Diagnostics – brakes

8.2 Diagnostics – brakes

annual safety test. The front or rear wheels are driven into a pair of rollers. The rollers drive each

8.2.1 Systematic testing

wheel of the car and as the brakes are applied the braking force affects the rotation. A measure of

If the reported fault is the hand brake not holding braking efficiency can then be worked out. proceed as follows.

1. Confirm the fault by trying to pull away with

8.2.3 Dial gauge (Figure 8.5)

the hand brake on.

A dial gauge, sometimes called a clock gauge or suggests the brake shoes and drums (or pads

2. Check the foot brake operation. If correct this

a dial test indicator (DTI), is used to check the and discs) are likely to be in good order.

brake disk for run out. The symptoms of this would

3. Consider this: do you need to remove the often be vibration or pulsation when braking. wheels and drums or could it be a cable fault?

Manufacturers recommend maximum run out

Chassis systems 173

figures. In some cases the disk can be re-ground

Callipers or

Free off or renew if in any

but often it is safer and more cost effective to fit

wheel cylinders doubt

new disks. This would also be done in pairs. sticking

Brake judder

Linings worn

Renew

Drums out

Renew

of round

8.2.4 Test results

Discs have

Renew

excessive

Some of the information you may have to get

run-out

from other sources such as data books or a work-

Squeaking

Badly worn

Renew

shop manual is listed in the following table.

linings Dirt in brake

Clean out with proper

cleaner Test carried out

drums

Replace and smear with Brake roller test

Information required

Anti-squeal

copper grease line brakes, 25% for second line brakes

Required braking efficiency: 50% for first

shims missing

at rear of pads and 16% for the parking brake. On mod-

ern vehicles half of the main system is the second line (dual line brakes). Old

8.2.6 Brakes fault diagnosis

vehicles had to use the parking brake as the second line, therefore it had to work

Table 2

at 25% Brake fluid

Possible cause condition

Manufacturers specify maximum

Symptom

moisture content

Brake fade

Incorrect linings Badly lined shoes Distorted shoes

8.2.5 Brakes fault diagnosis

Overloaded vehicle

Table I Excessive braking

Spongy pedal

Air in system Badly lined shoes

Symptom Possible faults

Suggested action

Shoes distorted or incorrectly set Faulty drums

Excessive Incorrect

Weak master cylinder mounting pedal travel

Adjust it! But check

Discs running out pushing pads back Poor

adjustment

condition as well

Long pedal

Distorted damping shims performance

Pad and/or

Renew

shoe linings Misplaced dust covers when stopping

worn Drum brakes need adjustment Seized calliper

Renew or free off if

Fluid leak

or wheel

Fluid contamination cylinders

possible and safe

Worn or swollen seals in master cylinder Contaminated

Blocked filler cap vent linings

Renew (both sides)

Brakes binding

Brakes or handbrake maladjusted No clearance at master cylinder push rod

Car pulls to Seized calliper

Seals swollen one side

Overhaul or renew if

Seized pistons when braking

or wheel

piston or cylinder

cylinder on

Shoe springs weak or broken one side

is worn

Servo faulty

Contaminated

Renew (both sides)

Incorrect linings side

linings on one

Hard pedal –

poor braking

Glazed linings Linings wet, greasy or not bedded correctly

Spongy pedal Air in the

Servo unit inoperative hydraulic system check for leaks

Bleed system and then

Seized calliper pistons Master cylinder

Worn dampers causing wheel bounce seals failing

Overhaul or renew

Seized pistons Pedal travels to Fluid reservoir

Brakes pulling

Variation in linings the floor

Refill, bleed system

Unsuitable tyres or pressures when pressed

empty

and check for leaks

Failed seals in

Loose brakes master cylinder

Overhaul or renew

Greasy linings Leak from a

Faulty drums, suspension or steering pipe or union

Replace or repair as

Worn disc pads Brakes

required

Fall in fluid level

External leak overheating

Shoe return

Renew (both sides)

springs broken Leak in servo unit

174 Advanced automotive fault diagnosis

Disc brake Worn retaining pins

energy conversion is taking place in the brake

squeal – pad Worn discs

system. This is the conversion of kinetic energy

rattle No pad damping shims or springs

to heat energy at the discs and brake drums. The

Uneven or Disc corroded or badly scored excessive

potential for this conversion process between a

Incorrect friction material

pad wear

tyre skidding, even on a dry road, is far less. A

good driver can pump the brakes on and off to

Brake judder Excessive disc or drum run-out

Calliper mounting bolts loose

prevent locking but electronic control can achieve

Worn suspension or steering components

even better results.

ABS is becoming more common on lower price vehicles, which should be a contribution to safety.

8.2.7 Brake hydraulic faults

It is important to remember, however, that for nor- mal use, the system is not intended to allow faster

Brake hose clamps will assist in diagnosing driving and shorter braking distances. It should be hydraulic faults and enable a fault to be located

viewed as operating in an emergency only. Figure quickly. Proceed as follows.

8.7 shows how ABS can help to maintain steering

1. Clamp all hydraulic flexible hoses and check control even under very heavy braking conditions. the pedal.

2. Remove the clamps one at a time and check

8.3.2 Requirements of ABS

the pedal again (each time).

A good way of considering the operation of a part of the system will now be apparent.

3. The location of air in the system or the faulty

complicated system is to ask: ‘What must the system be able to do?’ In other words, ‘what are the requirements?’ These can be considered for

8.3 Anti-lock brakes

ABS under the following headings:

8.3.1 Introduction

Fail safe system

In the event of the ABS system failing then conventional brakes must still

The reason for the development of anti-lock

operate to their full potential. In add-

brakes (ABS) is very simple. Under braking con-

ition a warning must be given to the

ditions if one or more of the vehicle wheels locks

driver. This is normally in the form of

(begins to skid) then this has a number of conse-

a simple warning light

quences:

Manoeuvrability

Good steering and road holding must

must be maintained

continue when the ABS system is

● braking distance increases;

operating. This is arguably the key

● steering control is lost;

issue as being able to swerve round a ●

tyre wear is abnormal. hazard whilst still braking hard is

often the best course of action

The obvious consequence is that an accident is

Immediate response

Even over a short distance the system

far more likely to occur. The maximum deceler-

must be available

must react such as to make use of the

ation of a vehicle is achieved when maximum

best grip on the road. The response

Figure 8.7 Advantages of ABS

Chassis systems 175

must be appropriate whether the driver applies the brakes gently or slams them on hard

Operational Normal driving and manoeuvring influences

should produce no reaction on the brake pedal.The stability and steering must be retained under all road con- ditions.The system must also adapt to braking hysteresis when the brakes are applied, released and then re- applied. Even if the wheels on one side are on dry tarmac and the other side on ice, the yaw (rotation about the vertical axis of the vehicle) of the vehicle must be kept to a minimum and only increase slowly to allow the driver to compensate

Controlled wheels In its basic form at least one wheel on each side of the vehicle should be controlled on a separate circuit. It is

Figure 8.8 ABS block diagram

now general for all four wheels to be controlled on passenger vehicles

Speed range of The system must operate under all

constant or allowed to increase. The maximum

operation speed conditions down to walking

pressure is determined by the driver’s pressure on

pace. At this very slow speed even when the wheels lock the vehicle will

the brake pedal.

From the wheel speed sensors the ECU calcu-

come to rest very quickly. If the

wheels did not lock then in theory

lates the following.

the vehicle would never stop! Other operating

Determined from the combination of conditions

The system must be able to recognise

Vehicle reference

aquaplaning and react accordingly. It

two diagonal wheel sensor signals. After must also still operate on an uneven

speed

the start of braking the ECU uses this road surface.The one area still not

value as its reference perfected is braking from slow speed

on snow.The ABS will actually Wheel acceleration This is a live measurement that is increase stopping distance in snow

constantly changing but steering will be maintained.

or deceleration

Although this cannot be measured This is considered to be a suitable

Brake slip

directly a value can be calculated from trade off

the vehicle reference speed. This figure is then used to determine when/if ABS

A number of different types of anti-lock brake should take control of the brake

pressure

systems are in use, but all try to achieve the require- ments as set out above. Vehicle deceleration During brake pressure control the ECU

uses the vehicle reference speed as the starting point and decreases it in a linear