Air Suspension Systems Basic Training

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Introduction If we look at the different suspension systems used in motor vehiclestoday, the most apparent difference between them is that they are eithermechanical or air suspension systems.

Both types are, of course, incapable of meeting all technicalrequirements. If they are, however, directly compared, it soon becomesapparent that air suspension offers major benefits compared withmechanical suspension systems.

As a result air suspension systems are used to an increasing extent incommercial vehicles.

Benefits of Air Suspension Systems 1. By changing the bellows pressure, depending on the load carried onthe vehicle, the distance between the road surface and the vehiclessuperstructure addresses the same level. This means that theboarding or loading height, and the headlight settings, remainconstant.

2. Spring comfort remains almost unchanged across the whole of theloading range; again this is achieved by changing the bellowspressure. The passenger on a motor coach will always perceive thesame pleasant type of oscillations. Sensitive loads can thus becarried without being severely damaged. The well-known jumpingof an unladen or partially laden trailer no longer occurs if an airsuspension system is used.

3. The stability of the steering system and the transfer of the brakingforces are improved since all wheels always have good adhesion tothe road surface.

4. The pressure in the air bellows, depending on the load the vehiclecarries, is ideal for use in controlling automatic load-sensitivebraking.

5. In the area of control for interchangeable platforms, air suspensionsystems are an excellent basis for cost-effective loading andunloading of containers.

6. The kneeling effect often required for routine buses can easily beachieved by venting the nearside air bellows.

Air Suspension Systems Basic Training 15

2Air Suspension BellowsBasic Training15Purpose Depending on the levelling valves control, the air suspension bellows

are designed to take up the required pressure in the bellows volume,depending on the load carried on the vehicle.

Air suspension bellows are used as elastic constructional elementsbetween the axle and the vehicles superstructure. Since its internalfriction is less than that of mechanical suspension systems, the air-sprung vehicle has to have shock-absorbers fitted.

Design types Today the following variants are mainly used:

Twin Concertina Bellows Twin concertina bellows show a favourable ratio of height versus springtravel, i. e. this type of bellows permits the lowest installation height.

The beaded heels around the bellows openings are held by metallicbead rings which are screwed against supporting consoles or plates.This causes part of the bellows heels to be deformed, thereby achievinga sealing effect.

Rolling Tube Bellows Rolling tube bellows achieve an excellent cushioning effect and offerexceptionally good lateral movement. For this reason they areparticularly suitable for use in buses and passenger cars but are alsoused on lorries and trailers.

In the course of their cushioning action, these bellows roll on a cylindricalor similar piston whose shape essentially effects the cushioningcharacteristic. This allows the natural frequency to be varied and thebest possible suspension for the vehicle to be achieved. For thispurpose, rolling tube bellows require no additional volume. The airvolume in the piston can also be used for cushioning.

These bellows are fairly easy to install and to seal. The bellows heelsare pushed onto conical fittings and assume their intended positionwhen connected to the air line.

Maintenance No maintenance is required beyond the checks required by law.

Testing Air suspension bellows merely need to be checked for any leakage, andfor mechanical wear.

3Purpose Levelling valves, also called air suspension valves, are used to controlthe suspension in air-sprung vehicles. Their purpose is the sensitivelygraded control of the compressed air for the air suspension bellows asa ratio of the vehicles load.

Design types

464 002 ... 0 Levelling valves with single or double-stage characteristic curve. Thedamping nozzles for energy delivery to the air suspension bellows varieswith the respective variants (1.3 mm resp. 3 mm). They use either a flatlever with a ball joint, or a linkage with a rubber transmitting member.

464 006 ... 0 Levelling Valve 464 006 ... 0Devives (in dual level characteristic) replace the designs 464 002 ... andhave a nominal width of 3 mm.

There are following design types:

Levelling Valve 464 006 00. 0 (without height limitation)The device is available in different variants (with or without lever resp.silencer).

They use either a flat lever with a ball joint, or a linkage with a rubbertransmitting member.

Levelling Valve with height limitation 464 006 100 0.This variant has an additional 3/2 way valve which closes, from a specialadjustable lever angle up, the pressure supply to the air bellows and turninto a venting position when the lever further is actuated.

Through this Height Limitation a lift up of the vehicle with the rotaryslide valve over the admissible level is prevented. This integratedsolution makes the former necessary seperate shutoff valve for strokelimitation superfluous.

Levelling Valves Basic Training 15

4Levelling ValvesBasic Training15Operation of Levelling Valve 464 002

a. Pressurizing Position If the vehicle is pressureless, its superstructure rests on the rubberbuffers of the chassis. The levelling valve has thus been reversed via thelinkage (10), causing the valve (5) on the inlet side to open. Thecompressed air from the air suspensions auxiliary air reservoir nowenters at port (1), opening the check valve (4), and flows through theopen valve (5) past the tappet (6) into chamber (a). Through thecalibrated nozzle holes (b), the compressed air flows to ports (21 and22) and from there to the air suspension bellows. As the vehiclessuperstructure rises, it simultaneously acts on the eccentric pin (8) viathe linkage (10). This causes the guide (7), together with the tappet (6),to be pulled downwards. When the level for loading the vehicle, or forboarding passengers, has been reached, the inlet side of the valve (5)closes, and the process of pressurizing is finished. Because of thegroove-shaped top of the tappet (6), the nozzle holes (6) are nowcovered.

b. When Axles Oscillate Any axle oscillations, caused by uneven road surfaces as a ratio of thevehicles speed, are transferred directly to the levelling valve. Althoughthis may cause the valve (5) to open, the air consumption is kept to aminimum because the nozzle holes (b) are covered by the tappet (6).

c. When Loading When the vehicle is being loaded, the existing bellows pressure is nolonger sufficient to keep the superstructure at its level. It movesdownwards, causing the tappet (6) to reverse via the guide (7). The inletside of the valve (5) opens, and the tappet (6) releases the nozzle holes(b), allowing compressed air at a higher pressure to flow to theconnected air suspension bellows. As described under a above, thelevelling valve reverses as the vehicles superstructure is raised.

5d. When Unloading When the vehicle is unloaded, the levelling valve is controlled in reverseorder. Its superstructure rises, moving the guide (7), together with thetappet (6), downwards via the linkage (10). As the tappet (6) is raised offthe valve (5), the nozzle holes (b) are released and the air suspensionbellows are connected with the levelling valves exhaust (3). As thepressure in the air suspension bellows is reduced, the vehiclessuperstructure drops and the levelling valve is set back to its originalposition in which its air intake and its exhaust are closed.

Testing Provided the air suspension bellows have the required pressure, thelevelling valve is only checked for any leakages, and for mechanicalwear of the linkage.

Important Note The factory setting of the levelling valve should not be changed in termsof its empty stroke via the adjusting screw (9), or the centering platesPhillips screws, since this would neutralize its basic setting.

Schematic for Testing and Installation

5

2, 6

712 4142

21

41 42

1

2, 6

12

4

3

8

1

88

5

7

4

3

8

22 211

Air-suspension bellows

Supply

of theService Braking System

Load sensing valve Load sensing valve

Levelling Valves Basic Training 15

6Adjustment of Levelling ValveBasic Training15After mounting the air levelling valve, the lever length isadjusted following the vehicle manufacturersinstructions. For adjusting the valve at the vehicle it isdecisive which total spring travel the axle permits.

When the air suspension bellows are pressureless, thevehicles superstructure rests on the rubber buffers of thechassis. As the bellows are pressurized, thesuperstructure is raised.

When the air suspension bellows unladen level (levelfor loading or for mounting passengers) has beenreached, the lever on the valve is moved to its neutralposition. To facilitate the installation and adjusting oflever and connecting linkage the levelling valve shaft canbe fixed by plugging a parallel pin of 3 mm in the idleposition.

If the vehicle is at a normal level, the connecting linkagecan be installed. The linkage has to be aligned vertically.

The space A between the fulcrum on the levelling valvelever and the fulcrum on the angle bracket should not beless than 150 mm. The linking 433 401 003 0 has to beordered seperately.

The relation lever length L / rod length A should be 1.2if the closing angle of max. 45 is not exceeded. The leverlength L should be 175 to 295 mm (following the

instructions of the vehicle or Achsherstellers) betragen. Ifa shorter lever has to be used, a higher air consumptionof the levelling valve has to be concerned.

Depending on the fitting positon various cranks of thelever are possible. By accordingly fixing or turning thelever for 180 the valve can be optionally operated fromright or left. Depending on the final installation positon -vertical or horizontal - the lever is to be placed throughone of the two bores in the operating shaft which arediplaced to each other for 90.

Variant ... 100 0 is adjusted to a closing angle of 30 2by plant. The pilot pressure is 15 - 45 bar. A closingangle of 15 is not permissible, otherwise the cross-section will reduce itself and this can lead to completeclosing.

For adjustment of the closing angle the rubber plugunderneath the 3/2-Directional Control Valve has to beremoved to adjust the adjusting screw with a Torx T30screwdriver:

- Counterclockwise means a reduction of the closingangle, clockwise means an increasing.

- A rotation = approx. 10 changing of the angle

After the vehicle has been lowered to its buffers with thehelp of a rotary valve the height of the chassis has to bemeasured. Then the chassis has to be raised by therotary valve. Should the permissible full suspension beachieved before the Height Limitation of the levellingvalve sets in, the raising has to be cancelled and thevehicle has to be lowered. Turning the adjusting screwcounterclockwise the closing angle and also thesuspension way are reduced.

If the height limitation sets in before the chassis is at therequired hub height, the vehicle has to be lowered also inthis case. Turning the adjusting screw counterclockwisethe closing angle and also the suspension way arereduced. This process has to be repeated until therequired suspension way (equal or less than themaximum suspension way given by the axle producer) isachieved. The final braking position has been reached.

Important

The transfer linkage and the lever of the levelling valveneed not to be in one line, because the linking is turningaround and this could cause damages on the levellingvalve.

7Rotary Slide Valves Basic Training 15

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Purpose:Control of raising / lowering of airsuspended interchangeableplatforms and semitrailer chassis(lifting device).

The rotary slide valve 463 032 1..includes the deadman controlmeeting the accident preventionrequirement of the metalprofessional association (GermanVBG 8, 8). An automatic return isrequired for chassis movement witha stroke more than 300 mm,measured at the axle.With these valves, the leverautomatically returns to raise/stoprespectively lower/stop, all otherfunctions are like described below.

Operation:In the driving-position of the handlever the lifting device is turned off.The rotary slide valve has an openpassage between the levellingvalves (port 21 to 23) and thebellows (ports 22 and 24)

Upon that, this device enables 4further lever park positions allowingpressurizing/ venting of the bellowsfor raise/ lower function.

To raise the chassis, the lever isdislocked by pressing it down axiallyand then turned across positionstop to position raise. This willclose ports (21 and 23) and connectthe bellows (22 and 24) withreservoir at port 1.

After reaching the required height,the hand lever is to be turned tostop position. In this position, allports to the levelling valve (21 and23) as well as those to the bellows(22 and 24) are closed. Supportarms can now be turned out.

The afterwards required lowering ofthe chassis under normal level andplace down a container or theloading platform and to drive out isdone with the hand lever positionlower. Like in the position raise ,the ports (21 and 23) are nowclosed. However, the bellows (22and 24) are vented over exhaust 3.

After driving out of the chassis thehand lever is to switched to drivingposition to switch back to levelcontrol with levelling valves. Afterdriving out of the chassis the handlever is to switched to drivingposition to switch back to levelcontrol with levelling valves.

Maintenance:No maintenance is required beyondthe checks required by law.

Installation recommendation:The rotary slide valve has to be fittedvertically or horizontally with 4 M8screws - exhaust 3 pointingdownwards. The unassembledsupplied plate with the leverpositions has to be installedunderneath the lever (see fittingdimensions).

Variant 463 032 1.. 0

212223

24

13STOP STOP

I II III IV V

8Air suspension systems (examples)Basic Training15Schematic for Testing and Installation for Trailers

Pos. Units Description Order number

5 1 Drain valve 934 300 001 0

6 1 Test Connection 463 703 100 0

7 2 Levelling Valve 464 006 002 0

8 2 Test Connection 463 703 . . . 0


1 1 Charging Valve withoutreturn flow 6.0 bar 434 100 125 0

2 1 Air reservoirs 950 . . . . . . 0

3 2 Clamps 451 999 . . . 2

4 1 Line filter 432 500 02 . 0


5 1 Drain valve 934 300 001 0


7 1 Levelling Valve 464 006 100 0

8 2 Test Connection 463 703 . . . 0

9 1 Rotary Slide Valve 463 032 . . . 0

for semitrailer (24V)(Raising/Lowering)


1 1 Charging Valve withoutreturn flow 6.0 bar 434 100 125 0

2 1 Air reservoirs 950 . . . . . . 0

3 2 Clamps 451 901 10 . 2

4 1 Line filter 432 500 02 . 0

2, 3 41 42

21

ALB-Regler

Vorrat

von derBetriebsbremsanlage 6

4

1

8

5

7

8

Luftfederbalg

212121

2, 3 41 4221

ALB-Regler

Vorrat

von derBetriebsbremsanlage 6

4

1

5

Luftfederbalg

8

1 21

7

8

2221

2423

191 21 2

222312

9Lift Axle Control Valves Basic Training 15

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Purpose:The lifting axle compact valve is incharge of lowering or raising thelifting axle(s) manually orautomatically, if the axle(s) that aredown have reached their maximumload.

Variants 463 084 000 0 mechanically

operated version

463 084 010 0 electricallyoperated version

463 084 020 0 fully automatedpneumatic version

Operation:For lowering the lifting axlecompressed air flows via port 21 (air-suspension bellows) through theduct (k), through the throttling port ofthe check valve (d) to port 41(expansion tank) and through duct(f) into chamber B. After reaching theswitch pressure which is adjusted bythe screw (c) the piston (e) is raised.The compressed air flows via duct(g) into chamber A and moves thetappet (b) into its superior finalpositon. Port 1 (supply) is closed.Port 20 and the chambers D and Eare connected with vent 3. Thetappets (h and i) move towards thelower stop and the bellows (21 with22) and (23 with 24) are connected.

For raising the lifting axle theactuation button (a) has to bepushed ( only possible when piston(e) is lowered) and the supply airflows via port 20 to thedownstreamed lifting bag. At thesame time the compressed air flowsvia duct (j) into the chambers D andE, moves the tappets (h and i)against the power of thecompression spring upwards. Theconnection of the bellows (21 with23) and (23 with 24) is closed andcompressed air from the bellows ofthe lifting axle (port 22 and 24) isventing through the tappets (h and j),chamber C and exhaust 3 toatmosphere.Funktion of port 42 refer to p. 75.

Maintenance: No maintenance is required beyondthe checks required by law.

Installation recommendation: The fixing can made with the help of3 M6 stud bolts [A] (torque 10 Nm)or 2 M8 screws [B], torque 20 Nm,(wholes 9 mm avaiable on theappliance).The fitting position for the lifting axlecontrol valve is shown on page 70.

32324

20II

I

22 21 1 42 41

43 44

32324

20II

I

22 21 1 4241

32324

20II

I

22 21 1 4241

Variant 000

Variant 010

Variant 020

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Lifting Axle Systems (examples)Basic Training15

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Lifting Axle Circuit electronically operated


6 1 Pressure limiting valve 475 010 . . . 0

7 1 Pressure switch 441 042 000 0


9 1 Switch

10 1 Levelling Valve 464 006 . . . 0


1 1 Charging Valve 434 100 125 0

2 1 Air reservoirs 950 . . . . . . 0

3 1 Line filter 432 500 020 0

4 1 Air reservoirs 950 410 004 0


5 1 Lifting axle valve 463 084 000 0

6 1 Pressure limiting valve 475 010 . . . 0


8 1 Levelling Valve 464 006 . . . 0

Lifting Axle Circuit mechanically operated


1 1 Charging Valve 434 100 125 0

2 1 Air reservoirs 950 . . . . . . 0

3 1 Line filter 432 500 020 0

4 1 Air reservoirs 950 410 004 0

5 1 Lifting axle valve 463 084 010 0

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Lift Axle Control Valves Basic Training 15

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Setting Instruction After the valve has been installedaccording to the fixing instructionand the scheme, the adjustment ofthe switch pressure has to be made.

1. 463 084 000 0 mechanically operated version(Diagram 841 801 448 0 see p. 32)

Actuation button (a) has to bepushed.The switch pressure for lowering thelifting axle has to be set according tothe pressure, at which it is madesure, that the permissible axle loadis not exceeded.

Therefore a test hose with pressuregauge and pressure reduction valvehas to be connected with testconnection 42. The compressed airflows via duct (f) into chamber B. Byincreasing the pressure of the testhose the switching point on whichthe actuation button springs out isdetected, port 20 is gettingpressureless (lifting axle lowers) andthe air supply of the bellows on thelifting axle sets in.If the switch pressure is too high, it

can be lowered by turning theadjusting screw counterclockwise. Ifit is too low, it can be raised byturning the adjusting screwclockwise.While checking the test pressurealways has to be raised from 0 baron because the hysteresis has to beswitched off.

After setting the adjustment screwhas to be locked and covered withthe enclosed cap.

2. 463 084 010 0 electrically operated version(Diagram 841 801 447 0 see p. 32)

According to the scheme thepressure switch 441 042 000 0(Range of Adjustment 1.0 to 5.0 bar)has to be connected.The setting of the pressure switch islike the setting of the mechanicallyoperated lifting axle valve.

3. 463 084 020 0 fully automated pneumatic version(Diagram 841 801 449 0 see page33)

2 switching pressures have to beadjusted.

At first the protection cap has to beremoved with SW30 (M = 45 5 Nm)and the Philips screw A (size2) hasto be turned in until stop.Now the adjustment of the switchpressure for the lowering of the liftingaxle (screw B) follows with a 12 mmwrench for hexagon head capscrews almost like the adjustment ofthe mechanically operated version.

Afterwards the adjustment of theswitch pressure for the automaticraising has to be made with the helpof a Philips screwdriver (size 2) . Forthis the test pressure of 8.0 bar hasto be lowered. The pressuredifference of the switch pressures forthe automatic lowering and raisinghas to be 0.4 bar higher than thedifference in pressures for the airsuspension bellows between liftedand non-lifted axle.

Enclosures

Wrench size 10M = 4 1Nm

Ran

ge o

f Adj

ustm

ent

Wrench size 12M max. = 15 Nm

Lowering

Lifting

stop

= 5 rotations^Size 2

M max. = 1.5 Nm

12

Picture 1: ECAS parts for vehicles

Picture 2: ECAS parts for trailers

Electronically controlled air suspensionsystem (ECAS)

Basic Training15

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Basic Training 15

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ECAS stands for

ElectronicallyControlledAirSuspension

ECAS is an electronically controlled airsuspension system with a large numberof functions included in the system. Theuse of an electronic control unit hasachieved major improvements over theconventional system:

q Reduction of air consumption whilstthe vehicle is moving.

q It is possible to maintain differentlevels (e. g. ramp operation) bymeans of automatic readjustment.

q In the case of complex systems,installation is easier because lesspipes are required.

q Additional functions like theprogrammable vehicle levels, tyredeflection compensator, overloadingprotection, tracion help andautomatic lifting axle control can beeasily integrated.

q Due to large valve diameters,pressurizing and venting processesare accelerated.

q Easy operations and maximumsafety for those operating the systemdue to one single control unit.

q Highly flexible system due to teh factthat electronics can be programmedvia operating parameters (trailingend programming).

q Distinctive safety concept anddiagnosis facility.

Unlike mechanically controlled airsuspension systems in which the valvewhich measures the height also controlsthe air bellows, ECAS achieves controlby means of an electronic control unit(ECU) which actuates the air bellows viasolenoid valves, using informationreceived from sensors.

In addition to controlling the vehicle'slevel, the ECU, together with the remotecontrol unit, also controls functionswhich if implemented with conventionalair suspension systems, requires a largenumber of valves.

Furthermore with ECAS additionalsystem functions are avaiable. ECAS is adjustable to suit the differenttypes of trailer. For trailers, power is supplied from theABS or the EBS system. In addition tothat the ABS system, provides ECASwith the so-called C3 signal, i.e.information on the vehicle's currentspeed.

To permit adjustment of the level of atrailer not connected to a towningvehicle, an optional facility for a storagebattery may be provided for an additionalpower supply on the trailer.

Introduction:

Sample function: Trailer without lifting axle

Basic system:1 ECU 2 remote control3 height sensor4 solenoid valve5 bellow no

min

alle

vel

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Electronically controlled air suspension system (ECAS)

Basic Training15

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Functional descriptionA height sensor (3) permanentlyevaluates the vehicles height and sendits readings to to the ECU( 1). If the ECUrecognises that the normal level is notbeing maintained, a solenoid valve (4) isactivated in such a way that - bypressurizing or venting - the level isadjusted accordingly.

Via a remote control unit (2) the user canchange the reference level (important fore. g. rampdriving) underneath a givenspeed threshold (during standstill).

An indicator lamp is used to notify thedriver that the trailer is outside its normalride height.

A flashing of this lamp indicates a faultwithin the systems which was discoveredby the ECU ( Electronic Control Unit).

Scheme of the basic system:1 ECU (Electronic)2 Remote control unit3 Height sensor4 Solenoid valve5 Air-suspension bellows

ECAS Electronic

Diagnostics

Supply module ECAS

ABSVario-C

24N ISO 7638 24S

brak

e lig

ht

C3

Sign

al

grou

nd

pin

31pi

n 15

15


Basic Training 15

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ECAS Electronic (ECU)446 055 . . 0

The Electronic Control Unit (ECU)The Electronic Control Unit is the heart ofthe system and is connected with thesingles components on the vehicle bameans of a 35-pole or 25-pole plug-interminal. The ECU is located in thedrivers cab.

Together with a plug-in terminal forconnecting the ECAS ECU for trailer's tothe other components, the ECU ismounted on the trailer's chassis in aprotective housing. This protectivehousing corresponds to the ABS-VARIO-C System. The ECU can be used forimplementing a large number of systemconfigurations. The plug-in terminal hasa connector for each height sensor,pressure sensor and solenoid valve.Depending on the system used, parts ofthe terminal may not be used.As in the ABS-VARION-C system thecables are fed through glands in thelower part of the housing.

FunctionThe ECu contains a microprocessorwhich processes digital signals only. Amemory managing the data is connectedto this processor.

The outlets to the solenoid valves and tothe indicator lamp are switched via dirvermodules.

The ECU is responsibe for constantly monitoring the incoming

signals

converting these signals into counts

comparing these values (actualvalues) to the values stored (indexvalues)

computing the required controllingreaction in the event of any deviation

actuating the solenoid valves

Additional responsibilities of the= Electronic control unit managing and storing the various

index values (normal levels,memory, etc.)

data exchange with teh RCU and theDiagnostic Controller

regularly monitoring the function ofall system components

monitoring the axle loads (insystems with pressure sensor)

plausibility testing of the signalsreceived (for error detection)

error recovery.

In order to ensure swift control reactionsto any changes in actual values, themicro-processor cyclically processes aread-only program within fractions of asecond (25 milliseconds), one programcycle meeting all of the aboverequirements.

This program cannot be modified and isfixed in a program module (ROM).

However, it accesses numerical valueswhich are stored in a freelyprogrammable memory. These values,the parameters, effect the computingprocesses and thus the ECU'scontrolling reactions. They are used tocommunicate to the computing programthe calibrating positions, the systemconfiguration and the other preset valuesconcerning the vehicle and functions.

ECU 35-pin:*

ECU 25-pin:*ECU for Trailers

ECU 35-pin:*

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Basic Training15

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ECAS solenoid valve472 900 05 . 0

Valve for axle with two height sensors

The solenoid valve shown in theillustrations below has three solenoids.One solenoid (6.1) controls a centralbreather valve (also known as a central3/2 directional control valve), the otherscontrol the connection between the twoair bellows (2/2 directional controlvalves) and the central breather valve.This valve can be used for establishingwhat is known as 2-point control in whichboth height sensors on both sides of theaxle separately control the level of bothsides of the vehicle so that the body kepthorizontal even when the load is notevenly distributed.

Design of the valve

Solenoid 6.1 actuates a pilot valve (1),and the actuating pressure from thisvalve flows through hole (2) and acts onpiston valve (3) of the breather valve.The pilot valve receives its pressure via

port 11 (supply) and connecting hole (4).This drawing shows the breather valve inits venting position in which air fromchamber (5) can flow to port 3 via thehole of the piston valve.As solenoid 6.1is energized, piston valve (3) is pusheddownwards, initially causing valve plate(6) to close the hole of the piston valve.The valve plate is then pushed off itsseat (hence the name seat valve'), andsupply pressure can flow into chamber(5).The other two valves connect the airsuspension bellows with chamber (5).Depending on which solenoids (6.2 or6.3) are energized, piston valves (9)or(10) are pressurized via holes (7) or(8), opening valve plates (11) and (12) toports 22 and 23. A solenoid valve for control of the otheraxle can be fitted to port 21.

solenoid valves Special solenoid valve blocks have beendeveloped for the ECAS system. Bycombining several solenoid valves inone compact block, both space andinstallation time are kept to a minimum. The solenoid valves are actuated by theECU as a control element; they convertthe voltage present into a pressurizing orventing process, e.g. they increase,reduce or maintain the air volume in thebellows.

In order to achieve a large throughput ofair, pilot valves are used. The solenoidsinitially actuate those valves with a smallnominal width, and their control pressure

is then passed to the piston surface ofthe actual switching valves (NW 10 andNW 7 respectively).

Different types of solenoid valves areused, depending on the application: Forcontrolling a single axle, one seat valveis sufficient whilst a complex slidingvalve is required for controlling the liftingaxle. Both types of solenoid valves are basedon a modular principle: Depending onthe application, the same housing isused to accommodate different parts ofvalves and solenoids.

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Basic Training 15

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This valve is similar to the valvedescribed above but it contains fewerparts.

Since port 14 is connected to port 21 ofthe valve described above, no breathervalve is needed and only one pilot valve(1) is used. The piston valves (3) of bothair suspension bellows valves arepressurized via two connecting holes (2)so that each pressurizing or ventingprocess is effected evenly for bothbellows via chamber (5).

If the solenoid is not energized, thevalves are closed, as shown in theillustration. At this time, the onlyconnection between the bellows is thelateral choke (7), through which anydifference in pressures can gradually becompensated.

The valve is connected to the air supplyvia port 12. This port is needed merely topermit the pilot valve to displace thepiston valve.


Valve for an axle with one height sensor

ECAS solenoid valve472 905 1 . . 0

Sliding valve with rear axle block and lifting axle block


Valve for the bus with kneeling function

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Electronically controlled air suspension system (ECAS)

Basic Training15

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By means of the RCU the driver caninfluence the vehicle's level within thepermissible maximum limits. However,this can only be done whilst the vehicle iseither stationary or has not exceeded thedriving speed parameter.

The control keys for changing the levelare accommodated in a handy housing.Contact with the ECU is established viaa coiled cable and a socket on thevehicle.There are different RCUs depending onthe type of system used. The aboveillustration shows a unit with the largestpossible number of functions. Thefunctions of this RCU are:

raising and lowering of the chassis

setting normal level

stop

storage and actuation of threepreference levels

raising and lowering the lifting axle

unloading or loading the trailer axle

Switching automatic lifting axleoperation on and off

activating of the Stand-By mode.

ECAS Heightsensor441 050 0 . . 0

From the outside, the height sensorlooks similar to WABCO's conventionallevelling valve which means that it canoften be fitted in the same location on thevehicle frame (the pattern of the twoupper mounting bores is similar to that ofthe levelling valve).The sensor housing contains a coil inwhich an armature is moved up anddown. Via a connecting rod, thearmature is connected to a cam on the

lever's shaft. The lever is connected tothe vehicle axle.As the distance between thesuperstructure and the axle changes, thelever turned, causing the armature tomove into or out of the coil. This changesthe coil's inductance.This inductance is being measured bythe electronic control unit at shortintervals and converted into a heightsignal.

ECAS Remote Control Unit446 056 . . . 0

446 056 0.. 0 446 056 1.. 0

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The pressure sensor produces a voltageoutput which is proportional to thepressure present. The measuring rangelies between 0 and 10 bar; a pressure of16 bar must not be exceeded.

The signal voltage is sent to the ECU viaa connecting plug. Furthermore, thesensor must receive a supply voltagefrom the ECU via a third conductor. Thecable harness must be encased in ahose or similar material in such a waythat the housing - which is otherwisewaterproof - can breathe.

Under no circumstances should thepressure sensor be connected to theconnecting line between air suspensionbellows and solenoid valve since thiscould result in wrong readings whenpressurizing or venting is in progress.

If air suspension bellows with twothreaded ports, as offered by renownedmanufacturers of air suspensionsystems, cannot be used, a specialconnector must be fitted.

This connector can consist of a T-shaped pipe union, with a small pipebeing welded into its pressure sensorconnection protruding into the inside ofthe bellows, thereby sensing thesettled bellows pressure.

A normal tee-piece can be used but onlywhen a high raise/lowering-speed is notrequired. Two examples:

One axle is sensed (drawbar-trailerwith one lifting axle). The feed pipefrom bellows to solenoid should besmall (nominal size 6) but theconnection between bellows andsensor large.

Two axles are sensed (3-axle-semitrailer with one lifting axle). Use 12 pipe between the sensedbellows. Fit the pressure sensor in atee piece next to one bellows. Theline from the solenoid valve shouldbe 9) entering the system at theother bellows.

Pressure sensor441 040 00 . 0

Air Suspension Systems Basic Training

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