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