel. Installation Location The steering-angle sensor is fitted to the steering spindle. Construction The steering-angle sensor is a potentiometer with 2 sliding contacts offset by 90°. The use of 2 sliding contacts allows the direction of turn to be recognized. How It Works DSC needs the steering angle for its function. The steering angles detected by the sliding contacts (2 voltage values) make up a complete turn of the steering wheel. Once the steering wheel has been turned through 360°, the voltage values are repeated. The steering-angle sensor thus also counts the number of turns of the steering wheel. The total steering angle is thus formed from the currently measured steering angle and the number of turns of the steering wheel. The signal is converted into a digital protocol (CAN message) by the steering-angle sensor. To ensure that the steering angle, including the number of turns of the steering wheel, is available when the ignition is ON, the steering-angle sensor is always under voltage from terminal 30. Open circuits would cause the number of turns of the steering wheel in the steering-angle sensor to be lost. To allow the steering-angle sensor to remain operational after an open circuit, the DSC control unit learns the steering angle by means of a plausibility check (initialization). The DSC control unit derives the steering angle from the wheel speeds on the front axle. The plausibility check is made by comparison with other sensor signals and recognizes when the vehicle is being operated with an incorrect steering angle reference. If initialization is unsuccessful after starting up to a speed threshold of approx. 20 km/h, then: DSC is deactivated, DSC warning light comes on, and a fault entry is recorded in the DSC control unit. NOTE: If the steering-angle sensor fails, ABS will remain active.
7. 2 Brake Pressure Sensors The two brake pressure sensors on the tandem-brake master cylinder measure the brake pressure generated by the driver (2nd brake pressure sensor as redundancy). Installation Location The two brake pressure sensors are installed on the tandem-brake master cylinder in the pressure connection to the front-axle brake circuit and to the rear-axle brake circuit. Construction The two brake pressure sensors are capacitive sensors. A plate capacitor serves as a pressure sensor. One plate of the plate capacitor is fixed. The other plate moves on a spring-loaded mount. How It Works Pressure in the hydraulic circuit exerts a force on the plate capacitor. This force causes movable capacitor plate to move, changing the distance between the plates. When the distance between the plates changes, the capacity also changes. The brake pressure sensor supplies an analogue signal in the measuring range 0 to 250 bar. For the DSC, the following are derived from the brake pressure sensors' signal: Approximation for factors such as lateral stability forces and brake force for all relevant vehicle operating conditions (unbraked, partially braked, ABS regulation range). Evaluation of pressure correction information for a wheel being braked by DSC if the brake is depressed during regulation.
8. DSC button

The DSC button is located on the center console switch cluster and has 3 switch positions

DSC in standby, DTC in standby, DSC completely switched off.

Additional measured variables from the following components

1. Brake fluid level switch (via reed contact in expansion tank) Insufficient brake fluid is detected and a signal sent to the DSC control unit. DSC is deactivated if there is insufficient brake fluid. There would otherwise be a risk of air being drawn into the brake system. A signal is transmitted to the display in the instrument cluster via the CAN interface.
2. Brake Light Switch Together with the signals from the brake pressure sensors on the brake master cylinder, braking actions are recognized.
3. Parking Brake Warning Switch Skidding actions that are deliberately initiated by the driver are recognized by the DSC and no regulation takes place. Reason: a handbrake turn should remain technically possible.

DSC System Functions

The DSC dynamic driving system is a lateral and transverse dynamic control system that actively intervenes in the actions of the brakes and engine control systems. DSC comprises a number of functions

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BRAKE FLUID LEVEL SWITCH SPECIFICATION

Anti-Lock Braking System ABS

ABS prevents the wheels from locking when the brakes are applied.

Advantage: short braking distances, the vehicle remains stable.

Brake pressure is regulated at all wheels to ensure that each wheel runs in the best possible slip range. When this happens, slip is controlled so that the maximum possible braking and lateral stability forces can be transmitted.

Electronic Brake Force Distribution EBV

EBV is a component of ABS and controls the brake force distribution between the front and rear wheels.

Advantage: regardless of the load state of the vehicle, the best possible braking distance is achieved while driving stability is maintained.

Modern vehicles have relatively large brakes on the rear axle to shorten braking distances. To prevent the rear wheels from being overbraked in certain driving situations, EBV permanently monitors the slip values. EBV controls rear axle slip in dependence of the front axle.

Cornering Brake Control CBC

CBC is an extension of ABS. CBC enhances driving stability if the brakes are applied when cornering.

Advantage: if the brakes are applied in a corner, optimum brake force distribution ensures the best possible tracking stability.

The shifting of wheel loads when cornering (even if brakes are only applied lightly) can adversely affect driving stability. If required, CBC generates a stabilizing load moment when the brakes are applied lightly outside the ABS intervention range.

Automatic Stability Control ASC

ASC prevents the wheels from spinning when the vehicle is accelerating.

Advantage: enhanced traction and the vehicle remains stable.

If one of the wheels of the drive axle is on a high-grip surface and the other is on a slippery surface, the wheel tending to skid is braked.

ASC is also able to intervene in the engine control (to reduce the ignition angle, injection quantity, throttle valve setting).

Dynamic Traction Control DTC

The functions of the DTC correspond to those of DSC with a slightly modified regulating strategy. DTC can be activated by deactivating DSC (DSC button). DTC intervenes in the braking actions to imitate the function of a conventional differential lock.

Advantage: higher traction is available with DTC.

Vehicle stabilization intervention (e.g. reduced power output) is made slightly later than with DSC. This enhances traction with a slight loss of driving stability. In particular when accelerating and driving uphill on a loose surface or in deep snow (coefficients of friction requiring increased slip), a compromise is needed between vehicle stability and traction: DSC provides a high degree of vehicle stability with adequate traction. DTC provides better traction with a slight loss of stability, and is thus only recommended in exceptional circumstances (e.g. when driving in deep snow).

Engine Drag Torque Control MSR

Engine drag torque control MSR prevents: the tendency of the drive wheels to lock on a slippery surface when a lower gear is engaged or load is abruptly changed.

Advantage: the drive wheels retain their lateral stability in overrun mode.

The wheel speed sensors tell MSR as soon as the wheels are about to lock. MSR then briefly reduces the engine overrun torque by opening the throttle slightly.

Dynamic Brake Control DBC

DBC supports the driver in emergency braking situations by automatically boosting the brake pressure.

Advantage: shortest possible braking distances in emergency braking situations by achieving ABS regulation on all four wheels.

The brake pedal is frequently not depressed strongly enough in emergency braking situations. ABS regulation is then not activated.

The return pump increases the brake pressure until ABS regulation is activated

1. if the brake pedal is depressed quickly but with insufficient force (from brake pressure sensor signal)
2. if the brake pedal is depressed slowly and then rapid deceleration is required (from brake pressure sensor signal), when one wheel reaches ABS regulation.

Which wheel locks first depends on load and coefficient of friction of the road surface.

Example of a typical situation

The traffic slows, making light braking necessary at first, but then demands as short a stopping distance as possible.

Operation

The DSC button is located next to the RPA button (RPA = tire failure indicator) in the center console switch cluster.

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DYNAMIC BRAKE CONTROL DBC SPECIFICATION

The DSC button has 2 functions that can be set by pressing the button for different lengths of time.

DYNAMIC BRAKE CONTROL DBC SPECIFICATION

Note. Reactivating DSC Pressing the button again reactivates the DSC function. The DSC indicator and warning light goes out (unless a fault has developed in the system).

The RPA button is used for initialization (= learning tire pressure) of the tire failure indicator.

The tire failure indicator is a function integrated into DSC (but not a dynamic driving function). Reason: the speed signals for all four wheels are already available in DSC.

Preconditions For Activation

DSC is in standby after each engine start.

Notes For Service Staff

Service staff should not the following points

1. General information: Calibration must be performed after the following operations: Replacement of DSC control unit, Replacement of steering-angle sensor, Replacement of a DSC sensor, Replacement of a brake pressure sensor, Adjusting work on the steering geometry, and Work on the front-axle steering A calibration of the DSC sensors involves calibrating the actual values.
2. Diagnosis
3. Encoding/programming
4. Car & Key Memory

Encoding The Steering-Angle Sensor

If the steering-angle sensor is replaced, the steering-angle sensor must be recorded. After encoding, calibration must be performed in the straight-ahead position. The steering-wheel position set is then stored in the steering-angle sensor as the straight-ahead position. For this reason, the steering wheel must be precisely in the straight-ahead position when the front wheels are calibrated. Calibration must be performed with the BMW diagnosis system.

The steering-angle sensor requires model-specific data for its internal calculations. These are transmitted during encoding. For this reason, the vehicle identification number from the instrument cluster is permanently stored in the steering-angle sensor during encoding. The vehicle identification number is checked each time the engine is started. In the event of non-agreement, calibration must be performed. This prevents a new or used steering-angle sensor from being installed with a non-calibrated steering-angle-sensor signal.

Replacement Of DSC Control Unit After replacing the DSC control unit, the DSC control unit must be encoded. After this, calibration must be performed for: Steering-angle sensor, DSC sensor, and Brake pressure sensors.

US National Version

In US national version, 2 additional pins on the DSC control unit are assigned on vehicles without navigation system.

The following signals are then transmitted to the telephone control unit

1. processed wheel speed, front right
2. processed wheel speed, front left

DSC tells the control unit the wheel speeds, front left and right.

Advantage

Assume the vehicle is involved in a crash in a tunnel. The stored wheel speeds can be used to calculate the vehicle position (including cornering by left/right comparison).

The telephone control unit transmits the computed vehicle position in an emergency call.

Subject to alteration due to misprints, errors and technical modifications.