hydraulic add-on abs system - wabco inform...
TRANSCRIPT
Hydraulic ADD-ONABS system
System description
8150004303
1st edition
� Copyright WABCO 2003
Vehicle Control SystemsAn American Standard Company
The right of amendment is reservedVersion 002/07.03
3
Page1. Introduction 42. General notes 4
2.1 Hydraulic brake system 43. System description 5
3.1 Abbreviations 53.2 System structure 53.3 Basic functions and tasks of the anti-lock brake system 53.4 An anti-lock brake system control loop 63.5 What the driver needs to know 7
4. Mode of function 84.1 Why modified individual control (MIC)? 84.2 How the anti-lock brake system operates 94.3 Purpose of electronic brake force distribution (EBD) 94.4 Purpose of electronic traction control (ETC) 104.5 Anti-lock brake system warning light 104.6 Electronic traction control light 104.7 Mode of operation without electronic traction control 114.7.1 Pressure buildup 114.7.2 Pressure holding 124.7.3 Depressurisation 134.8 Mode of operation during electronic traction control 144.8.1 Pressure buildup / pressure holding / depressurisation 144.8.2 Control with electronic traction control 15
5. Components 166. Diagnosis 207. Brake diagram / offer drawings 248. Workshop instructions 36
8.1 Brake system fault table 378.2 Changing brake fluid 388.3 Fitting a new modulator 398.4 Fitting a new speed sensor 398.5 Fitting a new ECU 40
9. Concept 419.1 Tasks of the brake 419.2 Physical principle 429.3 Formulae 429.4 Legal provisions 439.5 Classification of vehicles according to EC Directive 98/12/EC 439.6 Physical principles of braking 449.7 Time sequence of the braking procedure 459.8 Driving with the ADD-ON anti-lock brake system 469.9 Points to bear in mind 469.10 Impulse wheel specification 46
Table of contents HABS
4
Introduction / General notesHABS1.1. Introduction
This publication dealing with the WA-BCO hydraulic ADD-ON anti-lockbrake system 4S/4M 12 V (HABS),WABCO system part number 400050 ... 0, is intended for workshopengineers, drivers and companies in-stalling the system in their vehicles.
In addition to operating information, it
contains additional technical detailsand safety instructions. Furthermore,this publication is intended to helpthe workshop engineer to maintain,repair and diagnose faults in the sys-tem.
In this publication, WABCO is also of-fering some information for compa-
nies who install the system in theirvehicle, in order to assist them interms of vehicle design. More information about the systemcan be obtained from our ServiceHotline, telephone number 0180 2232337, or on the Internet at www.wab-co-auto.com.
2. General operating and safety instructions
2.1 Hydraulic brake system
Brakes are a primary safety compo-nent, incorrect work on brakes mayresult in them failing to operate. Onlytrained personnel are allowed to per-form any maintenance and repairwork on the brake system. Take careto maintain absolute cleanlinesswhen working on the brake system.Disconnect the negative battery ter-minal before starting any work on theelectrical system.
The driver of the vehicle is responsi-ble for performing the following work:
� Checking the brake fluid level
� Checking the effectiveness of thebrake system
Checking the fluid levelThe fluid level must always be be-tween the MAX and MIN marks.
A slight reduction in the fluid leveltakes place in the course of normaldriving and is caused by the auto-matic adjustment of the brake pads(wear).However, there may be a leak in thebrake system if the fluid level drops
significantly within a short period oftime.
Changing the brake fluidBrake fluid is hygroscopic, whichmeans it absorbs water. Excessivewater content in the brake fluid mark-edly reduces its boiling point. Thiscan result in brake failure if the wheelbrakes are very hot.
As a result, the brake fluid must bechanged in accordance with the vehi-cle manufacturer's instructions.
Brake fluid is poisonous!Consequently, always keep itsealed in its original container andin particular store it out of thereach of children.
Furthermore, bear in mind that brakefluid also attacks the vehicle's paint-work.
Due to the disposal problem, theneed for special tools and the techni-cal expertise required, you shouldonly have your brake fluid changedby authorised specialist personnel.
Wear eye protection when changingthe brake fluid. If brake fluid comesinto contact with any parts of yourbody during or after the necessarywork, immediately wash it off withsoap and water.
Seek immediate medical attention ifany is swallowed or you get any inyour eye.
Warning!
5
3. System description
This chapter describes the basicfunctions of the anti-lock brake sys-tem and its tasks, as well as the gen-eral structure of the system with itsmechanical and electrical features.
3.1 Abbreviations
To aid understanding, it is necessaryto present a few basic definitions be-fore explaining the functions of thesystem.
ADD-ON systemAn add-on system can be installedinto an existing, conventional dual-circuit service brake system
HABSHydraulic anti-lock brake system
ABSAnti-lock brake system
ECUElectronic control unit
EBDElectronic brake force distribution
ETCElectronic traction control (traction help)
BWLBrake warning light
Four-channel modulatorThe brake pressure on each individ-ual wheel is controlled by one outletand one inlet valve each, i.e. thereare four outlet and four inlet valves inthe modulator.
Drive shaftThis is the axle which carries thetorque from the engine to yourwheels. It may also be a steering ax-le, depending on the type of vehicle.
MICModified individual control(ABS control algorithm)
4S/4M4 wheels fitted with sensors and 4wheels with brake pressure control.
12VVehicle electrical system voltage V = 12 volts
3.2 System structure
The vehicle's brake system consistsof a hydraulic brake master cylinderwith a brake booster as the actuationdevice and four hydraulic wheelbrakes on the front and rear axles.
The WABCO ADD-ON ABS systemconsists of:
1. A four-channel brake pressuremodulator. The brake pressure ateach wheel is controlled by anoutlet and an inlet valve. If thebrake pressure drops during ABSbraking, a return pump pumps thebrake fluid back into the brakemaster cylinder (closed system),
2. four speed sensors with impulsewheels which constantly monitorthe speed of each wheel,
3. and a central electronic controlunit (ECU).
The ECU processes the four speedsignals from the wheels and causesthe solenoid valves to be actuated.The appropriate solenoid valves arepulsed according to the require-ments, in order to maintain, reduceor increase the brake pressure.
Furthermore, the ECU monitors itsown functions and checks the electri-cal components of the system. TheECU automatically switches over tosafe mode if a component fails. In
such cases, the normal function ofthe service brake is assured.
An ISO 9141 diagnostic interfaceforms part of the ECU. This permitsmessages stored during operation tobe downloaded from the ECU'smemory chip.
3.3 Basic functions and tasks of the anti-lock brake system
The principal task of the ABS systemis to guarantee that the vehicle canstill be steered and holds its courseduring braking.
Anti-lock brake systems (ABS) - gen-erally also referred to as anti-locksystems (ALS) - must prevent the ve-hicle's wheels from locking as a re-sult of excessively powerfulactuation of the service brake, mainlyon slippery road surfaces.
If the critical locking point of thewheels is reached during braking,the brake pads are pulsed (releasedand reapplied) within fractions of asecond. This permits the wheels tocontinue to turn and means that thevehicle can still be steered, even dur-ing full braking.
Despite the advanced developmentstatus of commercial vehicle brakes,potential accident situations oftenoccur when braking on slippery roadsurfaces. During full or even partialbraking on a slippery road, it may nolonger be possible to transmit all ofthe brake force onto the road due tothe low coefficients of friction be-tween the tyres and the carriageway.The braking force is excessive andthe wheels lock up.
When the wheels are locked up, theycease to offer any purchase on theroad and they transmit almost no cor-nering forces (steering and tracking
System description HABS 3.
6
forces). This often has dangerousconsequences.
– The vehicle cannot be steered
– The vehicle swerves despitecountersteering and starts to skid
– The braking distance issignificantly increased.
As a result, cornering forces onbraked wheels should be maintainedeven during full braking, so as toguarantee that the vehicle or vehiclecombination remains stable and canstill be steered as far as physicallypossible. At the same time, the fric-tion contact between tyres and thecarriageway should be fully opti-mised, therefore reducing the brak-ing distance and making the vehicledecelerate more rapidly.
Requirements on ABS:• ABS must work even at very slow
speed.
• ABS must adapt to the coefficientof friction of the carriagewaywithout delay.
• It must still be possible to steerthe vehicle when cornering.
• ABS must also detect aqua-planing and react accordingly.
• Engine braking must not influ-ence anti-lock brake control.
• If a fault is detected whichimpairs the ABS function, theunderlying brake system mustcontinue to operate correctly.
• A check lamp must clearly signalthe ABS failure.
Advantages of ABS:• Guarantees stable braking char-
acteristics on all road surfaces.
• Means the vehicle can still besteered and, as a rule, shortensthe braking distance.
• Reduces tyre wear.
Limits of ABS:Although ABS is an effective safetydevice, it cannot overcome the limitsof driving dynamics. Even a vehiclefitted with ABS will become uncon-trollable if driven too fast around abend.
Despite the above, you should notexpect that ABS will reduce the brak-ing distance under all circumstances.The braking distance may even beslightly longer when driving on gravelor new snow on a slippery base (grit/snow wedge) when in any case youshould only ever drive slowly andwith the utmost care.
As a result, ABS is not to be seen asa ”get out of jail free“ card for baddriving or failure to maintain the cor-rect safety distance.
3.4 An ABS control loop
1 = Impulse wheel
2 = Sensor
3 = Brake cylinder
4 = Brake disc
Operation:The stationary sensor continuouslypicks up the rotary motion of thewheel by means of the impulsewheel. The electrical pulses generat-ed in the sensor are sent to the elec-tronic control unit which uses them tocalculate the speed of the wheel.
At the same time, the electronic con-trol unit utilises an internal mode tocalculate a reference speed whichapproximates to the indirectly meas-ured vehicle speed.
Using all this information, the elec-tronic control unit continuously calcu-lates the wheel acceleration ordeceleration values as well as theamount of brake slip.
The modulator is activated whenevercertain slip values are exceeded.This causes the pressure in thebrake to be restricted, maintained oreven lowered. As a result, the wheelis kept within the optimum slip range.
5 = Brake calliper
6 = HABS modulator
7 = Master cylinder
8 = Electronic control unit
System descriptionHABS3.
1 3 5
2 4 6 8Fig. 1
7
System description HABS 3.Graphic representation of the ABScontrol cycle
Example: The recording relates tothe control of a wheel. The initialspeed of the vehicle is 80 km/h.
The horizontal axis (X-axis) showsthe control cycles over time. The bottom third of the vertical axis(Y-axis) shows the brake pressurewhile the reference and wheelspeeds are shown in the middle third.The pulses of the solenoid valve areshown in the top third.
The control procedure:The driver presses the brake pedal.The brake pressure increases. Thespeed of the wheel in question sud-denly drops more quickly than thereference speed of the ECU. Al-though the wheel is still within thestable braking range (i.e. between 10% and 30 % slip), the electronic con-trol unit already starts the control pro-cedure.
A corresponding trigger signal is sentto the integrated ABS solenoid valveinstalled in the modulator, which rap-idly reduces the pressure in thebrake cylinder of this wheel. Thewheel begins to accelerate again.
The electronic control unit changes
over the solenoid valve settings(modulator), thereby keeping thebrake pressure constant until thewheel is once again in the stable sliprange.
If more brake force can now be trans-mitted, the brake pressure on thebrake is increased further by pulsing(i.e. alternating between maintainingpressure and increasing pressure). Anew control cycle starts if the wheelspeed once again drops markedly inrelation to the reference speed of theECU.
The procedure repeats itself for aslong as the brake pedal continues tobe pressed down too much for thecondition of the road in question, oruntil the vehicle comes to a halt. Themaximum control frequency (valvepulses) which can be achieved in thiscase is 3 to 5 cycles per second.
3.5 What the driver needs to know
Notes:In case of faults in the brake sys-tem
• The ABS warning light comes on
• The vehicle swerves duringacceleration (if ETC is fitted)
• The wheels lock up duringbraking
• The ETC light comes on andremains lit
• The EBD light comes on andremains lit
always contact an authorised spe-cialist workshop. Because parts ofthe overall ABS system are not func-tioning, one or more wheels couldlock up when the brakes are applied.Take greater care. However, the brakes will still functionalthough without ABS.
In case of pulsation (vibration) ofthe brake pedal• ABS control is in progress, a
slight noise can be heard as thecontrol system operates
• The vehicle has reached anabsolute limit zone. Adjust yourdriving style and speed to matchthe road conditions.
• To achieve an optimum brakingeffect, continue to press down thebrake pedal as before even whenABS control is in progress and inspite of the pulsation felt at thebrake pedal. ”Pumping“ the brakepedal may reduce the brakingeffect of ABS and thereforeincrease the braking distance.
The brake pedal• As soon as a brake circuit fails,
the driver will notice that it is nec-essary to press the brake pedaldown much further. If this hap-pens, the pedal should bepressed through its idle strokerapidly.
• Make sure nothing impairs thefree movement of the pedal. Donot place any additional floormats or the like under the pedalsince they might restrict brakingin the case just described.
Modulator electrical signals
Reference speed
Wheel speed
Brake pressure
Time in s
100
bar
km/h
80
60
40
20
0
Fig. 2
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
8
4. Mode of function
This chapter takes a detailed look atthe mode of function of WABCOADD-ON ABS. The individual func-tions involved are MIC, EBD andETC.
Mode of function of the ADD-ONABS systemThe WABCO ADD-ON system en-sures the vehicle remains stable andcan still be steered during braking,and minimises the braking distanceunder most road conditions.
The ECU continuously registers thespeed of all wheels using the speedsensors. The ABS algorithm deter-mines whether any wheel is startingto lock up and this causes the ECU toactivate the corresponding solenoidvalves in the modulator in order to
adjust the brake pressure at thewheel brake. The brake pressure isset so the wheel continues to turnwhile still transmitting the greatestpossible brake force.As a rule, every wheel is individuallycontrolled depending on the adhe-sion between its particular tyre andthe carriageway. In case a carriage-way has varying coefficients of fric-tion, modified individual control (MIR)is performed in order to reduce theyawing moment on the front axle. Inthis way, an optimum compromise isreached between stability/steeringability and deceleration.
By way of example, a front axle is ex-amined which is braked with its leftwheel on dry asphalt and its rightwheel on ice (see Fig. 3).
4.1 Why modified individual control (MIC)?
Vehicles with a short wheelbase aredifficult to control during full brakingon carriageways with different coeffi-cients of friction on each side andwhen cornering if they have straight-forward individual control (IC). Sincethe braking forces which result aredifferent on each side, this producesa yaw moment and these factorsmake the vehicle hard to control.
Fig. 4 shows by way of example theprinciple of a control loop with themost important control parameters,the wheel deceleration threshold -b,the wheel acceleration threshold +band the slip thresholds ��1 and ��2.
The wheel undergoes continuous in-creasing deceleration or braking asthe brake pressure increases. Atpoint 1, the braking of the wheel isgreater than a value which cannot bephysically exceeded by the vehiclebraking. The reference speed corre-sponded to the wheel speed up tothis point, but afterwards it deviatesand reduces at a slight decelerationcompared to a theoretically specifiedhigh level of vehicle deceleration
Mode of functionHABS4.Driving on unconsolidated ground
• Actuate the brakes carefullywhen driving on surfaces with asoft, deep covering such as indeep powder snow, sand orgravel. The braking distance maybe longer under certain circum-stances. Under such conditions,the braking distance may beshorter if the wheels lock up as ina system without anti-lock brakes(due to the wedge of materialwhich forms). However, ABSoffers the advantage of ensuring
that the vehicle remains stableand can still be steered.
Driving on gradients
• Driving on steep gradients with alow coefficient of friction andbraking the vehicle to a halt mayresult in a situation where thevehicle starts to slide with itswheels locked up. This isbecause the ECU only gets infor-mation about the motion of thevehicle if one or more wheels areturning. In order to initiate ABS
control if the vehicle starts slidingfrom stationary on a steep gra-dient, you must quickly releasethe brake and apply it again sothat the ECU gets the informationthat the vehicle is moving.
Road safety does not just dependon a comprehensive range ofsafety features, but also requiresa responsible driving style.
ECUFR RR
FL RL
driving direction
ice
dryasphalt
Fig. 3 Vehicle on differentroad surfaces
9
Mode of function HABS 4.
The threshold value for wheel decel-eration -b is exceeded at point 2. Thewheel characteristics are now in theunstable range. The wheel has nowreached its maximum braking force.
Any further increase in the brakingtorque does not further increase thebraking of the vehicle but exclusivelycontributes to increasing the wheeldeceleration.
For this reason, the brake pressure israpidly reduced. The braking of thewheel is reduced. This delay time islargely determined by the hysteresisof the wheel brake and by the profileof the ��� slip characteristic in theunstable range.
Only once the wheel brake hystere-sis has been overcome (continuationof an effect after cessation of itscause) does continuing the pressurereduction actually lead to a reductionin the braking of the wheel.
The wheel deceleration drops belowthe threshold value -b at point 3, and
the brake pressure is keptconstant for a definedtime T1.
Normally, the wheel ac-celeration exceeds theacceleration threshold +bwithin this defined time(point 4). The brake pres-sure is kept constant foras long as this thresholdcontinues to be exceed-ed. If the +b signal is notreached within the timeT1 (for example on a low-friction surface), thebrake pressure is furtherreduced by means of theslip signal ��1. The higherslip threshold ��2 is not at-tained during this controlphase.
The value drops belowthe threshold value +b atpoint 5; the wheel is nowin the stable range of the��� slip characteristic.
The brake pressure is applied with asteep gradient for the time T2 in or-der to overcome the brake hystere-sis. Time T2 is defined for the firstcontrol cycle and then recalculatedfor every subsequent control cycle.Following the first steep-gradient ap-plication phase, the pressure is thenincreased by "pulsing" (alternatingbetween pressure holding and pres-sure application).
This logical principle as shown in thisexample is by no means a set proc-ess. Instead, it adapts to the dynamicbehaviour of the wheel in responseto varying coefficients of friction, i.e.it uses an intelligent system control.All threshold values are based onseveral different parameters such asthe vehicle's speed, the braking ofthe vehicle, etc.
4.2 How the anti-lock brake system operates
When the wheel brakes of the vehi-
cle are applied, ABS makes optimumuse of the surface friction for braking.It ensures that the wheels continue toturn so the vehicle remains manoeu-vrable. This is guaranteed by theanti-lock brake system control com-ponents. These components evalu-ate the rotation speed of the wheeland adjust the braking procedure tomatch the road conditions. Depend-ing on the requirements, the brakepressure is reduced, increased orkept constant using solenoid valves.This is so the wheels continue to turnand optimum braking is assured. Thevalues are controlled by the ECU inaccordance with the ABS algorithm.
Every wheel has two solenoidvalves, and inlet valve and an outletvalve. If the inlet valve is closed, thepressure in the wheel brake cylinderand in the line is maintained. Thepressure is reduced if the outlet valveis opened whilst the inlet valve isclosed. The pressure is increased ifthe inlet valve is opened whilst theoutlet valve is closed (providing thebrake pedal remains fully de-pressed).
During ABS control, the solenoidvalves are pulsed, which allows thebrake pressure of the hydraulic fluidto be set in finely spaced stages.
4.3 Purpose of electronic brake force distribution (EBD)
The EBD module is intended to guar-antee the necessary locking se-quence (front axle before rear axle)like a pressure reduction valve; it re-places the pressure reduction valveon the rear axle.
The optimised brake force distribu-tion during partial braking manoeu-vres (long before ABS controlintervention is required) increasesutilisation of adhesion on the rearaxle with less force being required atthe pedal.
Fig. 4
T2
T1
1 2 3 4 5 6 7 89pres
sure
in
whe
el b
rake
cyl
inde
rty
re c
ircum
fere
nce
acce
lera
tion
inlet valve
outlet valve
spee
ds
+ b
- b
λ1
λ2
wheel speed reference speed
vehicle speed
t
t
t
t
t
10
Mode of functionHABS4.How EBD operatesWhen the vehicle's brakes are ap-plied, EBD guarantees the specifiedlocking sequence is achieved (frontaxle before rear axle), so that the ve-hicle will remain stable and can besteered during partial braking ma-noeuvres. This is guaranteed by theABS control components which eval-uate the wheel speed and set thebrake pressure at the rear axle.
Every wheel needs a solenoid valvein order to function correctly. If the in-let valve is closed, the pressure inthe brake cylinder and in the line ismaintained. The pressure is in-creased if the inlet valve is openedwhilst the outlet valve is closed (pro-viding the brake pedal remains actu-ated).
4.4 Purpose of electronic traction control (ETC)
In a speed range from zero to 50 km/h, electronic traction control ETCguarantees the maximum possibletraction and constant vehicle stabilitywith minimum steering intervention.If braking takes effect on a wheelwhich is spinning, the correspondingtorque is transferred via the axle dif-ferential to the wheel with the highercoefficient of friction. The limits oftraction control are represented bythe maximum engine or brakingtorque and the available adhesionbetween the tyres and the carriage-way.
How the ETC system functionsThe ETC system uses largely thesame components as the ABS, i.e.wheel speed sensors, an ECU and amodulator with integrated ABS sole-noid valves for each wheel. Using theABS wheel speed signals, the algo-
rithm in the ECU calculates thespeed difference between thewheels on one axle. As a result, itcan determine when a wheel is slip-ping excessively. The electronic con-trol unit actuates the return pumpand the ABS solenoid valves of themodulator in order to direct hydraulicpower to the brake. The brake pres-sure is then adapted by the corre-sponding ABS solenoid valves of themodulator. The pressure at the spin-ning wheel is increased until thespeed of both wheels is once againsynchronised.This means a torque is transmitted tothe wheel which is not spinning. ETCis switched off as soon as the elec-tronic control unit detects a fault inthe vehicle wiring harness or in thesystem components.
4.5 Anti-lock brake system warning light
The purpose of the ABS warning lightis to display a malfunction of theelectronic or electrical ABS compo-nents or of the entire system. A self-test of the ECU and the connectedelectrical circuits is performed whenthe ignition is switched on.
Fig. 5 ABS warning light function (following initial start)
Function of the ABS warning lightThe light comes on when the ignitionis switched on. It then goes out forabout 2 s and comes back on until allwheels with sensors have exceeded
a road speed of 7 km/h for the firsttime. This provides an optical indica-tor that the system self-test has beenperformed successfully. If the 1 slight off phase does not take place,this means a fault has been detectedfirst and stored in the non-volatilememory. If a fault is currentlypresent, the light remains continu-ously lit even when the vehicle isdriving.
4.6 ETC light
The ETC function is available be-tween 0 and 50 km/h, as soon as theignition is switched on and the ECUhas completed its self-test. A yellowdisplay light indicates when the ETCfunction is operating.
If a fault occurs, both the ETC displaylight and the ABS warning light comeon.
Fig. 6 ETC and brake warning lightfunction
Zündung anGesc hwind igkeit allerRäder grö ßer 7km /h
1 sec
Ignition on Speed of all wheels greater than 7 km/h
1 s 1 s
Zündung an
3 s
Ignition on
11
Mode of function HABS 4.4.7 Mode of operation
without electronic traction control
At this point, drawings are used todescribe the three phases of pres-sure buildup, pressure holding anddepressurisation. First of all, the pro-cedures are explained with the helpof a modulator without ETC.Fig. 7 shows:
• Four inlet valves (solenoid con-trol valves) (1)
• Four outlet valves (solenoid con-trol valves) (2)
• Expander chamber (3)
• Pump system with pressure andsuction valves (4)
• Electric motor (6)
• Four brakes (7)
• Brake pedal with brake boostermaster cylinder (5)
in rest position.
4.7.1 Pressure buildup:If the driver now actuates the brake,a pressure is built up (displacementof volume) which is channelled di-rectly to the brakes by the medium ofthe brake fluid flowing through theopen inlet valves. The vehicle is de-celerated (no ABS control). Whenthe brake pedal is released, the pres-sure at the brakes is reducedthrough the inlet valves which remainopen. The brake fluid flows throughthe system via the master cylinderand back into the tank.
5
6
4
4 4
3
1
12
31
1
2 2
Fig. 7 Front axle7 7 7 7
Rear axle
12
Mode of functionHABS4.4.7.2 Pressure holding:If the ECU now detects via the speedsensors that one or more wheels (inthe example in Fig. 8, the rear leftwheel) are starting to lock up duringa deceleration procedure, the corre-sponding inlet valves are activatedand closed in order to prevent anyfurther pressure buildup (the wheelremains in the stable range). Theoutlet valves remain closed for the
moment. The driver continues topress down on the brake.
Fig. 8 Front axle Rear axle
13
Mode of function HABS 4.4.7.3 Depressurisation:As the wheel deceleration continuesto increase with the pressure heldconstant, the outlet valve is activatedand opened (in this figure, it is thewheel at the rear left). This meansthe brake fluid flows through thevalves into the expander chamberand is pumped by the pump systemthough the suction and pressurevalve back into the circuit against theforce applied by the driver at the ped-al.
If the driver continues to press downthe brake pedal, the outlet valve isclosed and the pressure at the wheelin question is built up again by puls-ing of the inlet valve. This entire pro-cedure from pressure buildup todepressurisation starts all over againif anti-lock brake system control isstill necessary.
Fig. 9 Front axle Rear axle
14
Mode of functionHABS4.4.8 Mode of operation
during electronic traction control
Secondly, the procedures are ex-plained with the help of a modulatorwith ETC.Fig. 10 shows the brake pedal withmaster cylinder and brake booster(1), the ETC valve with switch (2), thefilling piston (3), the pump system
(4), the suction and pressure valves(5), the dampers (6), the systemvalves (7), the expander chambers(8), the inlet valves (9) and the outletvalves (10) in their initial positions.
4.8.1 Pressure buildup / pressureholding / depressurisation
If the driver presses down the brake,a pressure is built up (displacementof volume) as in the case of the sys-
tem without ETC. This pressure ischannelled directly to the brakes us-ing the medium of the brake fluidflowing through the ETC valve andthe vehicle is decelerated. DuringABS control, the principle of the sys-tem without ETC takes effect. Whenthe brake pedal is released, the pres-sure at the brakes is reduced throughthe inlet valves which remain open.
Fig. 10
12
6 33
5
54
2
5
67
8
7
8
9
10
9
10 9
10
9
10
Front axle Rear axle
15
Mode of function HABS 4.4.8.2 Control with electronic
traction controlInformation is supplied to the ECUfrom the bar sensors. ETC controltakes effect if the ECU detects fromthis information that there are differ-ent wheel speeds in the systemwhen moving off. In this drawing, therear right wheel is subject to ETCcontrol. This involves the motor ofthe pump system starting and thethree inlet valves of the wheels whichare not spinning are closed. Brakefluid is drawn from the tank through
the master cylinder by the suctionvalve of the pump system. This brakefluid is pressurised in the pump sys-tem using an orifice in the filler pis-ton.
From there, the pressure is directedthrough the only open inlet valve tothe spinning wheel and causes thisto be braked. Pulsing of the inletvalve similar to the situation duringABS control causes this procedure tobe carried out until all wheels onceagain share the same speed. If this is
the case, the motor of the pump sys-tem is switched off by the ECU. Thepressure is dissipated through the or-ifice, through leakage and throughthe opened outlet valves. If the driverpresses the brake during ETC con-trol, ETC is immediately switched offand the closed inlet valves areopened.
Fig. 11
Suction pressure
System pressure
Front axle Rear axle
16
ComponentsHABS5.5. Components
In this chapter, the drawing below isused to explain the individual compo-nents of the WABCO ADD-ON ABSsystem. Furthermore, the description
focuses on the aspects of inspection,installation position and cabling.
Fig. 12 Cabling and piping diagram
1. ECU2. Speed sensor3. ADD-ON ABS (4M) modulator
1
2 2
22
3
ECU 446 044 ... 0446 109 ... 0
Task:The electronic control unit (ECU)uses the wheel sensor signals to cal-culate the road speed and the wheelspeeds as well as the wheel deceler-ation and acceleration values. If re-quired, it activates solenoid valves inthe modulator in order to prevent thevehicle's wheels from locking.
The 4-channel electronic controlunits have a dual-circuit structure.Each circuit monitors two diagonallyopposed vehicle wheels and can be
subdivided into four functionalgroups:
– Input circuit
– Master circuit
– Safety circuit
– Valve control
In the input circuit, the signals gener-ated by the speed sensors are fil-tered and converted into digitalinformation.
17
Components HABS 5.The master circuit consists of a mi-crocomputer. By means of a complexprogram, the control signals are cal-culated and used in logic functions.In addition, actuator signals are out-put for controlling the valves in themodulator.
The ECU signals to the driver with awarning light if there are any mes-sages and, if necessary, switches offthe control of one wheel, both diago-nally opposed wheels or, in certaincircumstances, the entire ABS sys-tem. The brake system remains fullyfunctional in this case, it is merelythat the anti-lock protection and theETC function are partially or fully un-serviceable.
Messages are permanently saved inthe electronic control unit for diag-nostic purposes. It is possible to readout and delete the message memoryusing the diagnostic connection (ac-cording to ISO standard).
The valve control units contain powertransistors (output stages) which areactivated by the signals coming fromthe master circuit and which switchthe current for operating the controlvalves.
The non-steered axle(s) is/are con-trolled individually (IC). Modified indi-vidual control (MIC) is used for thesteered axle.
Installation:
The electronic control unit is installedin the vehicle in a location where it isprotected from splashing water.
Inspection:The electronic control unit and theconnected solenoid valves, sensorsand the cabling are checked by theintegrated safety circuit and anyfaults are displayed. Any additional
inspection of the electronic controlunit itself is only possible on a specialtest rig in the manufacturing plant.
Note:Always switch off the ignition beforeremoving and installing the electroniccontrol unit, i.e. to disconnect or con-nect the ECU plug!
31
30 UBat
15 Ignition
K1, K2 = Relay for Car ApplicationGround
3
3
3
2
2
2
1
1
1
4
4
4
5
5
5
6
6
6
9
9
9
12
12
15
18
8
8
8
11
11
14
14
17
7
7
7
10
10
13
13
16
IV
IV
IV
IV
Ref.-GND
FR
RR
FL
RLOV
OV
OV
OV
Pump RelayK1
ABS Warning Lamp (UBat)
30
15
Diagnostics "K"
Recirculation Pump Monitoring
Sensor FL
Sensor FR Sensor RR
Sensor RL IG
IGM
IG
IG IG
IGM
IGM
IGM
25A
5A
ETC Info Lamp(UBat)
9 Brake Warning Lamp (UBat)
Shuttle Valve Switch
EnduranceBrake Relay
K2
Fig. 14 Cabling diagram for maximum System configuration
Fig. 13 Installation of the ECU in the Multicar in the area of the central tunnel
18
ComponentsHABS5.Speed sensor 441 032 ... 0 and impulse wheel:
Task:Using a proximity method, the sta-tionary sensor detects the movementof an impulse wheel which rotates to-gether with the vehicle's wheel. Im-pulse wheels for light and mediumcommercial vehicles have 80 teeth orfewer.
The output voltage of newer WABCOsensors has been increased for thesame wheel speed by modifying theinternal design of the sensor. As a re-sult, ABS operation can still be as-sured even given increased air gapsand even at very slow wheel speeds.These sensors are identified by an”S+“ on the sensor head.
Operation:The bar sensor operates inductivelyand mainly consists of a permanentmagnet with a round pole pin and acoil. The rotation of the impulsewheel connected to the wheel hubproduces a change in the magneticflux picked up by the sensor coil,thereby generating an alternatingvoltage. The frequency of this volt-age is proportional to the wheelspeed.
Design types:The speed sensor has been espe-cially developed to cope with themore exacting requirements of acommercial vehicle. Good tempera-ture stability and resistance to vibra-tion ensure that it operates reliablyeven under extreme conditions.
Sensor installationThe speed sensor is mounted using
clamping bush 899 760 510 4 (CuBe)or 899 759 815 4 (CrNi) and mount-ing grease in a hole in the axle stubor in a special sensor holder. It isheld in a sliding arrangement. On thefront and rear axles, the speed sen-sor is pushed into the clamping bushby hand up to the impulse wheel.The wobble of the impulse wheelpushes the speed sensor axially out-wards in the sensor hole after one ortwo turns of the hub by hand. It ismoved out to the maximum extent ofthe impulse wheel wobble.
Fig. 15 Installation position of the speed sensor
NoteThere is no need to set a minimumair gap for the sensor since it sets itsposition automatically during the firstcouple of wheel rotations as a resultof the wheel bearing play and the im-pulse wheel wobble.
Technical dataContact resistance
1150 +100/-50 ohms
Temperature range-40 °C ... +150 °C
LubricantIn applications which are exposed togreater contamination, we recom-mend using a clamping bush andsensor with a thermally stable greasewhich is also resistant to splashingwater. This is in order to guardagainst corrosion of the hole in theaxle stub and the penetration of dirt.
We recommend: WABCO sensorgrease
1 kg can Order no. 830 502 063 4
8 g tube Order no. 830 502 068 4
Maintenance:
Fig. 16 Installation position of the components (sectional view)
As well as regularly checking thewheel bearing play, you should pushthe sensor back in by hand as far asthe stop when working on the wheelbrake.
In order to adjust the speed sen-sor (if the air gap is too large) neveruse force or an unsuitable tool suchas pointed or sharp objects. Doing somay otherwise lead to damage to thesensor cap!
When replacing a speed sensorwe recommend replacing the clamp-ing bush as well.
Inspection:The resistance of the sensor coil, thecorrect setting of the air gap and theassignment of the sensor to the cor-responding wheel can be checkedusing the diagnostic controller andthe corresponding program card forthe system which is fitted.
19
Components HABS 5.ADD-ON ABS (4M) modulator 478 407 ... 0
TaskThe modulator allows the brake pres-sure applied by the driver via thebrake booster / master cylinder topass through to the wheel brake cyl-inders. In addition, it is responsiblefor safeguarding the two brake sys-tems in relation to one another in theevent of a failure of one of thesebrake circuits, and to ensure brakepressure control (4 wheel channels)for ABS brake control with 2/2-waysolenoid control valves.
Modulator installationThe modulator is installed in the vehi-cle's engine compartment below themaster cylinder. All hydraulic lines tothe master cylinder are to be routedupwards, while those to the wheelbrake cylinders are to be routeddownwards. The type, routing, lengthand dimensions of the line must beselected in consultation with WABCO.
WARNING NOTE
Replace the entire ABS modulatorunit if it malfunctions!
Do not expose the modulator toshock loads or excessive vibrationprior to its installation. Also, do not al-low any compressed air to enter thehydraulic connection.
Figs. 17 and 18 Modulator installed in the vehicle's engine compartment
Fig. 19 Functional diagram of a modulator without ETC
Modulator
20
DiagnosisHABS6.6. Diagnosis
The term ”diagnosis“ refers to the fol-lowing sub-functions:
• Startup at the manufacturingplant or during a modulatorchange
• Startup at the end-user'spremises, for example in theworkshop
• Fault storage, fault display• Periodical checks
• Access to stored data
Diagnosis with the DiagnosticControllerThe Diagnostic Controller is a com-puter which can exchange data withcontrol units (also a computer). Dataincludes the following:
– Stored fault messages in theECU.
– Commands sent by the controllerto the ECU which trigger certainprocedures there.
A special program is needed in orderto communicate with an ECU. Theprogram is stored on the correspond-ing program card.
The program card must match theECU!
The Diagnostic Controller set (446 300 331 0) consists of the following parts:
1. Diagnostic Controller 446 300 320 0
2. Carrier bag 446 300 022 2
Accessories:
3. Program card 446 300 . . . 0
4. Connection cable 446 300 329 2
5. Multimeter cable black 894 604 301 2(not illustrated)
Multimeter cable red 894 604 302 0(not illustrated)
6. Keyboard 446 300 328 0
1
2
3
4
6
Fig. 20 Diagnostic Controller set
For notes and information about which electronic controlunits can be checked, visit www.wabco-auto.com on theInternet or call the hotline number 0180 223 2337.
21
Diagnosis HABS 6.Connecting the Diagnostic Con-troller to the vehicle diagnosticsocket:Attach one end of the connection ca-ble to the Diagnostic Controller (Fig.21) and the other end to the diagnos-tic socket in the vehicle's switch cab-inet (Fig. 22).
Fig. 21 Diagnostic Controller with connection cable
Diagnostic socket
Fig. 22 Multicar switch cabinet
Diagnostic Controller connectionEnsure there is clear access to thediagnostic socket
Remove the protective cap
Use the connection cable (446 300329 2) to connect the DiagnosticController (446 300 320 0) to the di-agnostic socket
Release the handbrake, shift to neu-tral and switch on the ignition. Se-cure the vehicle to prevent it fromrolling away!
There is no need to provide a powersource for the Diagnostic Controllerproviding the ECU is powered by the
vehicle's battery or another externalvoltage source.
Before plugging in the diagnosticcard, ensure that the contact surfac-es of the diagnostic card are clean.Otherwise malfunctions may occur
Refer to the booklet provided with theDiagnostic Controller or the diagnos-tic card for detailed operating instruc-tions.
Respond to the questions, instruc-tions and information displayed bythe Diagnostic Controller.
To be on the safe side, check theidentification data in the first menuitem of the Diagnostic Controller inorder to avoid subsequent communi-cation messages. In this case, theABS warning light and optionally theETC light or the brake warning lightcome on.
Menu structure diagram taking the example of program card 446 300 784 0
Menu structure of hydraulic ABS D (4S/4M)
1 Diagnosis
1 Diagnostic memory
2 Control
1 Lights
1 ABS warning light2 Brake warning light *3 ETC info light *
2 Pump
3 Modulators
1 Front right2 Front left3 Rear right4 Rear left
4 Endurance brake cutoff relay *
3 Measurement values
1 Voltages2 Speeds3 Internal brake switch *4 ABD off switch *
4 ECU data
1 WABCO data2 Parameters3 Chassis number **
2 Startup
3 Multimeter
1 Direct voltage
2 Alternating voltage
3 Resistance
4 Options
1 Online help
2 Version
3 Supported ECUs
5 Special functions
* Only if fitted in the vehicle** Only available after entering the PIN
number
22
DiagnosisHABS6.Use the keyboard to select the corre-sponding menu in order to downloaddiagnostic messages from the mem-ory.
Warning noteCheck the brake fluid level if both theABS light and the brake warning lightcome on. Take care even if the fluidlevel is adequate, since the lockingsequence is not guaranteed.
The warning light should come on for3 seconds when the ignition isswitched on with the vehicle at astandstill. Otherwise the bulbs of thewarning light or its cabling is/are de-fective. Replace the bulbs immedi-ately in this case!
Note on the brake warning light
When the ignition is switched on withthe vehicle at a standstill, the ABSwarning light should come on as de-scribed, as well as the ETC andbrake warning lights. Otherwise thebulbs of the warning light or the ca-bling is/are defective. Replace thebulbs immediately in this case!Repairs in the area of the wheel sen-sors may result in increasing the airgap on individual sensors. To preventreleasing a vehicle for use in this sit-uation, it is a requirement to deletethe memory and to set the condition”Sensor fault in last ignition cycle“.During the next inspection cycle ofthe ignition, the ABS light remains onuntil the vehicle exceeds 7 km/h.
• The size of all interacting tyres isallowed to deviate from the nom-inal value set in the parametersby ±8 %.Individual tyres are allowed todeviate from the nominal value bymax. ±1.5 %.
• Modifications to the vehicle withthe objective of increasing theoverall speed or acceleration ormodifications to the brake systemmay impair the function of theADD-ON ABS system.
Electronic systemECU inputs and outputs
X 1 9-pin plug
X 2 18-pin plug
X 3 15-pin plug
Inputs ConnectionWheel speed sensor front left X 1 pin 1, X1 pin 2Wheel speed sensor front right X 1 pin 4, X1 pin 5Wheel speed sensor rear left X 1 pin 7, X1 pin 8Wheel speed sensor rear right X 1 pin 3, X1 pin 6Battery voltage supply X 2 pin 1Ignition voltage supply X 2 pin 2Reference earth X 3 pin 3Switch for shutoff valve X 3 pin 6Pump monitoring X 2 pin 8Electronics earth X 2 pin 12Outputs ConnectionBrake warning light X 2 pin 9ABS warning light X 2 pin 18Outlet valve front left X 3 pin 1Inlet valve front left X 3 pin 2Outlet valve front right X 3 pin 4Inlet valve front right X 3 pin 5Outlet valve rear left X 3 pin 7Inlet valve rear left X 3 pin 8Outlet valve rear right X 3 pin 10Inlet valve rear right X 3 pin 11ETC info light X 3 pin 13Pump relay X 3 pin 15Fault number ConnectionK-line X 2 pin 5
23
Diagnosis HABS 6.List of ECU messagesThis list can provide information about the type of fault and the method of rectification
Fault Repair manual
Electronic control unitCheck the cabling and the cable plugs. Fit a new ABS electronic control unit if the fault reoccurs after the fault memory has been deleted
ABS modulatorCheck the modulator cables. There is a continuous or temporary discontinuity / short circuit to battery voltage or earth in the inlet valve (IV) or outlet valve (OV) or a shared cable
ABS modulatorearth connection
Check the modulator cables. There is a continuous or temporary discontinuity / short circuit to positive in the earth connection of the modulator.
Sensor air gapAmplitude of the sensor signal too low. Check the bearing play and axial run-out of the impulse wheel, press the sensor against the impulse wheel. Check for a loose contact in the sensor cabling and plugs.
Sensor short circuit / open circuit
Check the sensor cabling. An open circuit, short circuit to battery voltage or earth or between the IG/IGM sensor cables is detected.
Pump monitoring defective or Check the pump operation and the pump cabling.
pump cannot be switched on Check the pump operation and the pump cabling.
Pump sticking Check the pump operation and the pump cabling.
Pump relay fault (sticking) Check the pump relay.
Internal fault Fit a new ABS electronic control unit if the fault reoccurs.
24
Brake diagram 400 050 006 0HABS7.
25
400 050 012 0 Brake diagram HABS 7.
26
Brake diagram 400 050 016 0HABS7.
27
400 050 017 0 Brake diagram HABS 7.
28
Circuit diagram 841 801 560 0HABS7.
29
841 801 562 0 Circuit diagram HABS 7.
30
Circuit diagram 841 801 564 0HABS7.
31
841 801 567 0 Circuit diagram HABS 7.
32
”Modulator 478 407 022 0“HABS7.
33
”Bar sensor 441 032 490 0“ HABS 7.
34
”Socket 899 760 510 4“HABS7.
35
”ECU 446 044 079 0“ HABS 7.
36
Workshop instructionsHABS8.8. Workshop instructions
This chapter describes tips and tricksfor use by authorised workshop per-sonnel.
Information aboutsystem maintenance
Daily checks
Check the fill level in the brake fluidtank.
Check the warning and info lightsfunction correctly when the ignition isswitched on.
Check the effectiveness of the brakesystem before setting off.
NoteSee the vehicle manufacturer'smaintenance schedule for the serv-ice intervals of the hydraulic brakesystem.
SAFETY NOTE
This modern, complex system offersan increased level of reliability andimproves both active road safety andthe braking performance of the vehi-cle.
As a result, we strongly adviseagainst conducting repairs on the
ADD-ON ABS system. Brakes are aprimary safety component, incorrectwork on brakes may result in themfailing to operate. Only trained per-sonnel are allowed to perform anymaintenance and repair work on thebrake system.
Make sure brake fluid does not comeinto contact with the vehicle's paint-work since it may result in separationof the paint.
Danger!Take care when handling brake
fluid.
Danger of poisoning� Always keep brake fluid sealed in
its original container and, inparticular, keep it safely out ofthe reach of children.
� The brake fluid should bechanged every 15,000 miles(25,000 km) or every 18 months.Always select the shorter of thetwo intervals. Only use approvedbrake fluid (see the MaintenanceManual).
Important
� Contamination of the brake fluidmay result in failure of the brakesystem! Take care to maintain absolutecleanliness when working on thebrake system!
� Follow the vehiclemanufacturer's instructionsconcerning checking and toppingup brake fluid!
Environmental!Collect draining or spilled
brake fluid in a suitable container and dispose of it in an environmentally friendly
manner!
37
8.1 Brake system fault table
A troubleshooting table has beenprepared to facilitate the task of trou-bleshooting.
This table does not show all faultswhich may occur. However, the faultsshown are representative of thosereported by our customers. We aregrateful for any further informationfrom users of the HABS system.
Fault Cause Remedy
Long brake pedal travel, springy pedal feel
Air in the brake systemNot enough brake fluidVapour bubble formation - old brake fluid
Bleed the brakeTop up brake fluidChange brake fluid
Braking effect decreasing
Line leakingPads wornBrake master cylinder defective
Check and seal lineFit new padsFit new brake master cylinder
Wheels lock during braking although ABS warning light is out
ABS warning light defective orsystem fault
Fit new bulbConnect Diagnostic Controller
Vehicle swerves although ETC display is out
ETC light defective orsystem fault
Fit new bulbConnect Diagnostic Controller
ETC display continuously lit
Failure of a componentNo correct ECU assignment Connect Diagnostic Controller
ABS display lit Malfunction of components Connect Diagnostic Controller
Pump runs continuously System fault Connect Diagnostic Controller
Brake acts on one side
Pads hardened / oilyBrake piston does not move freely
Brake calliper tubes dirty
Fit new padsRepair / replace brake pistonClean guides and callipers
Brake gets hot when driving
Brake piston does not move freely
Brake calliper tubes dirtyRepair / replace brake pistonClean guides and callipers
Brake pulsing ABS control activeFace run-out of brake disc
Normal functionCheck brake disc and repair / replace if necessary
Brake squeaks
Possibly due to high atmospheric humidityUnsuitable brake pad
Brake calliper tubes dirtyBrake piston does not move freely
No remedy required
Use genuine brake padsClean guides and callipersRepair / replace brake piston
Workshop instructions HABS 8.
38
Workshop instructionsHABS8.8.2 Changing brake fluid
It is necessary to maintain themost stringent cleanliness andprecision when working on thebrake system.
Tools required:
• See manufacturer
• We recommend an excess pres-sure or vacuum filling andbleeding system.
Brake fluid absorbs moisture fromthe air through pores in brake hosesand through the vent hole in the ex-pansion tank.As a result, the brake fluid should bechanged every 18 months or every25,000 km (15,000 miles).
� Never try to suck up brake fluidusing a hose and your mouth.Only fill brake fluid intocontainers intended for it.
� Do not allow brake fluid to comeinto contact with mineral oil.Even small traces of mineral oilrender the brake fluid useless.
Air may get into the brake systemduring any repair to the brake systemwhich involves opening the system.In this case, you must bleed thebrake system.If the brake pedal feels spongy, thisis also a symptom that there is air inthe lines.In this case, repair any possibleleaks or bleed the system.
If you do not have a bleeding deviceavailable, you must bleed the brakesystem by pumping the brake pedal(i.e. at least two people are requiredfor this job).
Bleed each wheel brake separately ifyou have to bleed the entire system.This is always the case if you havereason to believe that there is air inevery single brake cylinder.
If only one brake calliper has beenreplaced or overhauled, it is general-
ly sufficient to bleed the brake calli-per in question.
ImportantObserve the brake fluid level in theexpansion tank during the bleedingprocedure and top up the fluid if nec-essary.Make sure the level does not dropbelow the MIN mark, otherwise airwill be drawn into the system.
Only ever top up with new brake flu-id.
As a rule, bleed the longest brakeline first. Refer to the informationfrom the vehicle manufacturer for theprecise sequence for bleeding. Themost important factors in this caseare the lengths and assignments ofthe brake lines.
1. Remove the dust cap from thebleed valve. Clean the valve,connect a clean drain house andinsert the other end of the hoseinto a half-full bottle. Make surethe end of the hose is below thelevel of fluid in the bottle,otherwise air can get back intothe system.
2. Get an assistant to pump thebrake pedal until it is possible tofeel resistance at the pedal. Ifthere is adequate pressureavailable now, fully depress thebrake pedal and hold it there.
If you have a filling and bleedingdevice available, now connect itto the expansion tank. Refer tothe instructions provided by themanufacturer of the device fordetailed information.
3. Open the bleed valve on thebrake calliper until brake fluidemerges in a controllable flow.Ensure the end of the hosealways remains below the level ofthe brake fluid.
4. Close the bleeder valve as soonas the fluid pressure drops.
5. Repeat the pumping procedureuntil an appropriate pressure canonce again be felt at the brakepedal. Now depress the pedalfully and hold it there.
6. The system or the one brake linehas been bled if only lightcoloured brake fluid (old brakefluid is darker in colour) withoutair bubbles can be seen anylonger.
7. Following correct completion ofthe bleeding procedure, pull offthe bleed hose, wipe away sur-plus brake fluid, check the valvetightening torque is correct (seemanufacturer's specifications),check the valve is leak-tight andreplace the dust cap on the valve.
8. Bleed the other brake cylinders inthe same way.
9. After bleeding, park the vehicleon a level surface and top up thebrake fluid in the expansion tankto the MAX line.
Safety check� Have you tightened all brake
lines and their screw fittings?
� Have you secured the brakelines in the holders provided forthem?
� Have you topped up enoughbrake fluid (manufacturer'sspecifications)?
� Perform a leak test with theengine running, involvingpressing the brake pedal downwith a force of at least 200 N (20kg) and holding it for 10 seconds.
� Then check the brake system forleaks.
39
NoteSee the vehicle manufacturer'smaintenance schedule for the serv-ice intervals of the hydraulic brakesystem.
� Do not use mineral oil-basedbrake fluid
� Only top up with genuine brakefluid
� Note the vehicle manufacturer'sinstructions if the brake fluid levelis too low(check lamps light up)
� Check the brake fluid level everyday and top it up if necessary! The need to top up brake fluidrelatively frequently is anindication that there is a leak inthe brake system.
� Change the brake fluid at leastevery 25,000 km (15,000 miles)or every 18 months (note thevehicle manufacturer'sinstructions).
8.3 Fitting a new modulator
Removal:Note - The service brake is unserv-iceable during removal of themodulator.
� Release the parking brake, shiftto neutral, switch off the ignitionand secure the vehicle to preventit from rolling away.
� Disconnect the earth cable fromthe battery and remove it.
� Disconnect electrical cables tothe modulator and remove them.
� Place a container under thehydraulic lines beforeunscrewing them, in order tocollect any brake fluid whichleaks out.
� Wipe away surplus brake fluidfrom other engine components.
� Keep blotting material to hand incase of any brake fluid spills.
� Mark the ends of the pipe toidentify their connections on themodulator.
� Release and unscrew thehydraulic lines.
� Working carefully to avoidkinking the pipes, bend the linesaway from the modulator. Ifnecessary, release the pipe clipsand dismantle the pipe as far aspossible.
� Use suitable caps to close off theends of the lines in order toprevent dirt getting into them.
� Unfasten and unscrew themodulator retaining screws.
Installation:� Fit the modulator and tighten the
retaining screws.
� Fit the pipelines onto themodulator observing theidentification markings.
� Fit the pipe attachments.
� Wipe away any surplus brakefluid.
� Reconnect the electricalconnections to the modulator.
� Reattach the earth cable to thebattery.
� Bleed the brake system asdescribed in the section onchanging the brake fluid.
Safety check� Have you tightened all pipelines
and their screw fittings?
� Have you put the hydraulic linesand electrical cables back in theholders provided for them andhave you fitted them correctly?
� Have you wiped up surplus brakefluid?
� Have you topped up enoughbrake fluid (note themanufacturer's specifications)?
� Perform a leak test with theengine running, involvingpressing the brake pedal downwith a force of at least 200 N (20kg) and holding it for 10 seconds.
� Then check the brake system forleaks.
� Check the function of the brakesystem!
8.4 Fitting a new speed sensor
Removal:� Release the parking brake, shift
to neutral, switch off the ignitionand secure the vehicle to preventit from rolling away.
� Disconnect the earth cable fromthe battery and remove it.
� Set the steering to the requiredposition.
� Unscrew the wheel studs withthe vehicle still on the ground.
� Raise the vehicle and removethe wheel.
� Remove the speed sensor cablefrom its holders.
� Disconnect the speed sensorplug.
� Pull the sensor and clampingbush from their hole.
Workshop instructions HABS 8.
40
Workshop instructionsHABS8.Installation:� Clean the hole for the sensor
� Apply WABCO sensor greaseorder no. 830 502 063 4 or 830502 068 4 to the hole and theclamping bush
� Install the new clamping bush
� By hand, push the sensor intothe clamping bush as far as thestop on the impulse wheel (donot use a hammer)
� Reconnect the plug
� Fit the sensor cable back in itsholder. Take care that the cableis not looped around the brakecalliper and make sure there areno unsecured lengths of cable
� Clean the rim centring pin andapply a light coat of bearinggrease if necessary
� Fit the removed wheel back on
� Turn the wheel through onerotation by hand. The wobble ofthe impulse wheel pushes thesensor out of the hole by theequivalent amount
� Lower the vehicle back down
� Set the setting to straight ahead
� Tighten the wheel studs
� Attach the earth cable to thebattery
Safety check� Have you fitted the sensor cable
in all the holders?
� Have you fitted the sensor plugconnection?
� Have you fitted and tightened allwheel studs?
� Have you removed the jack?
� Check the function of the brakesystem!
8.5 Fitting a new ECU
Removal:� Release the parking brake, shift
to neutral, switch off the ignitionand secure the vehicle to preventit from rolling away
� Disconnect the earth cable fromthe battery and remove it
� Open access to the ECU
� Remove electrical cables fromtheir holders if necessary
� Disconnect the electrical cablesfrom the ECU and remove them
� Release and unscrew theretaining screws.
� Remove the ECU
Installation:� Insert the ECU
� Screw in and tighten theretaining screws
� Reattach the electrical cables tothe ECU
� Fit the electrical cables into theirholders
� Refit all other parts which wereremoved
� Attach the earth cable to thebattery
Safety check� Have you fitted the electrical
cables into their holders?
� Have you fitted the plugconnection onto the ECU?
� Check the function of the brakesystem!
41
9. Concept
This chapter presents information forcompanies installing the system intheir vehicles.
9.1 Tasks of the brake
The service brake of a vehicle mustperform the following tasks:
• It should reduce the speed of thevehicle, if necessary bringing it toa halt. The actuation force shouldbe low and the response timeshort.
• It should keep the vehicle speedat 30 km/h on long downhill gradi-ents (7 % downhill gradient, 6 kmin length) (endurance brake, thirdbrake).
Structure and function of thehydraulic brake systemFig. 23 shows the structure of themain parts of a hydraulic brake sys-tem with ABS. Brake fluid is used inthe hydraulic brake system in orderto transmit forces.
• It should prevent the vehicle fromrolling away from a standstill,even on a gradient (parkingbrake). The parking brake mustserve as an auxiliary brake in theevent of failure of the servicebrake.
Fig. 23 Structure of a hydraulic ABS system
Concept HABS 9.
wheel
sensor cable
sensor
ABS warning light
ABS modulator
master cylinder
hydraulic line hydraulic unit
42
9.3 Formulae
The laws of hydraulic force transmission applyin this case.
V = Displaced volume in cm3
A1 = Area of the master piston in cm2
s1 = Travel of the master piston in cm
A2 = Area of the slave piston in cm2
s2 = Travel of the slave piston in cm
P = Pressure in N/ cm2
F1 = Force acting on the master piston in N
F2 = Force acting on the slave piston in N
V A1 s1 A2 s2�=�=
PF1
A1------
F2
A2------= =
ConceptHABS9.9.2 Physical principle
The mode of operation of the hydrau-lic brake system is based on Pascal'sprinciple (Blaise Pascal, the Frenchmathematician and philosopher,1623 to 1662).
F: Piston force s: Piston travelA: Piston areaFig. 24 Hydraulic force transmission
When a force is applied to an enclosed flu-id, the resulting fluid pressure is equalthroughout the fluid.
Fluid pressure p
A1
A2
F1
F2
s2
s1
43
9.4 Legal provisions
The requirements, the nominal con-dition, the types and monitoring inter-vals of the brake system are definedin the motor vehicle constructionand use regulations (StVZO in Ger-many).
Excerpts from StVZO § 41
Regulations for the service and parking brake
Motor vehicles must have 2 inde-pendent brake systems or one brakesystem with two independent operat-ing devices, each of which can takeeffect if the other fails. It must beeasy to adjust the brakes or theymust have an automatic adjustmentdevice.
The brake (service brake) onmotor vehicles - except for mo-torcycles - must achieve an av-erage deceleration of at least2.5 m/s²; however, an averagedeceleration of 1.5 m/s² is suf-ficient for motor vehicles with amaximum design speed not inexcess of 25 km/h.
In motor vehicles - except for motor-cycles - the operating device of theother brake must be lockable. Theparking brake must attain an averagedeceleration of at least 1.5 m/s².
Excerpts from § 41b
Automatic anti-lock system
(1) An automatic anti-lock system isthat part of a service brake which au-tomatically controls the slip in the di-rection of rotation of the wheel onone or more wheels of the vehicleduring braking.
(2) The following vehicles with amaximum design speed in excess of60 km/h must be equipped with ananti-lock system:
1. Heavy goods vehicles and semi-trailers with a total weight of morethan 3.5 t,
2. Trailers with a gross vehicle weightof more than 3.5 t; this only applies tosemitrailers if the gross vehicleweight reduced by the vertical loadborne by the semitrailer tractor is inexcess of 3.5 t,
3. Buses,
4. Tractor vehicles with a gross vehi-cle weight of more than3.5 t,
Other vehicles with chassis designfeatures that are equivalent to the ve-hicles named in numbers 1 to 4 mustalso be equipped with an anti-locksystem.
9.5 Classification of vehicles according to EC Directive 98/12/EC
Class M: Motor vehicles designedfor passenger transport with at least4 wheels or with 3 wheels and a totalweight in excess of 1 t:
- Class M1 Vehicles designed forpassenger transport which have amaximum of 8 seats in addition to thedriver's seat;
- Class M2 Vehicles designed forpassenger transport which havemore than 8 seats in addition to thedriver's seat and the total weight ofwhich is not in excess of 5 t;
- Class M3 Vehicles designed forpassenger transport which havemore than 8 seats in addition to thedriver's seat and the total weight ofwhich is in excess of 5 t;
Class N: Motor vehicles designed forgoods transport with at least 4wheels or with 3 wheels and a totalweight in excess of 1 t:
- Class N1 Vehicles designed forgoods transport with a total weightnot in excess of 3.5 t;
- Class N2 Vehicles designed forgoods transport with a total weight inexcess of 3.5 t but not in excess of12 t;
- Class N3 Vehicles designed forgoods transport with a total weight inexcess of 12 t;
Class O: Trailers (including semi-trailers);
- Class O1 Trailers with a total weightnot in excess of 0.75 t;
- Class O2 Trailers with a total weightin excess of 0.75 t but not in excessof 3.5 t;
§§
Concept HABS 9.
44
ConceptHABS9.- Class O3 Trailers with a total weightin excess of 3.5 t but not in excess of10 t;
- Class O4 Trailers with a total weightin excess of 10 t.
9.6 Physical principles of braking
A vehicle has the kinetic energy E ata particular speed. The energy de-pends on the mass of the vehicle mFand the road speed v:
E = Kinetic energy in Nm
mF = Vehicle mass in kg
v = Road speed in m/s
Doubling the mass with the roadspeed constant results in twice thekinetic energy. Doubling the roadspeed with the mass constant resultsin four times the kinetic energy.
The braking procedure con-verts the kinetic energy into ther-mal energy (friction heat).
The friction heat results from press-ing the brake shoes against the rotat-ing brake drums or pressing thebrake pads against the rotating brakediscs.
The braking deceleration a is the dif-ference between the original roadspeed v1 and the road speed v2 afterbraking, divided by the braking time t.
a = Braking deceleration in m/s²
v1 and v2 = Road speed in m/s
t = Braking time in s
The vehicle attains the best deceler-ation values when the tyres remain incontact with the carriageway due tostatic friction.
Fig. 25 Forces on the wheel and on the brake
The wheels lock if the contact forcebetween the brake shoe and thebrake drum / brake pad and brakedisc increases so much that the stat-ic friction between the wheel and thecarriageway is no longer sufficient tomaintain the braking procedure. Dy-namic friction then occurs betweenthe carriageway and the tyres. Thefollowing disadvantages result whenthe wheels are locked:
• Steering of the vehicle is nolonger assured, the vehicle tendsto swerve (skid),
• less braking deceleration and
• greater tyre wear.
The brake force coefficient � B:The brake force coefficient (adhe-sion) between the wheel and the car-riageway determines what brakingforces can be transmitted. It dependson the brake slip between the tyresand the road and is also influencedby, amongst other factors:
– The condition of the road and thetyres
– The wheel or axle load
– The speed of the vehicle
– The temperature
– The tyre inclination or the amountof cornering force utilised.
The cornering force coefficient � S:Maintaining lateral stability is a majorprerequisite for ensuring that the ve-hicle can still be steered. The corner-ing force coefficient reduces muchmore rapidly than the brake force co-efficient does.
The brake slip �:The brake slip is the ratio betweenthe speed of the vehicle and thewheel speed expressed as a per-centage. The slip is defined by thefollowing equation:
E12--- mF v
2��=
av1 v2–
t----------------=
wheel cylinder brake drum brake shoe
wheel load
dynamic tyre rolling radiusbrake force on the wheel
peripheral force on the brake drum
brake drum radius
45
Where:VF = Vehicle speed
VR = Wheel circumferentialspeed
Explanation of slip curves (� B and � S):
Fig. 26 Slip curves
Fig. 26 shows the relationship be-tween the brake force coefficient �B,the cornering force coefficient �S andthe brake slip for different road condi-tions. Providing maximum adhesion hasnot yet been achieved, it is possibleto continue increasing the brakingforce in the ”stable“ range by increas-ing slip. In this case, there are alsosufficiently large cornering forcesavailable to keep the vehicle steera-ble and therefore stable. If the unsta-ble area of the �-� curve is reached(approx. 30% to 100%) due to exces-sive braking forces, the wheel isoverbraked and locks (100 % slip).The vehicle is then almost complete-ly unsteerable. To prevent this fromhappening, the ABS system keepsthe adhesion between 10% and 30%slip.
9.7 Time sequence of the braking procedure
The total braking time ttot is the sumof the time slices as shown in Fig. 27 The response time tR is the time be-tween when the danger is detectedand when the brake pedal ispressed.
It varies depending on the speed ofthe driver's reactions. Special affectssuch as tiredness,
Fig. 27 Time sequence of the braking procedure
the influence of alcohol, etc. canmake it significantly longer. The playin the brake system is overcome dur-ing the response time tA (e.g. air gapbetween the brake shoe and brakedrum / brake pad and brake disc).The buildup time tsw is the time whichpasses until the brake force reachesits maximum amount. The brakingtime t is made up of the responsetime ta, the buildup time tsw and thedelay time tv (the time it takes for thebrakes to take effect).
The length of the braking travel is de-pendent on the actuation force and,to a large extent, on the following pa-rameters:
• Vehicle speed
• Composition of the carriagewayand condition of the tyres
• Coefficient of friction between thebrake shoe and brake drum /brake pad and brake disc.
�VF VR–
VF----------------- 100%�=
Concept HABS 9.
Stab
le ra
nge
wet concrete
dry concrete
brake slip
corn
erin
g fo
rce
coef
ficie
ntbr
ake
forc
e co
effic
ient
A: danger is detectedB: driver starts braking
time t in s
brak
ing
dece
lera
tion
dist
ance
C: Braking system startsD: the brake force reaches
its maximum amountE: Vehicle speed = 0 km/h
46
ConceptHABS9.9.8 Driving with the ADD-
ON anti-lock brake system
After the ignition is switched on, theADD-ON system checks its functionautomatically for the first time. Thiscan be felt as a slight movement ofthe brake pedal indicating that thevalves and the modulator pump arebeing checked. In every braking situ-ation during which the decelerationof one or more wheels is relativelyrapid in comparison to the vehiclespeed, ABS checks whether thewheel is starting to lock and controlsthe brake pressure in order to keepthe wheel turning during the brakingprocedure.
The braking procedure is controlledby electronically activated valveswhen ABS is working. The function-ing of the valves and the modulatorpump can be felt as a movement orpulsing at the brake pedal. At thesame time, the driver hears the noiseof the modulator operating. If thepedal frequently vibrates wheneverthe brake pedal is pressed, this indi-cates a hazardous road surface orthat an inappropriately high level offorce is being applied at the brakepedal. Also, further downwardsmovement of the pedal is felt to behard from the point when ABS takesover control. However, the force ap-plied to the brake pedal can be al-tered to influence the brakingprocedure whilst ABS control is oper-ating.
The force applied to the brake pedalwith the foot should be maintained asduring non-ABS braking, since thefriction of the road surface maychange and ABS can react to this ac-cordingly. The EBD function also op-erates during partial braking if therear axle is tending to lock before thefront axle. The EBD function is notaudible, however the driver feels aslight pulsation at the brake pedal.
9.9 Points to bear in mind
• A four-channel ADD-ON ABSsystem controls the individualwheels in accordance with theamount of adhesion under eachtyre. As a result, it makes it pos-sible to keep control of thevehicle even when the coeffi-cients of friction are varying rap-idly and are markedly differentfrom wheel to wheel.
• The ADD-ON ABS system evenoperates at very slow speed(the slowest measurable speedis 1.8 km/h). As a result, thevehicle normally comes to a haltin the last control cycle.
• The ADD-ON ABS system pre-vents excessive tyre wear. Thisapplies in particular to bald spotswhich would otherwise be pro-duced due to emergency brakingon surfaces with a high coeffi-cient of friction.
• The vehicle can still be steeredand remains stable duringbraking on all surfaces wherebraking without ABS would leadto blocking of the wheels and aloss of control over the vehicle.ABS functions on all surfacesproviding there is sufficient fric-tion to allow the wheels to con-tinue turning in coasting orfreewheel mode.
• The safety circuit of the ECU con-stantly monitors that the elec-tronic and electrical functions areintact.
The system is partially or completelyshut off, depending on the nature ofthe fault.
9.10 Impulse wheel specification
WABCO itself does not offer impulsewheels in its product range. As a re-sult, some information concerningsuch wheels is presented here.
FunctionThe impulse wheel attached to therotating wheel induces an alternatingvoltage in the stationary speed sen-sor. The electronic control unit calcu-lates the wheel speed based on thefrequency of this voltage. Refer toWABCO Product Specification 895905 000 4 for dimensions and toler-ances.
MaterialWABCO recommends that impulsewheels should be made from ferro-magnetic materials.Sample materials:Free-cutting steels: 9S20K 9SMn28Annealing steels: C35K C45KSinterised material: Sint DO2Stainless steels: X6Cr17
X6CrMo17These designations are based on theold DIN standard.
Surface protectionSurface protection in terms of resist-ance to corrosion and high tempera-tures must be adapted to theconditions where the products areused (e.g. front or rear axle, disc ordrum brake, open or closed brake).Furthermore, it must offer adequateresistance to abrasion in order toprevent damage to the surface whenthe impulse wheel spins against thesensor.
47
General number of teethThe number of teeth relates to im-pulse wheels which rotate at thespeed of the wheel. Contact WABCOto define the number of teeth in caseof tyre circumferences which are out-side the specified ranges.
60 teethFor cars, light goods vehicles andtrailers
80 teethFor low-loader axles, light goods ve-hicles and trailers
100 teethFor buses, medium and heavy goodsvehicle and trailers
120 teethFor special goods vehicles such asmobile trains, open-cast miningdump trucks, etc. with very largewheels.
AssemblyThe impulse wheel adapter shouldbe configured so that work can beperformed on the hub bearing/sealsand other vehicle maintenance workcan be conducted near to the wheelbrake without having to remove theimpulse wheel. There is no need toarrange the impulse wheel inside thesealed area of the wheel hub, but thisis permitted. The impulse wheelshould normally be connected to thehub using a press fit H8/s7. The im-pulse wheel or the hub should beequipped with a suitably chamferededge in order to allow the wheel to bepressed on. The impulse wheel can
also be heated to facilitate installa-tion. Suitable equipment or toolsmust be used to ensure that there isno risk of damaging the tooth edges.This also applies to maintenancework, e.g. when removing the hub.
Concept HABS 9.