capacity due to increased traction; particularly advantageous on road surfaces

Transcription

1 Design and function of acceleration slip control (ASR I) A. General B. Driving with ASR I C. Overall function of ASR I D. Location of components E. Individual functions A. General The acceleration slip control (ASR) is an automatic system for improving the capacity for as of off and accelerating as well as driving stability. ASR is a logical extension of the anti-lock braking system (ABS). Whilst with ABS the wheels are prevented from locking when braking, thus maintaining steerability and driving stability as the vehicle decelerates, ASR prevents the driving wheels from skidding when as of off and accelerating. It thus provides optimum driving stability together with increased traction capacity. For ASR the basic ABS components are supplemented by an additional hydraulic unit, a pressure supply unit, a fourth wheel speed sensor, extended electronics and the electronic accelerator pedal with control unit position sensor and actuator. Advantages Improves starting off and acceleration capacity due to increased traction; particularly advantageous on road surfaces with different levels of adhesion and on corners. Increases active safety, since only a wheel which is not sliping can provide optimum propulsion without any loss of lateral control. Automatically adapts the engine torque to the ability of the wheels to transmit this to the road when the driver applies too much throttle. A function display provides information on the ASR control operation and provides a signal to the driver that he is driving the vehicle close to its limits. Cruise control (Tempomat) is also contained in the electronic accelerator control unit. B. Driving with ASR I ti Sivu 1, yhteensä 24

2 Displays with various symbols are located in the instrument cluster for ASR. A1e17 A1e21 A1e22 ABS indicator lamp ASR function indicator lamp ASR indicator lamp P Switching on the ignition When the ignition is switched on (ignition/starter switch "position 2") the ASR function indicator lamp (A1e21) and the ABS (A1e17) and ASR (A1e22) indicator lamps in the instrument cluster (bulb check) come on and go out when the engine is running. Driving mode In driving mode the ASR function indicator lamp (A1e21) comes on as soon as ASR is operating. The function indicator lamp remains on approximately 1 s after the ASR control mode is terminated. The function display does not come on if ASR control mode is in operation for a very brief period. Control mode The function indicator lamp provides information on the ASR control mode and provides a signal to the driver that he is driving the vehicle close to its limits. He can then better adapt his driving style to the prevailing road conditions. ti Sivu 2, yhteensä 24

3 Checking If the ASR indicator lamp (A1e22) comes on when the engine is running there is a fault in the ASR or in the electronic accelerator pedal. ASR is inoperative, the ABS continues to operate. If there is a fault in the electronic accelerator pedal it is possible to continue driving with reduced engine performance. The driver notices this due to changes in the force required to operate the accelerator pedal. If the ASR and ABS indicator lamps (A1e17 and A1e22) come on when the engine is running, there is a fault in the ABS, which also causes the ASR to be switched off, or in the ASR which also influences the ABS. Both systems are switched off. Snow chain switch (S76) In deep snow and when snow chains are fitted the sensitive, stability-oriented control of ASR can have a disadvantageous effect when starting off. Therefore a special control logic has been programmed in for the starting off range in such situations, which can be selected by the driver via the snow chain switch. The indicator lamp in the switch comes on when the snow chain switch is operated. After approx. 2.5 s of continuous ASR control mode a higher drive slip is permitted at the wheels so that the vehicle can bite its way out in the above named situations. Since this higher slip would have a negative effect on driving stability particularly at higher speeds, it remains limited to the speed range < 38 km/h. The higher drive slip remains stored in memory up to 60 km/h and is automatically brought back into operation when the vehicle speed falls below 38 km/h. If 60 km/h is exceeded the memory is erased and the indicator lamp in the switch goes out. P ti Sivu 3, yhteensä 24

4 C. Overall function of ASR I The speeds of all wheels are recorded and processed in the ABS/ASR control unit. If the ABS/ASR control unit recognizes from the wheel speed signals that one or both driving wheels are tending to spin, the ASR control mode starts with the braking moment control circuit and the drive moment control circuit. The following processes are initiated: Brake moment control circuit The spinning drive wheel is braked via the relevant wheel brake until it has achieved the most favorable drive slip range. In this way the other wheel can be driven to best effect (differential lock effect). To brake the spinning wheel part of the brake fluid under high pressure in the accumulator is channelled from there into the brake caliper via the ABS and ASR hydraulic lines (pressure build-up). A solenoid valve modulates the pressure characteristics in the brake caliper during brake moment control, in the pressure build-up phase, pressure holding stage, and pressure reduction stage. For pressure reduction the volume of brake fluid from the brake caliper is returned into the accumulator via the return pump. This control sequence is performed individually for each wheel brake of the drive wheels. Drive moment control circuit In order to reduce drive moment which is too high for optimum traction, there is an exchange of signals via the connection between the ABS/ ASR control unit and the electronic accelerator control unit. The throttle position is reduced by the actuator from the accelerator pedal position set by the driver. A check is constantly made in the ABS/ ASR control unit as to whether the control functions can be cancelled e.g. as a result of a sudden improvement in road surface adhesion, so that the drive moment demanded by the driver via the accelerator pedal can come back into operation as soon as possible. A distinction has to be made between two control ranges in ASR control mode: Control mode with one spinning drive wheel and at vehicle speeds <38 km/h. Here the brake moment control responds before the drive moment control. Priority is thus given to maximum traction. Control mode when both drive wheels spin or at a vehicle speed >38 km/h. In this case the drive moment control responds before the brake moment control. Priority is thus given to maximum driving stability. A function display lamp provides information that ASR is in control mode and signals to the driver that he is driving the vehicle close to its limits. The function of ABS is not impaired by ASR since the ASR control mode is interrupted when braking. ti Sivu 4, yhteensä 24

5 Brake moment control circuit The brake moment control circuit is based on the electric/electronic and hydraulic components of the service brake with ABS. Electrics/electronics All ASR functions are contained next to the ABS functions in a common ABS/ASR control unit (N30/1). Compared to the conventional ABS control unit, further input signals and two ASR microcomputers are present for ASR functions and additional output signals. Hydraulics It was necessary to supplement the ABS (A7) hydraulic unit with the ASR hydraulic unit (A7/1). The ASR hydraulic unit contains a switchover valve for changing from braking mode to ASR control mode. In addition, pressure limiting valves as hydraulic safety equipment and the solenoid valve for the right rear wheel brake are included. The brake pressure necessary for ASR control mode comes from the accumulator (13) which is filled by the pressurizing pump (M15). The speed signals of the wheel speed sensors are continuously processed to the following parameters in the ABS/ASR control unit. Drive slip Acceleration Vehicle speed (front wheel) If the values obtained exceed specified nominal values corresponding output signals affect the control of the three pressure phases (pressure build-up, pressure holding, pressure reduction) in the hydraulic units (brake intervention). The brake intervention starts above 4% slip with pressure build-up in the relevant rear wheel brake, depending on the vehicle speed. Pressure is held between 4% and 2% slip and reduced below 2%. V = speed V = speed differential between front and rear wheels Drive moment control circuit The drive moment control circuit is based on the electric/electronic components of ABS/ASR and is supplemented by the electronic accelerator pedal. The electronic accelerator pedal comprises the following electrical/electronic and mechanical components: potentiometer (R25) actuator (M16/1) electronic accelerator control unit (N4/1) control linkage and accelerator pedal free travel rod (redundancy rod) The speed signals from the speed sensors are constantly processed in the ABS circuits and ASR microcomputers in the ABS/ASR control unit (N30/1) to the following parameters: ti Sivu 5, yhteensä 24

6 drive slip acceleration vehicle speed (front wheel) If the values obtained exceed specified nominal values, the ABS/ASR control unit causes the electronic accelerator control unit to close the throttle. The actuator and thus the throttle are closed irrespective of the adjustment movements of the accelerator pedal. The throttle is closed above 4% slip, held between 4% and 2% slip and opened below 2% in accordance with the accelerator pedal position, depending on the vehicle speed. V = speed V = speed differential between front and rear wheels Driving mode An ASR control mode operates as follows irrespective of the road surface characteristics and quality. ti Sivu 6, yhteensä 24

7 If a drive wheel slips and at speeds <38 km/h the braking moment control overrides the drive moment control. The brake moment control circuit operates first as follows: the ABS/ASR control unit establishes an inadmissibly high degree of slip and transmits a signal to the solenoid valve in the hydraulic units. After this, accumulator pressure reaches the corresponding brake caliper so that the relevant wheel is held firmly. In this way the wheel on the road surface with good adhesion is supplied with torque via the differential. If the driver fully depresses the accelerator then the spinning wheel slips even more. Drive moment control operates at about double the slip of the brake moment control. The ABS/ASR control unit sends a signal to the electronic accelerator control unit. The throttle is now moved by the actuator against the accelerator pedal position. Drive moment is reduced. The throttle is slowly opened again as soon as the amount of slip falls below the lower limit. The maximum possible drive moment can thus be transmitted to the road. If the road surface quality changes then the control process starts again. If both drive wheels spin or at vehicle speeds Under some conditions the driver may detect the >38 km/h the drive moment control overrides the adjustment of the throttle by means of brief brake moment control. The ABS/ASR control changes in the accelerator pedal forces. The unit establishes an inadmissibly high degree of adjustment is performed until one wheel is below slip and transmits a signal to the electronic the critical slip threshold. The control process accelerator control unit. The throttle is moved by then continues as for the slippage of one drive the actuator against the accelerator pedal wheel. position set by the driver. The drive moment is therefore reduced until no slip can occur at the drive axle to impair driving stability. D. Location of components ti Sivu 7, yhteensä 24

8 /1 Electrics/electronics A1e17 ABS indicator lamp Hydraulics A1e21 ASR function indicator lamp A7 ABS hydraulic unit A1e22 ASR indicator lamp A7/1 ASR hydraulic unit K1/1 Overvoltage protection relay M15 Pressurizing pump K20 Pressurizing pump relay 4a Brake fluid reservoir L6/1 Left front axle speed sensor 13 Accumulator L6/2 Right front axle speed sensor 6a/6b Rear brake caliper with wear indicator L6/3 Left rear axle speed sensor L6/4 Right rear axle speed sensor Mechanics M16/1 Electronic accelerator pedal actuator 20 Free travel rod (redundancy rod) N4/1 Electronic accelerator pedal control unit N30/1 ABS/ASR control unit R25 Electronic accelerator pedal position sensor S9/1 Stop lamp switch S40 Tempomat (cruise control) switch S76 Snow chain switch with indicator E. Individual functions ti Sivu 8, yhteensä 24

9 Electric/electronic components /1 A1e17 ABS indicator lamp L6/3 Left rear axle speed sensor A1e21 ASR function indicator lamp L6/4 Right rear axle speed sensor A1e22 ASR indicator lamp N4/1 Electronic accelerator pedal control unit K1/1 Overvoltage protection relay N30/1 ABS/ASR control unit ti Sivu 9, yhteensä 24

10 K20 Pressurizing pump relay S9/1 Stop lamp switch L6/1 Left front axle speed sensor S76 Snow chain switch with indicator L6/2 Right front axle speed sensor Function of electrics/electronics The corresponding functions in the hydraulic solenoid valves in the hydraulic units units (brake moment control circuit) and the relays (pressurizing pump, solenoid valves, functions of the electronic accelerator pedal return pump) (drive moment control circuit) are controlled by electronic accelerator control unit (throttle the electric/electronic components. valve nominal value ) Voltage is supplied to the ABS/ASR electronic ASR function indicator control unit mainly via the overvoltage protection indicator lamp in the snow chain switch relay. ABS indicator lamp The ABS/ASR control unit receives input signals ASR indicator lamp from the following components: The ABS/ASR control unit decides which of the 4 wheel speed sensors three following operating modes have to be stop lamp switch engaged based on the input signals: electronic accelerator control unit (throttle actual value) normal mode snow chain switch ABS control mode pressure switch ASR control mode The input signals are processed in the ABS/ ASR control unit into output signals for the following components Overvoltage protection relay (K1 or K1/1) The overvoltage protection relay is located on the left of the accessories compartment. The 5-pin overvoltage protection relay (K1) used in ABS has been installed in ASR from the start of production. The 7-pin overvoltage protection relay (K1/1) has been phased in as of around Autumn P Wheel speed sensor (L6) Vehicles with ASR are equipped with four wheel speed sensors. The location and design of the two wheel speed sensors (L6/1, L6/2) on the front axle have been taken from the 126 models with ABS. The wheel speed sensor housing has a diameter of 18 mm. Sivu 10, yhteensä 24

11 Identification: double-edged tip. One wheel speed sensor is located on the left and the right of the rear axle housing. The speed sensor housing has a diameter of 15 mm. Identification: single-edged tip. Left rear axle speed sensor (L6/3) The left wheel speed sensor is straight. The cable has a connector (L6/3x) as a plug connection. P Right rear axle speed sensor (L6/4) The right wheel speed sensor is angled. A plug (L6/4x) is attached as plug connection to the cable in order to avoid confusion with the connection of the left rear axle speed sensor. P Sivu 11, yhteensä 24

12 The speeds of the rear wheels are measured separately at the serrations on the rear axle shaft flange. L6/3 Left rear axle speed sensor L6/4 Right rear axle speed sensor 1 Rear axle shaft flange with serrations P Stop lamp switch (S9/1) The stop lamp switch is located on the foot controls above the brake pedal. When the service brake is operated, the ABS/ASR control unit receives a signal from the stop lamp contact via the cable connector of the stop lamp switch/ electronic speedometer (X21/2) whereby any ASR function is terminated immediately. The change-over valve (A7/1y2) and the solenoid valves (A7/1y1, A7y2) for the ASR are switched to the original position at the same time. The contact pin 14 on the electronic accelerator control unit is connected directly to ground via the additional contact. When the brake pedal is operated, this ground connection is interrupted and the Tempomat function is switched off. Sivu 12, yhteensä 24

13 ASR snow chain switch (S76) The snow chain switch (arrow) is located in the instrument panel in the center console. It is designed as a touch switch. After the switch has been operated an indicator lamp in the switch comes on which confirms that a higher slip threshold is preselected in the ABS/ ASR control unit. P ABS/ASR control unit (N30/1) The ABS/ASR control unit is located in the center of the accessories compartment. The control unit comprises various printed circuit boards which are equipped with electronic components. Connection to the cable harness is via the 35-pin plug connection. P Apart from the ABS functions the ABS/ASR control unit also contains all the ASR functions. It is divided up into functions as follows: signal processing input amplifier logic sections safety circuits output stages Signal processing Sivu 13, yhteensä 24

14 The input signals from the four wheel speed sensors are transformed into digital signals in the signal processing sections and transmitted to the ABS circuits and ASR microcomputer and processed. Input amplifier There is one input amplifier for each of the input signals from stop lamp switch throttle valve actual value snow chain switch pressure switch (A7/18s1). Here the incoming digital signals are filtered and converted into values which can be evaluated in the logic section and safety circuit. Logic section All input signals are processed in the logic sections of the ABS circuits and ASR microcomputer. Wheel speed signals: The processed wheel speed signals are continuously compared with each other and then with the specified slip thresholds (upper and lower limit of differential speeds between rear and front wheels, depending on the vehicle speed. The following values are determined through this comparative process: If the values obtained exceed the specified slip thresholds then an ASR control mode is initiated. Stop lamp switch signal: When the brake is operated a signal is transmitted to the logic section. The solenoid valves are switched into the original position by the logic section via the respective output stages. In other words, ASR control mode is terminated immediately. Throttle valve actual value signal: In ASR control mode, the logic section controls throttle closure by the electronic accelerator pedal via the throttle valve actual value according to the specified throttle valve nominal value. Snow chain switch signal: If there is a signal from the snow chain switch to the logic sections, an increased slip threshold is used in the logic and a signal issued to the indicator lamp via the corresponding output stage. drive slip acceleration front wheel driving speed Sivu 14, yhteensä 24

15 Pressure switch signal: The pressurizing signal from the pressure switch causes the activation of the pressurizing pump relay via the corresponding output stage. The pressurizing pump starts to fill up the accumulator. Safety circuit It is the task of the safety circuits to recognize faulty signals within the electronic control units and faults in the relays in the hydraulic units as well as the wheel speed sensors. The ABS and ASR system is then switched off in the event of a fault. The safety circuit also continuously monitors battery voltage. If the battery voltage is < 10 V, the system is switched off, until the voltage has again attained its nominal value. The following faults are recognized: control unit defective solenoid valve defective wheel speed sensor defective or loose contact relay defective return pump defective pressurizing pump defective accumulator leaking pressure switch defective. The faults detected are displayed by the indicator lamps coming on. When the ignition is switched off all faults which have occurred briefly are erased again. Output stages The output signals from the ABS circuits and ASR microcomputers to the solenoid valves, relays, indicator lamps etc. are amplified by appropriate output stages. Indicator lamps/functional indicator lamp The ASR (A1e22) and ABS (A1e17) indicator lamps are located at the bottom right of the instrument cluster. The ASR function indicator lamp (A1e21) is located towards the top in the center of the speedometer. All three lamps come on to check the bulb functions with the ignition starter switch in "position 2" in the ignition lock. If there are no faults in the system the indicator lamps and the function indicator lamp go out as soon as the engine is running. P Indicator lamps come on when engine is running: if the ASR indicator lamp comes on there is a fault in the ASR or in the electronic accelerator pedal. The ASR is switched off and ABS remains fully operative. If there is a fault in the electronic accelerator pedal, the vehicle can continue to be driven with slightly restricted engine performance via the emergency running system. Sivu 15, yhteensä 24

16 If the ASR and ABS indicator lamps come on, there is a fault in the ASR which influences the ABS or a fault in the ABS. Both systems are put out of operation. The function indicator lamp (A1e21) is actuated via a 4-pin plug connection on the reverse side of the instrument cluster. When the lights are switched on the brightness of the function display is reduced. Pressurizing pump relay (K20) The pressurizing pump relay is located on the partition wall in the accessories compartment. If a signal is transmitted from the ABS/ASR control unit to the pressurizing pump relay terminal 85 via terminal 33 then the relay is actuated and switches positive to the pressurizing pump motor. Thus the pressurizing pump delivers brake fluid to the accumulator. P Hydraulic components Sivu 16, yhteensä 24

17 P A7 ABS hydraulic unit 4a Brake fluid reservoir A7/1 ASR hydraulic unit 6a Rear right brake caliper with wear indicator M15 Pressurizing pump 6b Rear left brake caliper with wear indicator 13 Accumulator Sivu 17, yhteensä 24

18 Hydraulics function The function of the hydraulics is divided up basically into three modes of operation, which are effective in the ABS and the ASR hydraulic units. normal mode ABS control mode ASR control mode Normal mode All solenoid valves are in the original position (position 0). The tandem brake master cylinder is connected directly to the front axle brake calipers via the left front (A7y1) and right front (A7y2) solenoid valves. The brake line to the rear axle brake circuit is connected to the ASR hydraulic unit at connection "HZ". The rear axle brake calipers are connected via the changeover valve (A7/1y2) and both the left rear (A7y3) and right rear (A7/1y1) solenoid valves. P Bleed screw 1 (test connection) A7 ABS hydraulic unit 2 Bleed screw 2 (pressurizing pump) A7y1 Left front axle solenoid valve 4 Brake booster with tandem brake master A7y2 Right front axle solenoid valve cylinder A7y3 Left rear axle solenoid valve 4a Brake fluid reservoir A7m1 Return pump 5a Right front wheel brake A7/1 ASR hydraulic unit 5b Left front wheel brake A7/1s1 Pressure switch 6a Right rear wheel brake A7/1y1 Right rear axle solenoid valve 6b Left rear wheel brake A7/1y2 Switchover valve 8 Pump input valve 7 Check valve M15 Pressurizing pump 9 Pump output valve SP3 Bleed screw 3 (accumulator) 10 Silencer 11 Low pressure accumulator 12 Pressure limiting valve - pressurizing pump 13 Accumulator 14 Pressure limiting valve Sivu 18, yhteensä 24

19 16 Pressure limiting valve 17 Pressurizing valve ABS control mode The following modifications have been made to the ABS without ASR: The brake pressure from the tandem brake master cylinder is led to the two left rear and right rear solenoid valves via the switchover valve (A7/1y2). Both solenoid valves (A7y3, A7/1y1) are actuated together synchronously for the rear axle circuit in ABS control mode (pressure build-up, pressure holding, pressure reduction). ASR control mode The control mode requires a full accumulator. The pressure switch constantly monitors the accumulator pressure. The brake pressure for acceleration slip control comes exclusively from the accumulator. If the ABS/ASR control unit recognizes that a wheel is tending to spin, the switchover valve (A7/1y2) is switched into "position 2" (and remains in this position until the control process is terminated). Thus the full accumulator pressure is present in front of the solenoid valves (A7y3, A7/1y1). Depending on how much and which wheel spins, the three P pressure phases, pressure build-up (position 0), pressure holding (position 1) and pressure reduction (position 2) are activated on at the solenoid valves (A7y3, A7/1y1). Solenoid valve - position 0, 1, 2 Brake fluid reservoir As previously, the brake fluid reservoir is located above the tandem brake master cylinder. The volume has been increased to 350 cm 3 for vehicles with ASR. The brake fluid reservoir has an additional line connection (arrow), to which the pressurizing pump and the ASR hydraulic unit at connection "BA" is connected. Two brake fluid level switches (S11) and two plugs are located on the brake fluid reservoir. P Sivu 19, yhteensä 24

20 Pressurizing pump (M15) The pressurizing pump is located in the pressure supply unit behind the plastic cover under the front left fender. Together with the accumulator it is fixed to a frame, which in turn is bolted on to the front end using rubber elements. 13 Accumulator M15 Pressurizing pump P A Accumulator The accumulator is located at the pressure supply unit behind the plastic cover beneath the left front fender. (Refer to figure "pressurizing pump (M15)"). It is designed as a piston-type accumulator. 1 Screen 2 Check valve 3 Storage chamber 4 Gas chamber 5 Twin seal piston 6 O-ring 7 Bore 8 Plastic cover 9 Bleed orifice 10 Overflow P Inlet line from pressurizing pump E Outlet line to ASR hydraulic unit Sivu 20, yhteensä 24

21 When bleeding the hydraulic system the twin seal piston must seat on the base of the storage chamber. The brake fluid thus presses any residual air via the bleed bore (9), directly to the outlet line "E" and from there to the bleed screw "SP3" (see bleeding the brake system ). ABS hydraulic unit (A7) The ABS hydraulic unit is located at the left front of the accessories compartment. In principle it is derived from the conventional ABS. The brake circuits for the front and rear wheel brakes remain separate. Three solenoid valves are located in the hydraulic unit: left front axle solenoid valve (A7y1) right front axle solenoid valve (A7y2) left rear axle solenoid valve (A7y3). (See hydraulics function, normal mode) The ABS hydraulic unit for ASR differs from the previous ABS hydraulic unit externally through modified connections, as follows: P I Left front wheel brake r Right front wheel brake V Front brake circuit 1 Bleed screw one (test connection) HL Left rear wheel brake SA Suction line H Rear brake circuit Sivu 21, yhteensä 24

22 On vehicles with ASR the left rear solenoid ASR hydraulic unit (A7/1) valve (A7y3) is used to actuate the left rear The ASR hydraulic unit is located next to the wheel brake. With ABS without ASR, as is ABS hydraulic unit. already known, this valve is used to regulate the It contains the following hydraulic components: rear wheel brakes together. 2 pressure limiting valves (14, 16) In an ASR control process the brake pressure 2 solenoid valves (A7/1y1, A7/1y2) supply for the left rear wheel brake is controlled 1 pressurizing valve (17) in the ABS hydraulic unit for ASR. The brake 1 pressure switch (A7/1s1) pressure is supplied exclusively from the accumulator (13), to the ASR hydraulic unit via (Refer to hydraulics function, normal mode). line "E", from there to the ABS hydraulic unit for The connections of the hydraulic lines are ASR via line "H". The brake pressure for the left marked with embossed letters, as on the ABS rear brake caliper is controlled in the left rear hydraulic unit for ASR. solenoid valve in three pressure phases by corresponding signals, which come from the ABS/ASR control unit: pressure build-up pressure holding pressure reduction HZ Brake master cylinder E Inlet (line from accumulator) BA Reservoir connection SP3 Bleed position no. 3 (accumulator) HR Rear right SA Suction line H Rear P Sivu 22, yhteensä 24

23 P Bleed screw 1 (test connection) A7 ABS hydraulic unit 2 Bleed screw 2 (pressurizing pump) A7y1 Left front axle solenoid valve 4 Brake booster with tandem brake master A7y2 Right front axle solenoid valve cylinder A7y3 Left rear axle solenoid valve 4a Brake fluid reservoir A7m1 Return pump 5a Right front wheel brake A7/1 ASR hydraulic unit 5b Left front wheel brake A7/1s1 Pressure switch 6a Right rear wheel brake A7/1y1 Right rear axle solenoid valve 6b Left rear wheel brake A7/1y2 Switchover valve 8 Pump input valve 7 Check valve M15 Pressurizing pump 9 Pump output valve SP3 Bleed screw 3 (accumulator) 10 Noise damper 11 Low pressure accumulator 12 Pressure limiting valve - pressurizing pump 13 Accumulator 14 Pressure limiting valve 16 Pressure limiting valve 17 Pressurizing valve The electrical connections are routed into the ASR hydraulic unit via a 4-pin round plug connection. The hydraulic components have the following tasks: Sivu 23, yhteensä 24

24 Pressure limiting valve (14) is a safety device and protects the control circuit from excessive pressures. Pressure limiting valve (16) protects the system against excessive accumulator pressure. If the accumulator pressure rises above 230 bar, the pressure limiting valve opens and permits the excessive pressure to flow back directly to the brake fluid reservoir via the "BA" line. The switchover valve (A7/1y2) is a solenoid The pressurizing valve (17) is actuated when valve and ensures the change-over from normal the brake pedal is operated by the pressure in mode to ASR mode. the rear axle brake circuit. In the event of ABS The switchover valve (as well as the left rear activation, a loss in pressure volume is prevented and right rear solenoid valves) switched to by the additional shut-off action of the check continuity in "position 0". When the brake pedal valve. When bleeding at position 1 the bleeding is operated the brake pressure is led directly equipment opens the check valve via line "BA" from the tandem brake master cylinder to the and thus the line "SA" is vented. brake calipers. In an ASR control mode the switchover valve is switched so that the The pressure switch (A7/1s1) monitors the accumulator is connected directly to the line to pressure in the accumulator (13). The the left rear and right rear solenoid valves. pressurizing process starts at a pressure of approx. 160 bar and when the pressure reaches The solenoid valve (A7/1y1) controls (on ASR approx. 180 bar the pressurizing process is and ABS) the brake pressure in the right rear complete. brake caliper in accordance with actuation signal from the ABS/ASR control unit in the three pressure phases Pressure build-up Pressure holding Pressure reduction Sivu 24, yhteensä 24