Brake System
ABS/ASC/DSC
The EMS2000 receives road speed signals from the ABS/ASC/DSC module for cruise functions (if equipped) and maximum vehicle speed limiting. Road speed signals arrives at the EMS2000 over the CAN BUS.
Requests for torque modification are also received from the ASC/DSC module over the CAN Bus, If either of the driven wheels loose traction the ASC/DSC control module will request a change in the torque output of the engine in order to regain traction. This occurs in either of the following situations.
- The wheels have broken traction due to excess torque being generated by the engine; the ASC Control Unit therefore requests a torque reduction from the engine.
- The wheels have broken traction due to excess engine braking when on low grip surface; the ASC Control Unit requests a torque increase from the engine under these circumstances. This is called the Engine Drag Torque Control (MSR) System.
Once traction has been regained then the brake or torque intervention will be removed allowing the engine to return to its appropriate torque output as determined by other inputs such as driver demand or cruise control. The requests for a torque increase or reduction are received from the ASC Control Unit via the vehicle CAN-bus.
Brake System
The braking system features disc brake and calipers on all four wheels. The hydraulic system has a dual circuit that is split diagonally. A brake booster is fitted to all models. The system incorporates Anti-Lock Brakes (ABS) as a standard with its sub systems of Electronic Brake Force Distribution (EBV), Cornering Brake Control (CBC) and Engine Torque Feedback Control (MSR).
Purpose of the System
The Brake System provides the stopping power for the MINI and a cable operated parking brake. System components include:
- Brake Booster.
- Master Cylinder with Reservoir.
- Front Wheel Brake Assemblies.
- Rear Wheel Brake Assemblies.
- Parking Brake.
System Components
Brake Booster
All cars are fitted with a 10" diameter vacuum brake booster. The booster is vented through an aluminum baffle plate, fitted between the booster and the bulkhead.
Master Cylinder with Reservoir
The tandem master cylinder is available in two versions one for ABS/ASC and one for DSC, both have a piston diameter of 22.2 mm. The visible difference between the two units is the single sensor fitted to the underside of the master cylinder on the DSC unit. This sensor informs the DSC control unit whether the driver has applied the brakes during DSC intervention.
The reservoir is secured to the master cylinder by a bolt connecting two mounting tabs. A sensor monitors fluid level, and supplies information to the ABS/DSC control unit. Two "0" rings seal the unit to the master cylinder. The reservoir also supplies fluid to the clutch hydraulics on manual gearbox models. There is an integral filter in the filler neck to prevent dirt contamination of the fluid.
Front Wheel Brake Assembly
Internally vented discs of 275 mm diameter with single 48 mm piston floating calipers are used at the front. The brake hose is fixed to the caliper using a Banjo type bolt.
Rear Wheel Brake Assembly
Solid brake discs with a diameter of 260 mm and single 32 mm piston floating calipers are used at the rear. The pad on the inside is secured to the caliper piston via a spring that locates in a groove around the outside of the piston.
Parking Brake Assembly
The parking brake operates through the rear caliper piston acting directly on to the disc using the pads. The parking brake is self-adjusting, with the adjustment being made on the piston itself. A tool is required to wind back the adjuster when fitting new pads. The parking brake is operated from the handbrake by two Bowden cables with a compensator fitted inside the cabin area and an adjustment nut under the handbrake lever in the center console.
Brake Fluid
Brake fluid meeting DOT 4 specification is the only fluid recommended for use with MINI. Brake fluid must be replaced at 24 month service intervals.
Principle of Operation
For normal brake operation, the brake booster assembly assists brake pedal pressure and the force is transmitted to the master cylinder assembly. The master cylinder assembly converts brake pedal movement to hydraulic pressure. Primary and secondary brake pipe circuits supply the hydraulic pressure via the ABS hydraulic unit to the brake calipers.
The primary circuit supplies the front right and rear left brakes while the secondary circuit supplies the front left and rear right brakes. The engine intake manifold provides the brake booster assembly with vacuum through a pipe and non-return valve.
Parking Brake Adjustment
The procedure to set up the handbrake correctly is as follows:
- Slacken the parking brake cable-adjusting nut at the handbrake lever. Wind back the rear brake caliper piston using the special tool. Fit brake pad linings into the caliper. Ensure the pad retention spring is fitted in the piston groove. Fit the caliper over the brake disc and secure it to the bracket.
- With the parking brake lever released, check whether the stop of the control lever makes contact with the grooved pin and the circular section at the end of the cable is correctly located in the caliper support. (Cable support has a torsion lock. With the correct fit, cable twisting is impossible.)
- Adjust the parking brake cable on the adjusting nut at the parking brake lever until the control lever on the calipers lift 1-2 mm from the grooved pin. The measurement between the grooved pin and control lever is carried out using a feeler gauge.
- Slowly and forcefully press the brake pedal several times (One shot adjustment). Forcefully pull parking brake and then release. Check wheels for freedom of movement.
Note:
Adjustment of the parking brake cable is only carried out:
- A. When the parking brake cable is replaced.
- B. When the brake caliper is replaced.
If linings or discs are changed, all that is necessary is to move back the piston.
Traction Control Systems
Safe vehicle handling and braking is primarily achieved by highly developed, 'State of the Art' chassis, suspension and brakes design.
However, even on a perfectly balanced vehicle, critical situations may occur during braking, accelerating and cornering which result in loss of directional stability. Because the majority of drivers would not be able to maintain control of the vehicle in these situations, MINI has developed the following systems to assist drivers in these extreme situations:
- ABS Anti-lock Braking System.
- EBV Electronic Brake Force Distribution.
- CBC Cornering Brake Control.
- MSR Engine Drag Torque Control.
- ASC Automatic Stability Control.
- DSC Dynamic Stability Control.
EBV, CBC and MSR are sub-systems of, and are controlled by, the ABS system. All versions of the MINI are fitted with ABS, and thus EBV, CBC and MSR as standard equipment.
ASC is standard on MINI COOPER S. The ASC option is fitted as a package with ABS, EBV, CBC and MSR systems.
DSC is not standard on any version of the MINI but is available as an option on all versions.
Purpose of the System
ABS
The ABS system prevents the road wheel lock-up during brake application, thus maintaining vehicle stability and steering control under emergency conditions. The system is fitted to all versions of the MINI.
The ABS function is permanently active when the vehicle is being driven and cannot be disabled by the driver. ABS is always fitted together with EBV, CBC and MSR.
Primary system components of the ABS System include:
- Hydraulic/Electronic Control Unit
- Wheel Speed Sensors
- Brake Light Switch
- Brake Fluid Level Switch
- Warning LED
Automatic Stability Control + Traction (ASC+T)
The ASC system prevents the driven front wheels from spinning when engine torque is applied and maintains an equal torque distribution to both wheels. This ensures that optimum traction and directional stability are maintained. ASC is always fitted together with ABS, EBV, CBC and MSR.
The main tasks of the ASC system are:
- To prevent loss of traction and possible resultant steering veer when engine torque is applied to the driving wheels.
- To minimize power induced under steer (front end running wide) when driving through a corner and the front (driven) wheels start to slip due to excessive torque application.
Components differing from ABS system are:
- Hydraulic/Electronic Control Unit
- Interface to EMS2000
- ASC ON/OFF Switch
Dynamic Stability Control (DSC)
The ABS and ASC systems sense and react purely to wheel slip (longitudinal data) but cannot sense the sideways forces (lateral data) acting on the vehicle. ASC therefore has limited influence on stability when cornering.
DSC is fitted as a package with ABS, EBV, CBC, MSR and ASC. DSC interfaces with all these systems but, additionally, monitors the following items:
- Steering wheel movements via the steering angle sensor.
- Lateral (sideways) forces via the acceleration sensor in the DSC sensor cluster.
- Vehicle rotational speed via the yaw rate sensor in the DSC sensor cluster.
System Components
ABS
Hydraulic/Electronic Control Unit
All versions of MINI are equipped with Teves Mk 60 ABS as standard, which includes the following:
- EBV - Electronic Brake Force Distribution
- CBC - Corner Brake Control
- MSR - Engine Drag Torque Control
The ABS Control Unit is mounted in the plenum chamber on the opposite side of the vehicle to the master cylinder. The aluminum body of the unit contains the hydraulic valves that control hydraulic pressure to the individual brake lines.
The cylindrical part of the unit contains the electrically driven hydraulic pump. The plastic molded cover on the unit contains the electronic control unit for the ABS (includes EBV/CBC/MSR).
The purpose of the switched orifices in the inlet valves to the front brakes is to allow a greater volume of fluid to reach the pistons during initial application. When the pad to disc clearances have been eliminated and the required volume of fluid transfer is reduced, the orifices are closed and fluid finds its way through a more restricted path in the inlet valves.
Wheel Speed Sensors
One Wheel Speed Sensor is located in each of the four wheel hubs. The sensors are of the magneto resistive type.
The active sensing of the Magneto resistive Sensor is particularly suitable for advanced stability control applications in which sensing at zero or near zero speed is required. A permanent magnet in the sensor produces a magnetic field with the magnetic field stream at a right angle to the sensing element.
The sensor element is a ferromagnetic alloy that changes its resistance based on the influence of magnetic fields. As the high portion of the pulse wheel approaches the sensing element a deflection of the magnetic field stream is created. This creates a resistance change in the thin film ferromagnetic layer of the sensor element.
The sensor element is affected by the direction of the magnetic field, not the field strength. The field strength is not important as long as it is above a certain level. This allows the sensor to tolerate variations in the field strength caused by age, temperature, or mechanical tolerances.
The resistance change in the sensor element affects the voltage that is supplied by the evaluation circuit. The small amount of voltage provided to the sensor element is monitored and the voltage changes (1 to 10Omv) are converted into current pulses by the evaluation module.
- Signal Low - 7mA
- Signal High - 14mA
The sensor is supplied 12V by the control unit. Output voltage from the sensor is approximately 10V. The control unit counts the high and low current pulses to determine the wheel speed.
Brake Light Switch
The electronic brake light switch supplies important information in hall effect format to the ABS control unit.
Brake Fluid Level Switch
If the hydraulic fluid level is correct, the switch contacts remain closed. Should the hydraulic fluid level fall below a certain point, the switch opens.
ABS Warning LED
Located in the instrument cluster the ABS Warning Led is amber in color. The LED is ON during the pre-Drive check or when the system is faulted.
ASC System
The ASC System, standard on COOPER S and optional on MINI COOPER, adds to the ABS system traction control functions. The Hydraulic/Electronic Control Unit while differing from the ABS unit is similar in appearance and in the same locations as the ABS unit. Additionally, a switch is added to the central switch pack in the center console to enable the driver to turn off the ASC functions.
ACS Switch
The ASC Switch provides an ON/OFF request to the ASC control unit for ASC operation.
ASC Warning LED
The ASC Warning LED in located in the instrument cluster and is amber in color. The LED is ON:
-During Pre-Drive Check
-When the system is switched off by the driver
-When a system fault is detected
The LED Flashes when the system is active.
DSC System
The DSC system, optional on both MINI COOPER AND COOPERS, replaces either the ABS or ASC system. Additional inputs allow the DSC control unit to analyze the desired motion of the vehicle and compare it with the actual motion and if instability is detected to take corrective action.
Additional input devices used by the DSC system:
- Steering Angle Sensor.
- DSC Sensor Cluster.
- Hydraulic Pressure Sensor.
Steering Angle Sensor
The steering wheel angle sensor is a potentiometer with two brushes positioned at 90 degrees to each other. The potentiometer provides data to the DSC control unit. When first fitted on the factory assembly line or when replaced in service, the sensor must be calibrated with the wheels in the straight ahead position. Thus, steering wheel angle is identified and analyzed by the DSC control unit.
DSC Sensor Cluster
The cluster contains a transverse acceleration sensor and a rotation speed (yaw rate) sensor. The acceleration sensor measures the lateral force acting on the vehicle when cornering. The yaw rate sensor measures the speed at which the vehicle is rotating around its vertical axis.
Hydraulic Pressure Sensor
The sensor provides fluid pressure information to the DSC control unit in the event of the driver applying the brakes during DSC intervention.
DSC Switch
The DSC Switch provides an ON/OFF request to the DSC control unit for DSC operation.
DSC Warning LED
The DSC Switch provides an ON/OFF request to the DSC control unit for DSC operation.
The DSC Warning LED is located in the instrument cluster and is amber in color.
The LED is ON:
-During Pre-Drive Check
-When the system is switched off by the driver -When a system fault is detected
The LED Flashes when the system is active.
Principle of Operation ABS
ABS
The four ABS wheel sensors supply information to the ABS control unit that monitors the rotational speed of each wheel. Should one or more wheels decelerate at a rate that exceeds a predetermined value, the ABS control unit activates the ABS hydraulic unit and in turn reduces the hydraulic brake pressure supplied to the affected wheel(s). The ABS control unit and hydraulic unit are assembled together as a single unit.
When the control unit determines that ABS operation is required, it energizes the inlet and outlet solenoid valves within the hydraulic unit corresponding to the relevant hydraulic brake lines and starts the return pump. The inlet valve closes to isolate the brake line(s) from master cylinder pressure and the outlet valve opens to release pressure from the brake line into the accumulator. Thus the brake is released and the relevant wheel begins to accelerate. The control unit then opens and closes the inlet and outlet valves as necessary to modulate the hydraulic pressure and achieve the maximum possible braking effort without locking the wheel.
The return pump takes its fluid supply from the master cylinder reservoir and maintains pressure behind the inlet valves so that when the inlet valves re-open, the pressure in the brake lines is quickly re-established.
During anti-lock brake operation the driver will experience 'feed-back' in the form of a pulsating brake pedal that is accompanied by solenoid/pump noise.
Sub-systems of the ABS system are:
- EBV.
- CBC.
- MSR.
These Sub-systems are also included in the ASC and DSC Systems.
EBV (Electronic Brake Force Distribution)
EBV is a development of the ABS system which controls the distribution of hydraulic brake pressure between the front and rear wheels. This is necessary to optimize the efficiency of the brakes under varying vehicle load conditions. It also allows larger rear brakes to be specified and ensures that front and rear brake linings wear at a similar rate.
The EBV system is fitted to all versions of the MINI and cannot be disabled by the driver.
If the vehicle is carrying 2 passengers in the rear seats, the luggage compartment is full and the fuel tank is full, the point at which the rear wheels start to lock under braking will correspond approximately with that of the front wheels. Thus a high amount of rear brake force can be applied safely and the vehicle's braking performance optimized.
If, however, a vehicle is carrying only the driver and the fuel tank is nearly empty, the rear brakes will start to lock well before the front brakes. This could result in the vehicle becoming unstable. The EBV system is designed to prevent this situation from occurring.
During braking, the ABS control unit detects a potential rear wheel slip situation and instructs the hydraulic unit to close the rear brake inlet valves. This prevents any further increase in hydraulic pressure being applied to the rear brakes while allowing pressure to the front brakes to increase and maximize the overall braking force. If the relative speed of the front and rear wheels return to within p re-determined limits, the ABS control unit opens the inlet solenoid valves in a series of steps which allows a progressive increase in hydraulic pressure to the rear brakes. The slip limit at which EBV operates varies with vehicle speed.
The ABS control unit manages EBV and, while operation is similar to that of ABS, EBV intervenes at lower wheel slip conditions and operates on the rear wheels only. The driver detects operation of EBV when the inlet valves close as a slightly stiffening pedal and a slight pulsing of the pedal when the valves open.
Brake force applied to individual wheels in unloaded and fully loaded conditions.
CBC
CBC is a further development of the ABS system which, together with EBV, helps to keep the vehicle under control under all braking, vehicle loading and steering conditions. The CBC system is fitted to all versions of the MINI. It is permanently engaged and cannot be disabled by the driver. When the brakes are applied, weight is transferred to the front axle of the vehicle. If this occurs during cornering or directional change, the self-steering properties of the vehicle have a tendency to turn the vehicle into the curve (slight over steer). This tendency is normally compensated for by the design characteristics (elastokinematics) of the suspension and by intuitive steering adjustment by the driver. The CBC system adjusts hydraulic pressure to each individual wheel under these circumstances to optimize directional stability. Application of adequate rear axle brake force is largely responsible for vehicle stability when braking in a corner or during directional changes. The CBC system provides the precondition to enable rear brakes with larger pistons to be fitted to the vehicle (pistons with a greater diameter will apply greater brake forces for a given brake pressure). Overall braking efficiency and pad life are also optimized with the fitting of larger rear brakes. The CBC system is controlled by the ABS control unit and operates using the same slip detection and brake modulation principles as the ABS system. However, CBC intervenes at relatively lower system pressures.
Note: CBC operates only when the brakes are applied by the driver as opposed to DSC which operates even when the brakes are not applied. CBC operates about 1% of the time during braking operations.
Brake force applied to individual wheels during CBC operation.
MSR
If the vehicle is coasting with a low gear selected, or during a downshift, excessive engine braking could be applied resulting in wheel slip. This would result in vehicle instability. In this situation, the MSR system intervenes and sends a command to the engine management system to cancel decel fuel cut-off and open the throttle valve. Thus engine drag torque is converted into positive engine torque and wheel slip is eliminated. The system is controlled by the ABS control unit and uses the same slip detection principles as the ABS system.
ASC
The system is controlled by the ASC control unit (which replaces the ABS control unit) and is permanently active when the vehicle is being driven. The driver can disable ASC through the switch panel mounted switch. ASC is fitted as standard to MINI COOPER S version and is available as a factory fitted option on all other versions of the MINI.
The prevention of wheel spin is achieved by engine torque reduction and/or brake application. To prevent one wheel spinning, brake application to that wheel is employed and to prevent both wheels spinning, engine torque reduction is employed. These processes occur within milliseconds.
The ASC control unit checks for wheel spin by comparing the speed of each front driven wheel with the speed of the rear wheel on the same side. With this information, the ASC control unit determines whether brake application or engine torque reduction is appropriate. If brake application is required, the hydraulic unit isolates the brake line to the appropriate driven wheel and hydraulic pressure is applied in that brake line until the speed of the wheel returns to a value acceptable to the control unit. The return pump that takes its fluid supply from the master cylinder reservoir provides hydraulic pressure.
If engine torque reduction is also required, the ASC control unit sends the necessary command to the Engine Management System (EMS) control unit. A torque change request from the ASC control unit takes priority over any other torque change request received by, or generated internally within, the EMS control unit. Torque reduction is limited to the minimum value necessary to prevent wheel spin. The ASC control unit constantly updates the EMS control unit to ensure that 'driver demand' for acceleration is restored as soon as conditions permit.
The ASC warning lamp in the instrument cluster will flash when the system is operating. The warning lamp is illuminated permanently if the driver has switched off the ASC system or if the ASC control unit has detected a fault with the system. The need to switch off the ASC system should only be necessary under unusual circumstances such as driving away in snow or muddy conditions. The system must be switched off when snow chains are in use.
DSC
When the lateral cornering forces acting on the vehicle overcome the available grip on the road, under steer or over steer will occur.
If the DSC system senses that under steer is occurring, it will apply brake force to the rear wheel that follows the inside radius of the curve thus applying a counter force which helps to bring the vehicle back onto its original course. (A much lower braking force may also be applied to the front wheel on the inside of the curve.)
If the DSC system senses that over steer is occurring, it will apply brake pressure to the front wheel that follows the outside radius of the curve. (A much lower braking force may also be applied to the rear wheel on the outside of the curve.)
At the same time as with ASC, engine torque reduction will be applied.
DSC operates whether or not the driver applies the brakes and, if necessary, during ABS controlled braking.
The DSC warning lamp in the tachometer (or center mounted speedometer for USA vehicles) will flash when the system is operating. This is the same lamp and symbol as that of a vehicle equipped with the ASC system. The warning lamp is illuminated permanently if the DSC control unit has detected a fault with the system. The driver through the switch panel may disable the DCS system. When DSC is disabled, the DSC warning light is illuminated.
Special DSC Brake Lines
The flexible sections can be seen adjacent to the bulkhead on the same side of the vehicle as the hydraulic unit and are a useful visual indicator that DSC is fitted. On vehicles equipped with DSC, short lengths of flexible hose are situated in the rigid hydraulic brake lines from the master cylinder to the hydraulic unit. Their purpose is to reduce the rigidity of the master cylinder brake lines (which are of a larger diameter on DSC equipped vehicles) and also to minimize transmission of system noise.