1995 LAND ROVER DISCOVERY brakes

70BRAKES
4
DESCRIPTION AND OPERATION System description
The brake system consists of dual hydraulic circuits in
a front - rear split. That is individual circuits to front
and rear axles.
Wheelspeed signals are provided by the fitting of
exciter rings to axles and speed sensors.
An ABS stop light switch is fitted to provide a braking
signal to the ECU as well as operate the stop lights.
Electrical wiring is provided as necessary with system
relays and fuses. Discovery Electrical Troubleshooting
Manual gives the location and wiring circuits of these
components.
Visual warning of system malfunction is provided via a
warning light in the instrument panel.
Hydraulic circuit ABS
Key
A = Primary hydraulic circuit
B = Secondary hydraulic circuit
C = Pressure reducing valve
D = ABS ModulatorABS System operation
During normal braking the feel of the brake pedal on
vehicles equipped with ABS will be the same as that
on non ABS vehicles. During ABS cycling the driver
will experience feedback in the form of a pulsating
brake pedal and solenoid/pump motor noise from the
Modulator.
The system is fully automatic in operation both on and
off road, and will only act when the ECU detects that
ABS cycling is necessary. The ECU continually
monitors the operation of the system.
When starting the ABS system will go through a series
of self checks, including a check of the solenoid
valves/pump motor.
WARNING LIGHT
The warning light will be on during the self check
(1.3-2 secs). If no faults are stored in the memory the
light goes out for 0.5sec,then comes on again. It will
then remain on until the vehicle is driven faster than 7
kph/5mph. This is the only time that the light will be on
without indicating a problem.
Any faults detected by the ECU will cause the warning
light to illuminate indicating that the vehicle may not
have full ABS control.

BRAKES
5
DESCRIPTION AND OPERATION BRAKE PIPE LAYOUT ABS
Left hand drive shown. On right hand drive vehicles the master cylinder and servo unit only move
position.
KEY
1. Connector double elbow
2. Rear left brake
3. Modulator to rear left brake
4. Modulator to master cylinder
5. Modulator to pressure reducing valve upper
6. Electrical connection
7. Modulator unit
8. Nut, mounting bush and cup
9. Modulator to front left brake
10. Front left brake
11. Electrical connection12. Modulator to front right brake
13. Modulator to pressure reducing valve lower
14. Master cylinder to pressure reducing valve lower
15. Front right brake
16. Bolt
17. Pressure reducing valve
18. Modulator to pressure reducing valve upper
19. Master cylinder
20. Servo unit
21. Modulator to rear right brake
22. Rear right brake

70BRAKES
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DESCRIPTION AND OPERATION BRAKE PIPE LAYOUT NON ABS
Left hand drive shown. On right hand drive vehicles the master cylinder and servo unit only move
position.
KEY
1. T-Connector
2. Rear left brake
3. Master cylinder to pressure reducing valve lower
4. Front left brake to pressure reducing valve upper
5. Front left brake
6. Front right brake
7. Master cylinder to pressure reducing valve lower8. Pressure reducing valve
9. Front right brake to pressure reducing valve
upper
10. Master cylinder
11. Servo unit
12. Pressure reducing valve upper to both rear
brakes
13. Rear right brake

BRAKES
7
DESCRIPTION AND OPERATION BRAKE SERVO UNIT
Description
The power assistance for the brake system is
provided by a twin diaphragm suspended vacuum
servo.
A 'Suspended Vacuum' servo is one where, with the
engine running vacuum is present both sides of the
diaphragm or diaphragms when the brakes are fully
released or partially applied. The principal of having
vacuum both sides of the diaphragm has two main
advantages. First, it ensures instantaneous servo
response to the brake being applied or released and
second, it provides very accurate control of the
position of the diaphragm and therefore, the degree of
partial brake application.At full pedal depression, vacuum is present in front of
the diaphragm with atmospheric pressure on the rear.
Operation
Brakes released
With the brakes released depression is present both
sides of the diaphragms, in chambers 1,2,3 and 4.
The chambers comunicate by ports A, B, D via the
valve C which is shown fully released in RR3640M.

70BRAKES
8
DESCRIPTION AND OPERATION Brakes applied
When the brake pedal is depressed the pedal pushrod
operates valve C situated in the plastic diaphragm hub
9 and 10. Inital depression of the the brake pedal,
push rod 6 and plunger 7 compress a rubber pad 8.
This slight movement causes valve C to close port D
sealing off the vacuum to chambers 2/4 and allows
atmospheric pressure to enter the chambers through
the air filter E.
With depression in chambers 1/3 and air pressure in
chambers 2/4 the servo hub will apply the brakes via
the master cylinder push rod 13.Atmospheric pressure over the large area of the
diaphragms multiples the force applied to the master
cylinder piston, to provide the power assistance. If the
brake pedal is only partially depressed, the servo hub,
diaphragms and master cylinder push rod will stop
moving, when valve C comes to rest on the plunger 7.
At this point the valve will balance the pressure in all
the chambers, to the applied effort at the pedal and
provide proportional braking.
It is only when the brakes are fully applied that the
valve does not balance the pressures in all the
chambers, but ensures that maximum available
depression is in chambers 1/3 and full atmospheric
pressure enters chambers 2/4.
When the brake pedal is released, the pressure in all
chambers is equalised and the servo is returned to the
rest position by spring 12.

BRAKES
9
DESCRIPTION AND OPERATION Petrol engine vacuum system
The vacuum necessary to operate the brake servo on
petrol engine vehicles is provided by the engine inlet
manifold. During normal operation of the brakes only a
small volume of air is drawn from the servo and into
the inlet manifold or plenum chamber. However any
excessive air leaks due to damage in the brake servo
system or manifold hose connection, will effect the
mixture strength and ECM fuelling calculations on
engines equipped with an MFI system.
Diesel engine vacuum system
As the diesel engine inlet system does not produce
depression sufficient to operate the brake servo, it is
necessary to install an engine driven vacuum pump.
During normal operation of the brakes only a small
volume of air is drawn from the servo by the vacuum
pump. However as the pump exhausts air into the
engine crankcase atmosphere, an air leak due to
damage in the brake servo system or pump hose
connection, may cause excessive crankcase
pressure.

BRAKES
1
FAULT DIAGNOSIS ABS FAULT DIAGNOSIS
If a fault has occurred, or has been identified by ECU
self diagnostic function and ABS warning light is
illuminated. The system and components must be
checked to locate and rectify fault, using Testbook
diagnostics.
NOTE: If warning lamp has indicated a
fault in system, and no fault code has been
stored in memory, cause of fault is:
a) Failure in electrical supply
b) Bad ECU ground
c) Faulty warning light relay
d) ECU not connected
Before commencing fault diagnosis procedure
following items must be checked:
1.Inspect all exposed cables for damage or
abrasion.
2.Check ground on ABS system.
3.Battery - state of charge.
4.Check hub end-float.
5.All ABS fuses and electrical connections.
Fault rectification
1.Complete harness should be replaced if faults
are found in wiring harness.
2.DO NOT use unspecified cables or connectors,
as this could jeopardise safe function of ABS.
3.DO NOT attempt to open sealed 35 way
connector to ECU.FAULT DIAGNOSIS PROCEDURE
NOTE: If ABS warning light illuminates due
to large sensor air gap, fault will be
retained by the ECU memory. Where wheel
sensors have been pushed fully home prior to
test, The ECU will indicate a fault that has been
rectified.
NOTE: After any steering adjustment,
bearing replacement/adjustment, brake
disc replacement: Check hub end-float and
sensor clearance.
RELAYS AND FUSES ABS
The location and identification of ABS electrical relays
are given in the Electrical Troubleshooting Manual
For location and identification of ABS electrical fuses.
See ELECTRICAL, Repair, Fuse Box - Interioror.
See ELECTRICAL, Repair, Fuse Box - Engine
Compartment

70BRAKES
2
FAULT DIAGNOSIS ABS FAULT AND REMEDY CHART
SYMPTOM POSSIBLE CAUSE CHECK REMEDY
ABS warning light on ABS electrical fault Check ABS electrical
circuit with Testbook
diagnostic equipmentChange component if
necessary
High sensor air gaps Push in sensors
Brake fluid warning light
ONFluid loss Check reservoir fluid level
and inspect system for
leaksRectify leakage, refill
reservoir
Reservoir fluid level
switch malfunctionCheck fluid level switch Change reservoir
cap/switch
Pedal travel increased,
foot pressure normalAir in system Rebleed brake system
Master cylinder
malfunctionChange master cylinder
Pedal can be moved
downwards under
constant pressureSeal leaking in master
cylinderInspect system for leaks Change master cylinder
Seal leaking in servo unit Change servo unit