part safety, security and draft only...

p a r t 7Safety, security and

convenience

30 Braking: ABS, traction control and vehicle stability

31 Air conditioning

Simpson_Ch30.indd 596 10/05/14 8:27 AM

DRAFT ONLY

30 Braking: ABS, traction control and vehicle stability

31 Air conditioning

Mechanic’s profile: Kylee rumpf

I became interested in studying mechanics after doing some work experience at school. Not long after my time at Ian Weigh Toyota, I was offered a school-based traineeship and from then on I undertook a school-based apprenticeship. After completing Year 12 I became a full-time apprentice and finally finished as a qualified mechanic.

Throughout my apprenticeship I had to work hard to prove that I wanted to become a mechanic. There were many learning curves and even a few hilarious blunders along the way. On my first day I was told to check the headlights of a car and had to lean in really close to check if they were working. I hadn’t realised that the headlights had a washer on them and when I put my

face up close the mechanic sprayed me with water straight in the face! He couldn’t stop laughing at me and from then on I learnt to look at the headlights before I put my head too close to them! That didn’t stop me from going on to win the Capricorna Apprentice of the Year and Student Vocation Prize of the Year, though, as well as finishing as runner-up for Toyota’s Apprentice of the Year award.

More recently, on completing my apprenticeship, I was offered the technical service advisor position. Gradually I began to understand customer concerns and how best to explain why some things happen and how they can be fixed. Now I am fully confident in diagnosing problems and pinpointing areas of concern to help my colleagues during repairs. Sometimes I’m able to do a quick fix before a car enters the workshop to help customers cut down on costs. Being a qualified mechanic also helps me to explain things to customers. People really appreciate the knowledge and special skills you pick up during your traineeship – everything you learn, see and do is a valuable experience and will prepare you for all sorts of scenarios in the workplace.


DRAFT ONLY

• Overview of ABS

• Main system components

• Hydraulic system

• Electronic system

• Hydraulic operation

• Complete four-channel system

• Summary of ABS operation

• Electronic brake-force distribution (EBD)

• Traction control system (TCS)

• Traction control with vacuum components

• Electronic stability program (ESP)

• Service points

• Technical terms

• Review questions

chapter 30Braking: aBS, traction control and vehicle stability


DRAFT ONLY

chapter 30 Braking: ABS, traction control and vehicle stability 599

Antilock braking systems (ABS) are fitted to vehicles to prevent the wheels from locking during heavy braking conditions. This will generally reduce the stopping distance and provide safer driving. ABS can also help with steering control when the brakes are applied.

This chapter will also include developments of ABS including traction control system (TCS), electronic brake-force distribution (EBD) and vehicle electronic stability program (ESP).

Overview of aBSAn antilock brake system (ABS) controls the hydraulic pressure of all four wheels of a vehicle during sudden braking and when braking on slippery or uneven road surfaces.

Without ABS, when the brakes are suddenly applied, the wheels can lock and the tyres will skid. This will cause the vehicle to veer to the side where the tyres have the greatest friction with the road surface.

With ABS operating, the wheels are prevented from locking during braking. The vehicle stays straight, and the stopping distance is generally reduced.

ABS prevents the brakes from locking and the wheels from skidding by modulating the hydraulic pressure in the brake system. It can hold the brake pressure, decrease brake pressure or restore brake pressure to provide the most effective braking for the road conditions.

rear wheelspeed sensors

ABScontrolmodule

RHF wheelspeed sensor

ABS owarninglamp stop lamp

switchmastercylinder

hydraulicmodulator LHF wheel

speed sensor

Figure 30.1 ABS with four channels fitted to a rear-wheel-drive vehicle with independent rear suspension holden ltd

Its operation is similar to a driver pumping the brake pedal so that the brakes are applied and released to prevent skidding. ABS does this automatically and a lot more effectively than the driver.

ABS and its electronic controls enables traction control (or torque control) to be fitted to a vehicle. This uses the ABS components in a somewhat opposite way to braking. The brakes are still used, but they are applied to prevent wheel spin during acceleration.

Generally speaking, ABS does not reduce the braking distances. It does, however, allow the driver to retain better control by preventing lock-up of the road wheels. In some circumstances, it is possible for braking distances to be increased, as the brakes may require more distance depending on the particular environment.

Main system componentsThe location of the main parts of an antilock braking system on a vehicle are shown in Figure 30.1. The master-cylinder assembly and the disc-brake assemblies at the wheels are normal brake-system components. The ABS parts of the system are:

1 a hydraulic control unit2 an electronic control unit (ECU)3 speed sensors at the wheels4 an electrical circuit.


DRAFT ONLY

600 part 7 Safety, security and convenience

A separate electronic control unit is shown in the illustration, but many systems have the electronic control unit combined with the hydraulic control unit. This forms a hydraulic-electric control unit, which is sometimes referred to as an HECU.

The main components of an antilock braking system are shown in Figure 30.2. These are: the hydraulic control unit and the ECU, the master cylinder, brake lines and the wheel-speed sensors.

hydraulic control unitThe hydraulic control unit (or hydraulic modulator) is mounted in the engine compartment adjacent to the master cylinder (Figure 30.2(a)).

It has two hydraulic lines that connect it to the master cylinder and, being a four-channel system, there are four hydraulic lines that connect it to the calipers of the brake assemblies.

The hydraulic control unit contains the solenoid valves, hydraulic pumps, non-return valves, accumulators and connecting passages.

Master cylinder and linesThe master cylinder and booster are the same as those used for a brake system without ABS. The difference with ABS fitted is that the brake lines from the primary and secondary parts of the master cylinder are connected to the hydraulic control unit and do not go directly to the brake assemblies at the wheels.

brakemastercylinder

ECU

hydraulicmodulator

brake linesto wheels

brake linesfrom mastercylinder

sensor leadretainer

sensorattachingscrew

sensor pulsering

(a) (b)

Figure 30.2 Components of an antilock brake system (a) master cylinder and hydraulic modulator (b) wheel-speed sensor for an independent rear suspension holden ltd

electronic control unitThe electronic control unit is attached to the hydraulic control unit assembly. A multipin connector joins the ECU to the wiring harness.

The ECU receives signals from the sensors at each wheel, processes them and then signals the hydraulic control unit to make the necessary changes in hydraulic pressure. It also operates the ABS warning light on the instrument panel.

Wheel-speed sensorsThe sensors and pulse rings shown in Figure 30.2(b) are used with an independent rear suspension. The pulse rings are mounted to the differential housing and the pulse rings are fitted to the drive shafts. Independent rear suspension enables a four-channel system to be used.

The wheel hub or drive shaft has a pulse ring (toothed rotor) that rotates with the wheel and a sensor that is mounted close to it. As each tooth of the pulse ring passes under the sensor, a small voltage pulse is induced in the sensor. The pulses are sent as input signals to the electronic control unit.

The frequency of the pulses is related to road speed, and the control unit uses these signals to determine the rate of deceleration which could produce skidding. If the brake is about to lock, the pressure is relieved for a moment and skidding is prevented.


DRAFT ONLY


Front-wheel sensorsFigure 30.3 shows how a passive front-wheel sensor and pulse ring are mounted to a front hub. Sensors can be located at the wheel hubs, or on drive shafts.

Active wheel-speed sensors are a further development. These look like passive wheel sensors, but are more accurate at lower speeds and can detect very low speeds. They can also detect if the vehicle is moving in reverse.

sensor

toothed rotor

Figure 30.3 ABS sensor for a front wheel ford

ECU

LF LR

RF RRprimary

brake booster

pedal

secondary

wheelsensors wheel

sensors

electroniccontrol unit

hydraulic unit

Figure 30.4 Arrangement of an antilock braking system

The trigger wheel is either integrated into the wheel bearing seal or included as part of the wheel bearing assembly (see Figure 30.21).

Note that trigger wheel, pulse ring and toothed rotor are all names that are used for these gear-type rotors.

Information: the following names are interchangeable: rotor, toothed ring, pulse ring.

General operationBriefly, when the brakes are applied, the parts of the system covered above work like this: the sensors supply wheel-speed information to the electronic control unit, the electronic control unit operates the hydraulic unit and the hydraulic unit adjusts the pressure at the brakes to prevent the wheels from skidding.

Figure 30.4 is a diagram of a simplified antilock braking system. There are two main subsystems: the hydraulic system and the electronic control system. ABS can be used with diagonally split brake systems, or front–rear split systems.

hydraulic systemThe hydraulic system has a conventional master cylinder with primary and secondary pistons and conventional disc brake assemblies at the wheels.


DRAFT ONLY


Information: With a rigid rear axle, each rear wheel has its own sensor, but there is only one channel to serve both wheels. So, if the pressure at one wheel has to be altered, the pressure at both wheels will be altered.

The hydraulic control unit is used to control brake pressure. It has eight solenoid valves – two for each hydraulic circuit of a four-channel system.

For all normal braking, when the ABS is not operating, pressure from the tandem master cylinder reaches the hydraulic control unit and passes on through the brake lines to the calipers. The braking system acts like a conventional system.

When the ABS comes into operation, fluid pressure from the master cylinder is controlled by the solenoid valves in the hydraulic control unit. The pressure is controlled by closing and opening the solenoid valves at the appropriate times, so that it provides maximum braking, but at the same time prevents skidding.

three-channel systemVehicles with a rigid rear axle have a three-channel system, while those with independent rear suspension have a four-channel system. Channels refers to the lines, or circuits, to the wheels.

A three-channel system has a hydraulic line and a sensor at each wheel. The hydraulic control unit has a pair of solenoid valves for each front wheel, but only one pair of solenoid valves for both rear wheels.

This means that a lock up sensed at one of the rear wheels will cause a reduction in hydraulic pressure to both wheels. This occurs even if one wheel is on a loose gravel surface where it would easily skid, and the other wheel is on a hard surface and unlikely to skid.

Information: Where a vehicle has independent suspension at both front and rear, the pressure at each of the wheels is adjusted separately.

Information: the system includes a test function, a monitoring function and a warning function.

Four-channel systemWith independent suspension at both the front and the rear of vehicle, a four-channel system can be used. Each wheel has its own sensor and its own hydraulic line. The hydraulic control unit has separate solenoids and valves for each channel, therefore each wheel can be controlled independently. (The diagram in Figure 30.7 is of a four-channel system.) There are two solenoid-operated valves for each channel.

electronic systemThe electronic control system has sensors at the wheels and an electronic control unit in the engine compartment or other convenient position. The wheel sensors send speed signals to the control unit, which monitors them and decides when wheel lock is about to occur.

Before the wheel locks, the electronic control unit operates a solenoid in the hydraulic control unit. The solenoid valve reduces the hydraulic pressure to the brake of that particular wheel and so prevents the brake from locking and the wheel from skidding.

When the possibility of skidding has been overcome, the solenoid is again operated to restore normal brake pressure at the wheel.

Basic operationFigure 30.5 shows the basic operation of the electronic system and how it is part of a closed-loop system. The wheel sensors measure rotational speed of the wheels and signal this to the electronic control unit.

The ECU processes the signals to determine the deceleration and skid of the wheels. It then signals the hydraulic modulator, which influences brake pressure in the system.

hydraulic operationThe arrangement of the hydraulic system is shown in more detail in the diagrams that follow. These show the different pressure conditions that can occur during brake operation. They are referred to as modes or phases.

Operation of the hydraulic systemFigure 30.6 shows part of a hydraulic system schemati-cally. This is a front–rear split brake system and, to keep things simple, only the rear-wheel brakes are shown. The front brakes operate in a similar way.

The parts of the hydraulic control unit are enclosed by a border line. In the actual unit, the parts inside the border form the hydraulic control unit.

The illustration shows the following parts:1 Master-cylinder reservoir – holds the hydraulic

brake fluid for the system.2 Return pump and electric motor – returns hydraulic

fluid to the master cylinder.


DRAFT ONLY


3 Accumulator – holds fluid under pressure, during certain conditions.

4 Inlet valves – solenoid-operated valves that control fluid to the brakes.

5 Outlet valves – solenoid-operated valves that control fluid from the brakes.

6 Bypass valves – help return fluid to the reservoir when the brakes are released.

Operating modesWith ABS, the hydraulic system has three operating modes as far as fluid pressure in the system is concerned. In operation, the pressure is quickly changed from one mode to the other, as required, to obtain the maximum braking effect without locking the wheels.

The fluid pressure modes are:1 non-ABS braking pressure2 holding pressure3 reducing pressure4 increasing (restoring) pressure.

These pressures will be outlined in relation to Figure 30.6 for one rear brake only. (A diagram of the complete system is shown as Figure 30.7.)

The inlet and outlet valves control the fluid in the hydraulic control unit. They are solenoid valves that can be opened or closed by energising their solenoids. This is done by an electrical signal from the electronic control unit.

ECU

calculate controltest

monitorwarn

ABSindicator/warning

light

mastercylinder

hydraulicmodulator

in�uencebrake

pressure

condition of road surface

brakepressure

brakecaliper

wheelspeed

wheelspeedsensor

Figure 30.5 ABS schematic showing the basic operation of the electronics and hydraulics holden ltd

master cylinder

inletvalve

outletvalve outlet

valve

inletvalve

accumulator

motor

pump

left rearwheel

right rearwheel

bypassvalves

Figure 30.6 hydraulic arrangement for the rear wheels of a four-channel ABS ford


DRAFT ONLY


3 The fluid flows to the accumulator and to the pump.

4 The accumulator holds fluid under pressure and the pump sends fluid back to the master cylinder.

The inlet valves are normally open, but are closed when their solenoids are energised. The outlet valves are normally closed, but are opened when their solenoids are energised. Each valve can be operated independently by signals from the electronic control unit.

Information: the inlet valves control pressure fluid to the brakes, and the outlet valves control pressure fluid from the brakes.

Non-aBS braking pressureThe following descriptions should be related to one wheel of the diagram in Figure 30.6 – for example the right wheel. The other wheel operates in a similar way. Each wheel can operate independently of the others, so there could be a different pressure at each wheel if this was needed for the particular braking conditions.

When the brakes are being used normally and there is no ABS action required, the system acts as follows:

1 The inlet valves are held open and the outlet valves are held closed.

2 When the brake pedal is pressed, fluid flows through the open inlet valve to the calipers to apply the brakes in the normal way.

3 When the brake pedal is released, fluid flows back through the inlet valve to release the brakes.

4 Braking occurs in the usual way, with pressure in the system dependent on the force applied to the brake pedal by the driver.

holding pressureWhen the electronic control unit detects the sudden deceleration of a wheel, indicating that the wheel is about to skid, it signals the hydraulic control unit to hold the pressure at that brake:

1 The outlet valve is already closed and the signal from the electronic control unit closes the inlet valve.

2 Fluid under pressure is then held in the part of the circuit between the inlet valve, the caliper and the outlet valve.

3 This pressure will remain constant regardless of any change in pressure at the master cylinder as long as the outlet valve remains closed.

reducing pressureIf the electronic control unit detects that the pressure at a wheel needs to be reduced to prevent skidding:

1 The outlet valve is opened by a signal from the electronic control unit.

2 Fluid flows through the outlet valve to reduce pressure at the brake.

Information: Pressure can be decreased and increased in the braking circuit by ABS action, but it cannot be increased above the master-cylinder pressure.

Reference: See the section ‘Accumulator and pump operation’ below.

Increasing pressureAfter the pressure at the wheel has been reduced and the wheel is no longer about to skid, the pressure can be increased:

1 The outlet valve is closed so that fluid cannot pass to the accumulator and pump.

2 The inlet valve is opened so that pressure fluid from the master-cylinder circuit can again reach the caliper.

3 The braking effort is restored.

The cycle of reducing and increasing the pressure at the brakes to prevent skidding will continue as long as the brake pedal is held depressed.

accumulator and pump operationThe accumulator is basically a spring-loaded piston in a cylinder. It comes into operation during the pressure-reduction mode. Fluid from the outlet valve is temporarily stored in the accumulator. The pump starts to operate at this time and fluid is returned to the master cylinder.

By providing somewhere for the fluid to flow, the accumulator causes a quick reduction of pressure at the brakes. The fluid cannot flow directly back to the master cylinder because the master-cylinder pressure is higher than the brake pressure at this time.

The master-cylinder pressure is blocked from entering the pump by the pump outlet valve, but when the pump comes into operation, it creates a fluid pressure that is a little higher than master-cylinder pressure. This enables the pumped fluid to pass through the pump outlet valve and be returned to the master cylinder.


DRAFT ONLY


hydrauliccontrol unit

damper

pump

inletvalve(left)

outletvalve(left)

left rearwheel

right rearwheel

left frontwheel

right frontwheel

outletvalve(right)

outletvalve(left)

inletvalve(right)

inletvalve(left)

motor

master cylinder

rear front

damper

pump

accumulatorinletvalve(right)

outletvalve(right)

accumulator

Figure 30.7 diagram of the hydraulic control of a four-channel ABS ford

complete four-channel systemFigure 30.7 is a diagram of the hydraulic system of a four-channel braking system. The following should be identified in the illustration:

1 The components of the hydraulic control system are contained within a border.

2 There is an inlet solenoid valve and an outlet solenoid valve for each channel.

3 Because it is a four-channel system, each brake can have its pressure modified independently of the others.

4 The bypass valves only allow fluid to flow away from the calipers.

Summary of aBS operationFollowing is a summary of ABS hydraulic operation as shown in Figure 30.8. This has four simplified diagrams of the hydraulic circuit for a right rear wheel. The parts shown in the diagram are: the master cylinder, one caliper, an inlet valve, a bypass valve, an outlet valve, the accumulator and the return pump and its valves.

The hydraulic actions shown by the diagrams are:1 Non-ABS braking. The fluid flows through the

open inlet valve to the caliper to apply the brake (Figure 30.8(a)). When the brake pedal is released, fluid returns to the master cylinder through the open inlet valve and also through the bypass valve to release the brake. (The outlet valve is closed.)


DRAFT ONLY


Figure 30.8 Simplified ABS operation: only the pressure modes are highlighted – there is pressure in other parts of the system, but not shown

outletvalveclosed

inletvalveopen

outletvalveclosed

inletvalveclosed

(a) Non-ABS (b) Holding pressure

outletvalveopen

inletvalveclosed

inletvalveopen

outletvalveclosed

(c) Reducing pressure (d) Increasing pressure

accumulator

pump

P M P M

P M P M


DRAFT ONLY


2 Holding pressure. When the wheel is about to skid, the inlet valve is closed (Figure 30.8(b)). This holds fluid under pressure in the caliper and prevents any further increase in pressure. (Both valves are closed.)

3 Reducing pressure. If the pressure at the caliper would cause the wheel to skid, then the outlet valve opens to reduce the pressure (Figure 30.8(c)). (Inlet valve is closed.) Fluid flows into the accumulator and the pump returns fluid to the master cylinder.

4 Increasing pressure. When the skid has been prevented, the pressure at the caliper can be increased by opening the inlet valve (Figure 30.8(d)). (Outlet valve is closed.)

electronic brake-force distribution (eBD)Some vehicles are provided with electronic brake-force distribution (EBD). This is a means of distributing pressure between the front and rear brakes, as required, for maximum braking effect. EBD is an extension of the antilock braking system and is carried out by using the ABS components.

Hydraulic brake systems that do not have ABS have a way of distributing the hydraulic pressure (and the braking force) between the front and rear of the vehicle. This can be by means of a hydraulic valve in the master cylinder, or other valves in the system.

With EBD, the brake-system’s ECU is provided with extra programming to carry out brake-force distribution. The EBD program monitors the rear wheels through the ABS wheel sensors and compares this with information from the front-wheel sensors.

If rear-wheel slip (skid) is detected, the rear inlet valves in the ABS hydraulic unit are switched to hold so that there is no further increase in rear-wheel pressure. In this way, the electronic controls act to proportion the pressure between the front and rear brakes.

Distribution of brake forceThe distribution of brake force between the front and rear brakes is shown as a graph in Figure 30.9. The ideal relationships between the front and rear are shown as dotted lines – there is a curve for full load and one for a light load. There are also lines on the graph that show how EBD can keep the actual brake force close to the ideal.

eBD operationElectronic brake-force distribution takes place ahead of any antilock braking action. Its electronic program

operates the inlet valves to the rear brakes. The valves will be opened and closed, as required, to control the fluid pressure applied to the rear wheels.

EBD action will occur by using its computer program to operate the ABS components. The antilock braking function of the ABS will not be in operation. This will only come into operation if it is required for the particular driving or braking conditions that exist at the time.

Braking with eBDWhen the brakes are applied on a level road which has a normal prepared surface, there is a natural shift in force from the rear to the front of the vehicle. The front of the vehicle tends to drop and the rear to rise. This action is resisted by the suspension and shock absorbers, but there is still an effective transfer of force to the front of the vehicle. The extent to which this occurs will depend on the speed of the vehicle, the force applied to the brake pedal by the driver and how quickly the vehicle is slowed or stopped.

Without some means of adjusting the brake force between the front and the rear brakes, the rear brakes could lock and the rear wheels skid. EBD prevents this by adjusting the pressure to the rear brakes.

With EBD, the rear-wheel pressure is controlled and the vehicle stops in the direction in which it is being driven. Without EBD (or some form of hydraulic proportioning valve in the system), the rear wheels could lock and skid and provide reduced braking as a result.

Safety tip: If the rear wheels were to lock, the rear of the vehicle could become unstable and slide across the road.

Rear

bra

king

forc

e

Front braking force

heavy load

light load

ideal with EBD

Figure 30.9 Graph shows how the rear braking force is reduced for a light load and increased for a heavy load


DRAFT ONLY


traction control system (tcS)The main function of a traction control system is to prevent wheel spin during acceleration. If the driver accelerates excessively, the driving wheels could spin, particularly on soft, slippery or gravel road surfaces. Under these conditions, traction control will come into operation to reduce engine torque and so reduce the traction between the tyres of the driving wheels and the road surface.

Traction control can also assist steering. A vehicle accelerating on a curve will tend to understeer and the driver must compensate for this. To control the vehicle, the driver will turn the steering wheel more, or slacken off the accelerator, probably both. Traction control with a steering sensor can be used to reduce traction automatically and improve driving safety.

front-wheelsensor

ABS/TCS hydraulicmodulator

master cylinder

rear-wheelsensors

ABS/TCS ECUaccelerator-pedal sensor

PCMfront-wheelsensor

engine(fuel andignition)

Figure 30.10 Schematic arrangement of the components of ABS/tCS BoSCh

Reference: the tCS is the basis of the electronic stability program (eSP) that is discussed later.

Information: the ABS/tCS electronic control unit is shown as being separate from the hydraulic modulator, but they can be combined.

traction control system arrangementFigure 30.10 is a schematic that shows the parts of a traction control system. The ABS/TCS hydraulic modulator is the same as the modulator for ABS, except that it has additional valves.

The ECU contains the electronic program. It receives inputs from the front- and rear-wheel sensors and also from the engine’s power control module (PCM). Its output signals go to the ABS/TCS modulator and to the engine’s PCM.

The PCM receives signals from the accelerator pedal switch and from other sources. For the traction control system, it sends signals to the engine’s fuel and ignition systems.

traction control operationTraction control operates by reducing engine torque and this reduces traction between the tyres and the road. It can also use ABS to apply the brakes to reduce wheel spin.

Engine torque can be decreased in three ways:1 By reducing or cutting off fuel from the injectors.2 By retarding the ignition.3 By closing the throttle valve.

Where traction control is fitted, the ABS electronic control unit has extra programming for traction control and often extra valves in the modulator.


DRAFT ONLY


TC on–o� switch

warning/indicator lights

Figure 30.11 traction control switch and warning/indicator lights for ABS and tCS holden ltd

When driving conditions are such that traction control is needed, the ABS/TCS electronic control unit sends a traction control signal to the engine PCM telling it to reduce engine torque. The engine ECU then reduces the fuel from the injectors, retards the ignition or, in some systems, closes the throttle valve.

Traction control is active whenever the engine is running, but it only comes into operation when it is needed. While it is actually in operation, the traction control (TC) indicator light flashes (Figure 30.11).

An on–off switch is provided so that traction control can be switched off when it is not required. The indicator in the instrument panel will then light to show that traction control is turned off.

electronic throttle control for tcSElectronic throttle control, or drive by wire, is a device that can be used with traction control. This does away with the conventional throttle linkage or cable.

Figure 30.12 shows the way that such a system could be arranged. The accelerator pedal is connected to an electronic sensor which signals the throttle position to the engine ECU (or PCM). The engine ECU then operates a servo motor which is connected to the throttle valve.

For normal operation, when traction control is not working, signals pass through the engine ECU, and the servo motor opens the throttle valve in proportion to accelerator pedal movement.

With the traction control in operation, the engine ECU takes into account signals from the antilock braking system/traction control ECU and adjusts the throttle valve opening accordingly.

traction control and aBSThe antilock braking system can operate in conjunction with traction control. With ABS, the hydraulic modulator uses its internal hydraulic pump to boost or reduce the master cylinder pressure.

For traction control at lower speeds, hydraulic pressure from the pump is used without the driver applying the brakes. The ABS/TCS electronic control unit processes inputs from the wheel sensors and applies a brake when needed. The ABS/TCS hydraulic modulator has extra valves for traction control. These are the priming and switching valves. They are opened and closed by the ECU to control the brake fluid flowing to and from the master cylinder.

Information: With tCS, there are other inputs to the engine eCU, for example, engine speed and temperature.

ABS/TC

M

wheel speed

enginespeed

control circuit

temperature

1

23

4

5 6

Figure 30.12 Concept of electronic throttle control (drive by wire) for use with traction control 1 eCU for antilock brake system/traction control, 2 engine PCM, 3 accelerator pedal sensor, 4 servo motor, 5 throttle valve, 6 wheel speed sensor BoSCh

traction control with vacuum componentsThe parts of a traction control system are shown in Figure 30.13. For this system, the manufacturer uses the


DRAFT ONLY


electronic program that is part of the electronic control system of the vehicle.

A brief comparison of an electronic stability program (ESP) with the other systems is: the antilock braking system (ABS) prevents the wheels from locking and skidding; the traction control system (TCS) prevents the wheels from spinning and the electronic stability program (ESP) provides steering stability.

ESP can stabilise steering by intervention in two different ways: by braking selected wheels, or by accelerating the driving wheels. This helps to keep the vehicle stable on the road.

1

11

10

9

8

6

5

43

2

7

Figure 30.13 Components of a traction control (tCl) system 1 stop-lamp switch, 2 tCl switch, 3 steering-wheel sensor, 4 ventilation solenoid, 5 vacuum solenoid valve, 6 tCl eCU, 7 engine and automatic transmission eCU, 8 accelerator position sensor, 9 vacuum tank, 10 ABS eCU, 11 vacuum actuator MItSUBIShI

abbreviation TCL for traction control. This has the usual electronic sensors and control units, but uses a vacuum actuator on the throttle valve. It has the normal ABS functions as well as traction control (TCL). This system has two operating modes: slip control and trace control. Slip control prevents wheel slip (wheel spin) and trace control assists with keeping the vehicle on track when cornering.

electronic stability program (eSp)The electronic stability program (ESP) is an active safety system. It is a further development of ABS and TCS. It uses additional electronic controls to operate the brakes, in conjunction with ABS, to provide vehicle stability. This is applied particularly during cornering. ABS has the capacity to sense the need to apply or release the brakes on each wheel independently and ESP makes use of this facility.

Outwardly, there is little to be seen with ESP because most of the components that it uses already exist for the ABS. Its main feature is hidden, because it is the

Information: Simply, intervention is a term that is used with these systems to denote that a different action has been introduced.

eSp closed-loop systemA schematic arrangement of the main components that are used in an ESP is shown as Figure 30.14. There are sensors that send signals to the ESP control unit, controllers for ABS, TCS and ESP within the ESP control unit and actuators that receive signals from the ESP control unit.

The sensors for ESP that are additional to those in ABS and TCS are the yaw sensor, the steering-wheel angle sensor and the brake-pressure sensor. These are used to sense the direction of vehicle travel (see Figure 30.15).

The various parts and sensors for ESP have the following functions:

1 Yaw sensor. This detects any tendency for the vehicle to rotate about its axis. It also detects any sideways movement (lateral acceleration) of the vehicle.

2 Steering wheel angle sensor. This registers the turning angle of the steering wheel. It provides an input from the driver that indicates the direction in which the vehicle is intended to be steered.

3 Pressure sensor. This sensor monitors the brake pressure in the system as applied by the driver.

4 Wheel sensors. These signal the wheel speeds.5 Hydraulic modulator. This is responsible for

applying the brakes and controlling the brake pressure.

6 Brakes. These are selectively applied by the hydraulic modulator.

7 PCM. This engine control module is directed to control the engine functions.

8 Ignition timing. Controlled by the PCM, the ignition timing is retarded to reduce engine torque.


DRAFT ONLY


yaw

steeringwheel angle

pressure

wheels

brakes

ignitiontiming

fuel injection

ESPcontroller

ABS/TCS controller

hydraulicmodulator

PCM

throttlevalve

Sensors ESP control unit Actuators/�nal controls

Figure 30.14 eSP components and closed-loop system BoSCh

9 Fuel injection. Also controlled by the PCM, injection is reduced to decrease engine torque.

10 Throttle valve. This is closed to reduce engine torque, also controlled by the PCM.

Vehicle stabilityFigure 30.15 shows the various directions in which it is possible for a vehicle to travel and which are controlled as far as possible by ESP.

Normally, the desired direction, or track, that the vehicle travels is controlled by the driver. However, with different types of operating conditions, other forces can take over and the direction of travel will be different to that desired.

The illustration shows three axes about which movement can occur. These are the vertical axis, the transverse axis and the longitudinal axis. In adverse conditions, vehicle movement could occur in any of these directions and the possibility of movement in these directions even exists during normal driving. Actual movement would only be apparent in situations where control of the vehicle became difficult.

Information: engine eCU and power control module (PCM) are different terms for the same component and both terms are used by vehicle manufacturers.

Forces acting on a vehicleSome of the conditions that influence vehicle operation are:

1 Centrifugal force. This is an outward force on curves that increases with speed. It can cause lateral instability and produce an overturning force.

2 Weather conditions. Poor conditions reduce visibility and create slippery road surfaces. Side winds can cause lateral instability.

vertical axis

longitudinalaxis

transverse axis

Figure 30.15 directions in which vehicle travel could take place BoSCh


DRAFT ONLY


(a) With ESP (b) Without ESP

Figure 30.16 Comparison of the actions of a vehicle with eSP and without eSP under particular driving conditions BoSCh

3 Road surfaces. There are various types of road surfaces and these can have different frictional effects with the tyres. This can affect braking, traction, side slip when cornering, and general driveability.

4 Driver fatigue. Driver fatigue causes drowsiness and slow reactions.

5 Load conditions. The forces and vehicle stability will be different with light and heavy loads.

6 Other. Wind effects of larger passing vehicles can cause instability. Animals, objects on the road or the behaviour of other drivers might require a panic stop or evasive action. This could affect general vehicle stability.

Steering: understeer and oversteerTwo terms are used to refer to the steering characteristics of a vehicle: oversteer and understeer.

1 Oversteer is where the vehicle is over-responsive when cornering and tends to move further into the curve than it is actually being steered.

2 Understeer is the opposite effect. The vehicle is not as responsive to steering as it should be and tries to keep out wide on a curve.

Oversteer and understeer are not directly related to the vehicle’s steering system, but are due more to the overall vehicle design – the suspension system,

distribution of weight, vehicle load, type of drive and so on. It is unlikely that any one factor will be responsible for a vehicle’s characteristics. It is usually a combination of features that gives it an understeer or oversteer tendency.

A slight tendency to understeer is usually considered more desirable than oversteer, but for normal speeds and conditions of driving, the driver is unlikely to be aware of one condition or the other.

Vehicles with and without eSpAn appreciation of how ESP works is illustrated in Figure 30.16. This shows the possible tracks taken by a vehicle both with and without ESP.

Without ESP, a vehicle driven into a curve at high speed could become unstable. The front of the vehicle is steered into the curve and the rear tends to catch up. Attempts at correction might fail and the vehicle could become uncontrollable. The vehicle could finally skid off the road.

With ESP, the brakes are automatically applied to help steering and correct the direction. The right-front brake is applied (as shown by the arrow) on a left bend in the road – this helps to keep the vehicle in the centre of the lane. Then the left-front brake is applied on a right bend and this again helps with a steering correction. In this way, the vehicle maintains a regular track within its lane.


DRAFT ONLY


Service points

aBS-ecU check functionThe ECU of the ABS will perform an initial check when the vehicle is started and a monitoring check while the vehicle is in operation.

Initial checkThe ECU will carry out an initial operational check of the system each time the ignition is switched on and the vehicle is driven. The ECU will carry out a check when the vehicle reaches a speed of approximately 6 km/h. It will check the operation of the solenoid valves and the pump unit of the hydraulic unit. If a malfunction is detected, the ABS will be isolated and the warning lamp will be switched on.

When the ECU receives signals from the wheel sensors, it checks the sensor operation. If an incorrect signal is received, or if there is no signal, then the ABS will also be isolated and the warning lamp switched on.

Monitor checkThe ECU will constantly monitor the system. The ECU has two micro-processors that receive identical input signals and should therefore have identical output signals. If this is not the case, then there is a malfunction in the system and ABS operation will be shut down.

Components that are checked are: the stop-light switch, the sensors and the circuits. Also, the speeds of the sensors are checked against a reference value.

Fail-safe functionIf a failure is detected by the system’s self-diagnosis function, the ABS will be isolated. The brakes will act like a normal system that does not have ABS. The warning lamp will light and remain on.

Fault diagnosisPossible ABS faults can be separated into hydraulic faults, electrical faults and electronic faults.

ABS hydraulic faults are the same as those likely to be encountered in a system without ABS. These include faults in the master cylinder, calipers, hoses, leaks and so on. Electrical faults could be related to cables and connections and might be found by a close inspection. Electronic and component faults can only be located with instruments or by the self-diagnosis function of the system. An ABS warning lamp in the instrument cluster will light up when ABS faults are present (see Figure 30.17).

Fault codes can be accessed through a diagnosis connector, as shown in Figure 30.18. These are read with a scan or multifunction test instrument. Table 30.1 shows typical fault codes using an OBDII-compatible scan tool.

diagnostic connector

1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16

Figure 30.18 diagnostic connector ford

ABS warninglight

Figure 30.17 ABS warning light on an instrument panel

holden ltd

aBS checksThere are many items that will affect the operation of ABS, TCS and ESP apart from the basic systems components themselves. The systems work through the vehicle’s brakes so a thorough inspection of the braking system should be carried out before ABS components are replaced. Particular attention should be paid to:

1 tyre pressure (recommended pressures are important)

2 tyre size – all tyres must be the same size with no significant difference in tyre wear (differences in circumference will indicate to the ECU difference in wheel speed)

3 disc pad and rotor condition4 brake hoses and pipe condition5 wheel bearing condition and adjustment6 master-cylinder operation7 brake fluid level and condition.

Checking wheel sensorsPassive wheel sensors have a permanent magnet built into them and will therefore attract iron particles or objects. Sensors should be cleaned and tested with a multimeter if a fault code is detected. When the wheel is rotated by


DRAFT ONLY


hand an AC voltage should be present. The voltage can be measured at the sensor two-pin connector or at the ECM connector (shown in Table 30.2 and Figure 30.19).

For example, an electrical test of the left front wheel-speed sensor would require location of the correct pins from Table 30.2. These are pins 6 and 7. A multimeter could then be applied to pins 6 and 7 at the ABS ECM connector shown in Figure 30.19. Rotation of the left-hand front wheel with the ignition ON should produce and AC voltage or waveform as shown in Figure 30.20.

Active wheel speed sensors consist of an integrated circuit that produces a DC, square-wave voltage signal. These sensors are tested with the ignition ON and by checking reference or supply voltage to the sensor using a DC voltmeter. A typical wheel-speed sensor circuit is shown in Figure 30.21. This particular sensor should be tested for a reference voltage of 8 volts. Signal voltage can also be tested by turning the wheel and checking for a square wave pattern. This test can be used to check the electronic operation of the sensor or to identify trigger-wheel faults (as shown in Figure 30.22).

Brake bleedingIf air is present in the ABS module a compatible scan tool is normally required to correctly remove air from the

20211112131415 4678910 1235

29302223242526

272831 19 18 17 16

31 pin ABS module (backloading view)

Figure 30.19 eCM connector ford

Figure 30.20 ABS sensor waveform – passive sensor

Figure 30.21 A typical active wheel speed sensor circuit

hall element

trigger inwheel bearing

8 V

magnet

Table 30.1 ABS fault codes

B1342 eCU fault

B1676 Battery voltage out of range

C1093 traction control switch failure

C1095 Pump motor fault

C1145 right front wheel-speed sensor continuity fault

C1148 right front wheel-speed sensor output plausibility fault

C1155 left front wheel-speed sensor continuity fault

C1158 left front wheel-speed sensor output plausibility fault

C1165 right rear wheel-speed sensor continuity fault

C1168 right rear wheel-speed sensor output plausibility fault

C1175 left rear wheel-speed sensor continuity fault

C1178 left rear wheel-speed sensor output plausibility fault

C1222 Wheel-speed sensor frequency fault

C1266 Valve relay/valve relay power supply fault

C1288 hydraulic modulator internal pressure sensor valve

P1571 Brake lamp switch failure

U0073 CAn communication BUS fault

U2501 CAn message from PCM failure

U2503 hybrid electronic CAn message failure


DRAFT ONLY


Table 30.2 the eCM connector

expected outputs from the eCM connector

Action result

1 Wheel-speed sensor right rear negative Ignition on Wheel turning slowly

AC waveform

2 Wheel-speed sensor right rear positive Ignition on Wheel turning slowly

AC waveform

4 Wheel-speed sensor right front negative Ignition on Wheel turning slowly

AC waveform

5 Wheel-speed sensor right front positive Ignition on Wheel turning slowly

AC waveform

6 Wheel-speed sensor left front negative Ignition on Wheel turning slowly

AC waveform

7 Wheel-speed sensor left front positive Ignition on Wheel turning slowly

AC waveform

8 Wheel-speed sensor left rear negative Ignition on Wheel turning slowly

AC waveform

9 Wheel-speed sensor left rear positive Ignition on Wheel turning slowly

AC waveform

11 diagnostic connector terminal 7 Serial data n/A

12 AtCP module (if fitted) BUS negative Serial data

13 AtCP module (if fitted) BUS positive Serial data

14 Brake lamp switch signal Ignition on Brake pedal at rest Brake pedal depressed

<0.5 volts Battery volts

15 Ignition power Ignition on Battery volts

16 Ground All times <0.5 volts

17 Valve relay power supply All times Battery volts

18 Valve relay power supply All times Battery volts

19 Ground All times <0.5 volts

20 ABS warning lamp no specifications available

n/A

21 eBd warning lamp no specifications available

n/A

22 Vehicle speed signal to PCM Ignition on Vehicle moving

Voltage pulse

29 CAn low Serial data n/A

30 CAn high Serial data n/A

31 traction control off switch Ignition on Switch open Switch closed

high volts low volts


DRAFT ONLY


cracked

cracked

damaged

damagedblocked

blocked

normal

32 teeth

Figure 30.22 Abnormal waveform patterns caused by trigger-wheel faults

system. Manufacturers’ instructions must be followed as pressures in the fluid higher than normal could be encountered. Replacement of brake fluid can be carried out in the usual manner.

Safety: on hydraulic braking systems that use a pressurised accumulator, do not loosen fittings or open hydraulic lines until the system has first been depressurised. the manufacturer’s service and safety instructions must be followed. Some systems operate at pressures of up to 17 000 kPa.

One way to depressurise the system, used by a number of manufacturers, is to pump the brake pedal 30 or so times with the ignition turned off. The pedal feel will get harder as the pressure in the system drops. The system must be fully depressurised before attempting any work of the hydraulic system.


DRAFT ONLY


technical termsAntilock, antilock braking system, ABS, traction control system, slip control, trace control, tCS, electronic brake distribution, eBd, electronic stability program, eSP, intervention, closed loop, hydraulic modulator, modulated, three-channel, four-channel, diagonally split, hydraulic reservoir, ABS/tCS, accumulator, traction, understeer, oversteer, vacuum actuator, vacuum solenoid valve, electronic throttle control, drive by wire, servo motor, load sensing, fail safe.

review questions1. What is meant by ABS?

2. Why is ABS fitted to motor vehicles?

3. What are the three main parts of an antilock braking system?

4. name the main parts of the hydraulic system of ABS.

5. What is meant by channel in relation to ABS?

6. What is the difference between a three-channel and a four-channel brake system?

7. What is the function of the wheel sensors?

8. What part does the eCU play in an antilock braking system?

9. What is the hydraulic modulator?

10. Briefly, how does a hydraulic modulator influence brake pressure?

11. What is the purpose of the solenoid valves in the hydraulic modulator?

12. What is traction control?

13. how does traction control prevent wheel spin (briefly)?

14. how is engine torque reduced by the traction control system?

15. What is meant by eSP?

16. What is the purpose of a steering wheel sensor in an eSP?

17. how is steering assisted by eSP (briefly)?

18. If an ABS warning light remains on, what does this tell the driver?

19. If the tC light or symbol flashes, what would this indicate?

20. What is the purpose of the accumulator in the hydraulic control unit?

21. What is meant by the term fail safe?

22. how is the driver made aware that an ABS has a fault?

23. how could a sensor be checked for serviceability?

24. What visual checks can be carried out on an ABS system?


DRAFT ONLY

part safety, security and draft only...

Documents