DSC5.7 is a traction-control and handling stability system that is fitted on the E65 as standard. It is a development of the previous DSC5.7.
The tandem brake master cylinder is fitted with an integral DSC back-pressure restrictor[?] (pushrod/baffle[?] design) and a brake fluid reservoir.
The single precharger pump ensures that adequate hydraulic pressure is available quickly enough under all operating conditions, specifically with regard to the increased viscosity of brake fluid at low temperatures.
The single precharger pump has a separate connection to the brake fluid reservoir, from which it draws in fluid, and a separate connection to the tandem master cylinder, to which it delivers fluid.
The hydraulic control unit consists of the following components:
The pump motor operates the pump elements (one for each brake circuit) of the self-priming return-flow pump. When brake pressure is released, the return-flow pump pumps the brake fluid returning from the brake calipers back to the tandem master cylinder via integral flow dampers, damper chambers and accumulator chambers. When the DSC system is actively intervening in braking system operation, the return-flow pump generates the required brake pressure.
The damper chambers (one for each brake circuit) reduce pump pressure fluctuation at the brake calipers.
The accumulators (one for each brake circuit) act as initial brake fluid collectors when pressure is released during active operation of the ABS/ASC/DSC.
The integrated flow dampers (one for each brake circuit) reduce pump pressure fluctuation at the tandem master cylinder. This reduces noise levels and feedback of vibration to the brake pedal.
The inlet and outlet valves (one pair per brake caliper) control the flow of brake fluid to and from the brake calipers during the pressure modulation phase.[?]
The high-pressure modulator valves, which are closed when de-energised, and the change-over valves, which are open when de-energised (one pair per braking circuit), control braking system pressurisation during active intervention by the DSC.
The pressure sensor measures the brake pressure in the front-wheel circuit. It has a detection range of 0...250 bar. Its zero point is continuously calibrated when the brake light switch is off and the ignition is on.
The add-on control unit is the ABS/ASC/DSC control centre. To secure the functions, the system is subject to permanent self-monitoring. Faults that occur are stored in a fault code memory and, if required, displayed to the driver.
The wheel speed sensors detect the speed of each wheel individually. On the E65, they also detect the direction of rotation.
The yaw rate sensor detects the speed of rotation around the vehicle's vertical axis (yaw) and the lateral acceleration sensor the sideways acceleration. That data is used to determine whether the vehicle is following a course in keeping with the steering angle or whether it is starting to slide sideways. The two sensors are housed in a single unit.
The steering angle sensor and the vehicle's road speed provide the information from which the vehicle's intended course is calculated. On the E65, the steering angle sensor is integrated in the steering column control centre.
The brake fluid warning switch monitors the brake-fluid level in the brake fluid reservoir of the tandem master cylinder. If the brake fluid warning switch trips, the ASC/DSC function is deactivated to prevent air being drawn into the hydraulic system.
The brake light switch informs the DSC system when the brake pedal is being operated by the driver. This triggers/disables calibration and plausibility checking procedures and cancels braking system intervention during active ASC operation.
The DSC calculates the vehicle's actual movement on the basis of measured data and compares it with a defined ideal. By so doing, it establishes whether the vehicle is following the course intended by the driver. If an unstable condition is detected, the DSC keeps the vehicle under control by throttling back the engine and/or operating the brakes on individual wheels. Intervention on the part of the DSC takes precedence over ABS and ASC activity.
The purpose of the ABS is to maintain vehicle handling stability under braking.
In the event of failure of the DSC sensor systems or CAN faults, only the ABS remains functional.
The ASC controls the behaviour of a wheel that is about to spin in order to maintain the vehicle's handling stability. This is done by reducing the engine power and braking the spinning wheel.
The ASC dynamic[?] traction-control (DTC) mode is used to improve power transmission when driving in a straight line on a road surface with low grip. This is achieved by varying the ASC response thresholds up to a speed of 70 km/h.
Abrupt load changes on the drive train can have a braking effect on the rear wheels which can cause a rear-wheel skid. The MSR function counteracts this effect by slightly increasing the drive torque. The MSR function is active above a minimum speed of 15 km/h. If the DSC is deactivated manually or as the result of a fault, the MSR is not functional either.
Sufficiently rapid application of the brakes by the driver is interpreted by the system as an emergency stop. It then actively increases the brake pressure until the ABS is activated on all four wheels. If the driver reduces the pressure applied to the brake pedal, the hydraulic assistance is proportionately reduced.
CBC increases the driving stability in the case of light braking in curves with medium to high lateral acceleration. It does so by reducing the braking force on the rear wheel that is on the inside of the bend. CBC comes into action before the ABS or DSC. If the DSC fails, the CBC remains functional. The CBC is only deactivated if there is an ABS failure.
EBV prevents the ABS coming into action on the rear wheels before the front wheels when braking. This ensures greater driving stability. In the event of an ABS failure, the emergency EBV function prevents excess braking of the rear wheels even if the signals from up to two wheel speed sensors or the pressure sensor have been lost.
When the engine is running, operation of the parking brake is performed hydraulically with the aid of the DSC system. If the ASC/DSC fails, only the ”Automatic Hold” function is lost. The remaining hydraulic functions remain available unless the ABS fails. For a more detailed description, refer to the description of the parking brake function.
When the ACC is active, deceleration commands are issued which are carried out by the DSC. When doing so, the DSC also switches on the brake lights.
The brake temperature is calculated by the DSC control unit individually for all four discs. The input variables used are the wheel speed, the individual brake pressure for each wheel, time and ambient temperature.
At brake temperatures upwards of about 600°C, the brakes are protected against overheating by
Once the brake temperature drops below approx. 500°C, the engine torque is allowed to gradually increase again over time and intervention by the control systems is allowed again.
If the brake temperature has exceeded 600°C, an entry is made in the fault memory. This is for the information of the service department - not that there has been a fault, but that a safety mechanism has been activated.