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Student Workbook

LV41 Braking Systems (3)

kap all phase 2 & 3 6/11/03 11:37 am Page 41

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV41 BRAKING SYSTEMS (3)

Contents

Introduction 3 ABS Actuator/Modulator: 18

Compact 3 – position solenoid

ABS Components: 8

-1-

Wheel Speed Sensor Checks: 47

Typical speed sensor code chart 47

Mechanical Diagnosis: 49

Introduction

Anti-lock brakes have been in use for a number of years Their origins can be traced back as far as the 1920’s The first systems were introduced in the 1930’s but it took until the 1960’s/1970’s before they became more widely used and then only on top of the range prestigious cars From the mid to late 1980’s, Anti-lock Braking systems became cheaper to produce and were then made available on entry-level cars

During this course of study, we will look at the principles of Anti-lock Braking Systems

as well as the component parts and diagnosis of these

What is ABS?

Braking is achieved through the medium of two types of friction:

• the friction between the brake linings and the brake drums/disc’s

• the friction that exists between the tyre and the road

Tyre (locked up) Tyre (rotating)

Braking can be controlled in a stable manner as long as the friction between the brake linings and brake drums/discs, is less than the friction created between the tyre and the road surface In other words the amount of braking does not exceed the stopping capacity of the tyre to road contact

-3-

If the braking force is greater than the tyre to road contact can handle, then the

wheels will lock up If the front wheels lock up, then the vehicle will become

impossible to steer If the rear wheels lock up, then the vehicle will lose grip at the rear and a 'tail spin' will occur

The ABS controls the hydraulic pressure acting on individual wheel cylinders/brake callipers to prevent the wheels from locking up under heavy braking This will allow the driver to maintain control when steering might have otherwise been lost

Operating principles of ABS

When a vehicle is being driven along a road in a straight line its wheels rotate at virtually identical speeds The vehicle’s body also travels along the road at this same speed When the driver applies the brakes in order to slow the vehicle the speed of the wheels becomes slightly slower than the speed of the body, which is travelling along under its own inertia The difference in speed is expressed as a percentage, and is called the ‘slip ratio’

The way to calculate the slip ratio is as follows: -

The easiest way to understand this principle is to look at the extremes e.g

• 100% slip ratio is the equivalent of a locked wheel In other words the vehicle body is still moving at 30 mph but the wheels are not rotating and so are

considered to be stationary

• 0% slip ratio is the equivalent to a freely rotating wheel The vehicle is travelling at

30 mph and the wheels are also travelling at 30 mph

When braking, if the slip ratio becomes too large then the vehicle will begin to lose control (as previously mentioned) The ideal slip ratio for the best possible braking would be between 10% and 30% The reason for this is a small amount of slip will create heat, which will improve the tyres’ grip However, if the slip ratio exceeds 30% then the braking force will begin to decline

In addition to braking force, we need to consider cornering force without which we would not be able to steer the vehicle The same slip ratio of 10% - 30% is also ideal The ABS system is designed to maintain a slip ratio of between 10% and 30%, which will explain why under ABS operating conditions it is sometimes noted that the tyres appear to skid slightly

It should also be noted that when a vehicle is driven on slippery or snowy roads, the vehicle might actually have a longer stopping distance than a vehicle that is not

equipped with ABS This is due to the fact that a vehicle without ABS locks its wheels and therefore creates a ‘snow plough effect’ i.e snow builds up in front of the locked tyre slowing it down, which cannot happen on an ABS equipped vehicle

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Slip Ratio

The slip ratio is not only affected by the condition of the vehicle’s tyres and its

physical weight, but also by the condition of the road surface

ABS - basic operation

The wheel speed is monitored via the wheel speed sensors and the signal is sent to the ABS control unit

-5-

The control unit monitors the individual wheel speeds and calculates the overall

vehicle speed

In a panic situation the brake light switch informs the control unit that the brakes are being applied The sudden reduction in wheel speed is noted by the control unit, which instructs the hydraulic brake actuator to provide the optimum brake fluid

pressure to each brake

The hydraulic brake actuator operates on commands from the control unit to ‘reduce’,

‘hold’ or ‘increase’ the brake fluid pressure as necessary in order to maintain the ideal slip ratio of between 10% and 30% and avoid wheel lock up

Types of ABS control

It is important to understand that the ABS is an addition to the existing conventional brake system It does not replace any existing components

Hydraulic brake circuits appear in various differing layouts:

Diagonally split circuit

The two rear brakes are not connected directly together (or the two front brakes) The ABS actuator has to provide two separate outlets for the rear brakes (and two for the front brakes) This is known as ‘four solenoid control’

Front to rear split circuit

In the case of front to rear split circuits, the actuator could have just one outlet for the rear brakes (the steered wheels are always controlled independently because of their importance to the maintenance of control) If just one were used for the rear brakes then this would be known as ‘three solenoid control’

In addition, the electrical control of the ABS varies from vehicle to vehicle:

• individual control of the front wheels whilst controlling the rear wheels together This is known as ‘three channel control’

-7-

ABS Components

A typical front wheel drive vehicle set up

ABS ECU (Electronic Control Unit)

Controls the entire system It monitors wheel speed and determines wheel lock up It sends commands to the hydraulic actuator to reduce, hold or increase the brake fluid pressure It carries out a self-check of the system at start up and informs the driver of any abnormalities via the dashboard ABS warning light It stores any diagnostic information for later retrieval by a technician

Wheel speed sensors

These detect individual wheel speeds and send this information to the ABS ECU

Sensor rotor

Attached to the hub or drive shaft, it has teeth that when passed in front of the ABS wheel speed sensor causes a signal to be generated

ABS actuator/modulator

This controls the hydraulic brake fluid pressure to the individual brakes dependent upon signals received from the ABS ECU

ABS warning light

This alerts the driver to system malfunctions It can also be used as a diagnostic

code indicator on some makes of vehicle

Control relays

Usually two relays are required to provide electrical power to the ABS The relay is the actuator pump relay and the other one is for the actuator solenoids They can be located on the actuator itself or an adjacent fuse/relay block

Diagnostic check connector

Various types of check connector have been used over the years but they all have basically the same function, which is to allow access to ABS diagnostic codes

-9-

Typical rear wheel drive vehicle set up

The vehicle pictured above has only one rear speed sensor and it is fitted to the gearbox This means that the rear wheels will be controlled together regardless of the number of control solenoids fitted in the actuator, (either three or four)

The vehicle pictured above is also rear wheel drive but uses individual wheel speed sensors for all four wheels

Typical four-wheel drive vehicle set up

Some four-wheel drive vehicles use a deceleration sensor This is due to the specific characteristics of four-wheel drive systems under braking

If a wheel tries to lock up, the four-wheel drive system can have the effect of actually transmitting the lock up to other wheels (through differential action)

The control unit uses the deceleration sensor to assist in carrying out its calculations

-11-

Components in detail

Wheel speed sensors

Wheel Speed Sensor Sensor Rotor

The front and rear wheel speed sensors consist of a gear-shaped sensor rotor and a sensor element

The element contains a bar magnet around which a copper coil is wound The sensor

is installed in the backside of the brake disc/rotor

Alternating current is generated in the copper winding as the teeth of the rotor pass the sensor

The frequency and voltage increase as the rotational speed of the rotor increases It

is the frequency of this signal that the ABS ECU interprets as rotational speed of the wheel

Electronic integrated circuit type

Some manufacturers built sophisticated electronic sensors in the early days of ABS The concern was that the signal from the sensor to the control unit could be subject to interference and cause system failures

-13-

The idea was to convert an analogue signal into a digital signal via a small integrated circuit built into the sensor

This type of sensor is no longer used due to its cost, plus the reliability of traditional sensors has now been proven

The location of the sensors can be varied; they can be fitted onto driveshafts, inside brake drums or inside transmission cases The illustration above also demonstrates that a sensor can be located either square on to the rotor, or at 90 degrees to it

This particular sensor consists of a swinging plate with slits cut into it, placed on a pivot between two sets of LED’s, (light emitting diodes) and two phototransistors The whole unit is mounted along the vehicle’s longitudinal axis and will swing when the brakes are applied The rate of deceleration determines how far the plate will swing forward Light will shine through the slits depending upon the position of the plate When light hits the phototransistors they will turn on By monitoring the ‘ON’s’ and

‘OFF's’, of the phototransistors the control unit can determine deceleration rate

-15-

Semi-conductor type sensor

The later semi-conductor type sensor has the advantage of being a solid-state

component and therefore has no moving parts

The semi-conducting element experiences a change in resistance in accordance with any flexing experienced by this element (the flexing is caused by the movement of weights during deceleration) The change in resistance alters a voltage based signal into the ABS ECU, which is then interpreted as a given deceleration rate

-17-

ABS Actuator/Modulator 3-position solenoid valve type

The traditional ABS actuator is made up of three or four ‘3 position solenoid valves’, a pump and a reservoir

As the name implies, the solenoids have three different operating positions:

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The ABS ECU regulates the current flowing through the solenoid windings This puts the valves in one of three different positions The valves then control the brake fluid pressure in the brake calliper/wheel cylinders accordingly

The pump is provided to move surplus fluid back to the master cylinder during ABS activation

The reservoir is a temporary store for the excess brake fluid whilst it waits for the pump to remove the fluid from the circuit

Compact 3-position solenoid actuator

The later 3-position type actuator has been significantly reduced in size

The 3 solenoids can be clearly seen at the bottom of the assembly represented by the diagram above

Circuit diagram

The diagram above shows the overall layout of a four 3-position solenoid system It can be seen by the way in which the brake assemblies are paired that the system is diagonally split

Note: The two circuits do not share a single fluid passageway No matter what fault occurs the two circuits remain independent of one another

Reservoir and pump

The cam rotates when the control unit and relay turn on the electric motor

The singular cam operates two horizontally opposed pump plungers (one for each hydraulically split circuit)

The reservoir is simply a piston with a compression spring fitted behind it It gives the high-pressure brake fluid somewhere to go when the pump cannot remove the

excess fluid fast enough

-21-

Overview of operation

Pressure increase mode

This mode is used during normal braking and also during pressure increase (ABS operating) The only difference between the two conditions is that the pump will be running during pressure increase, as it is too slow to stop and start again during ABS operation

Hold mode

The control unit uses the ‘Hold’ position to allow time to see what effect if any, an adjustment to the brake fluid pressure has had

Note: The current applied is 2 amps, which puts the valve into its middle position i.e

no more high-pressure brake fluid is supplied from the brake master cylinder to the brake calliper, whilst the pressure in the brake calliper is maintained at a constant level

The ABS ECU has the choice to either switch to pressure increase or pressure

reduction depending on what the wheel speed is doing If it were still slower than the other three wheels by more that 30% then it would select pressure reduction If the wheel speed is within 30% of the other wheels then it would continue to hold or select pressure increase

-23-

Pressure reduction mode

The control unit achieves a pressure reduction mode by applying 5 amps to the

solenoid windings

This will move the solenoid to the top, blocking the high pressure from entering the valve and therefore the brake calliper, whilst also opening the port at the bottom of the valve This allows the brake fluid in the brake calliper to escape to the reservoir and be returned to the master cylinder by the pump

ABS Operation

ABS not activated

This mode is should be considered as ‘normal braking’ The ABS has no effect on brake operation whatsoever It might as well not be there! The brake fluid travels from the brake master cylinder, through the three-position solenoid valve and down to the brake caliper When the driver releases the brake pedal the fluid travels back to the master cylinder in the normal way No 3 check valve operates as a one-way valve and ensures that fluid return is not restricted

Note: Both the solenoid and the pump are turned off

-25-

ABS activated (pressure reduction mode)

Under emergency braking, wheels will start to lock up The first thing the system must do is reduce the brake fluid pressure to the affected wheel(s) To do this, the pump must run and 5 amps would be applied to the three-position solenoid valve controlling the locking wheel(s) This will move the valve to its upper-most position, which will block the incoming high-pressure fluid from the master cylinder (driver still applying the brakes for all he is worth), whilst also opening the port which leads to the reservoir and pump The pump will continue to run for a pre-determined time

regardless of the position of the three-position valve because it cannot stop and start

as quickly as the solenoids can switch position