DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
2 Using a suitable vacuum gauge, check the
vacuum pressure.
• Is the vacuum pressure above 40.5 kPa (0.4 bar) with the brake booster non-operational?
zYe s
VERIFY the customer concern.
zNoINSTALL a new brake booster check valve
vacuum hose. TEST the system for normal
operation.
PINPOINT TEST C : BRAKE LOCK UP DURING LIGHT BRAKE PEDAL FORCE
DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
C1: CHECK THE BRAKE BOOSTER
1 Check the brake booster push rod alignment
and pedal travel.
• Is the push rod and pedal travel OK?
zYe s TEST the brake pedal application. GO to
Pinpoint Test D.
zNo
INSTALL a new brake booster. TEST the
system for normal operation.
PINPOINT TEST D : EXCESSIVE BRAKE PEDAL EFFORT
DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
D1: CHECK BRAKE APPLICATION
1 With the engine off, apply and release the brake
pedal five times to deplete all vacuum from the
brake booster. Apply the brake pedal, hold with
light pressure. Start the engine.
• Does the brake pedal hold?
zYe s GO to D2 .
zNo
GO to D3 .
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DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
D2: CHECK THE BRAKE BOOSTER FOR LEAKS
1 Run the engine at approximately 1000 rpm,
release the accelerator pedal and turn the
engine off. Wait 90 seconds and apply the
brakes. Two or more brake applications should
be power assisted.
• Does the brake booster work?
zYe s VERIFY the customer concern.
zNoGO to D4 .
D3: CHECK THE BRAKE PEDAL LINKAGE
1 Disconnect the actuator rod from the pedal pin
and fully depress the brake pedal.
• Did the pedal move freely?
zYe s VERIFY the customer concern.
zNoINSTALL new brake pedal bushings. TEST
the system for normal operation.
D4: CHECK THE BRAKE BOOSTER CHECK VALVE
1 Disconnect the brake booster check valve
vacuum hose at the manifold.
2Blow into the hose attached to the brake booster
check valve.
• Does air pass through the valve?
zYe s INSTALL a new brake booster check valve.
TEST the system for normal operation.
zNoGO to D5 .
D5: CHECK THE BRAKE BOOSTER CHECK VALVE VACUUM
1 Run the engine at idle.
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DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
2 Using a suitable vacuum gauge, check the
vacuum pressure.
• Is the vacuum pressure above 40.5 kPa (0.4 bar) with the brake booster non-operational?
zYe s
GO to D6 .
zNo
INSTALL a new vacuum hose and fittings.
TEST the system for normal operation.
D6: CHECK THE BRAKE BOOSTER
1 Check the brake booster. REFER to the Brake
Booster Operation Check in this procedure.
• Is the brake booster OK?
zYe s VERIFY the customer concern.
zNoINSTALL a new brake booster. TEST the
system for normal operation.
PINPOINT TEST E : BRAKE NOISE
DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
E1: CHECK FOR PEDAL NOISE
1 Run the engine at idle for 10 seconds or longer.
2Apply the brake pedal and listen for noise.
3Compare results with a known good system.
• Was a noise present?
zYe s GO to E2.
zNoVERIFY the customer concern.
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DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
E2: CHECK THE BRAKE BOOSTER
1 Check the brake booster push rod alignment
and travel.
• Is the push rod and pedal travel OK?
zYe s BLEED the brake system. REFER to: (206-00
Brake System - General Information)
Brake System Bleeding (General Procedures),
Brake System Pressure Bleeding (General
Procedures),
Component Bleeding (General Procedures).
TEST the system for normal operation.
zNo
INSTALL a new brake booster. TEST the
system for normal operation.
PINPOINT TEST F : SLOW OR INCOMPLETE BRAKE PEDAL RETURN
DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
F1: CHECK FOR BRAKE PEDAL RETURN
1 Run the engine at approximately 1200 rpm
whilst making several brake applications.
2Pull the brake pedal upwards with approximately
44.5 N (10 lbs) force.
3Release the brake pedal and measure the
distance to the floor panel and note the reading.
4Make a hard brake application.
5Release the brake pedal and measure the
distance to the floor panel and note the reading.
6Compare the measurements.
• Did the brake pedal return to its original posi- tion?
zYe s
VERIFY the customer concern.
zNoGO to F2.
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DETAILS/RESULTS/ACTIONS
TEST CONDITIONS
F2: CHECK FOR BRAKE PEDAL BINDING
1 Check the brake pedal for free operation.
• Did the brake pedal operate freely?
zYe s INSTALL a new brake booster. TEST the
system for normal operation.
zNoINSTALL new brake pedal bushings. TEST
the system for normal operation.
Component Tests
Hydraulic Leak Check
NOTE: There is a common clutch and brake fluid
reservoir, therefore it is possible that a clutch leak
can lead to reduction in the reservoir level.
It is possible that all evidence of fluid leakage may
have washed off if the vehicle has been operated
in rain or snow, as brake fluid is water-soluble.
Refill the system, bleed then apply the brakes
several times. Examine the system to verify that
the reservoir fluid level is actually dropping. Locate
and repair the external leak. If the fluid level drops
and no external leak can be found, check for a
brake master cylinder bore end seal leak.
Brake System Check
Brake Pedal Reserve Check
Where a low brake pedal or the feel of a
bottomed-out condition exists, check for brake
pedal reserve.
1. Operate the engine at idle with the transaxle in the NEUTRAL position.
2. Apply the brake pedal lightly three or four times.
3. Allow 15 seconds for the vacuum to replenish the brake booster.
NOTE: This increased resistance may feel like
something has bottomed out.
4. Apply the brake pedal until it stops moving downward or an increased resistance to the
pedal travel occurs. 5. Hold the brake pedal in the applied position and
raise the engine speed to approximately 2000
rpm.
NOTE: The additional movement of the brake pedal
is the result of the increased engine manifold
vacuum which exerts more force on the brake
booster during engine rundown. This means that
additional stroke is available in the brake master
cylinder and the brake system is not bottoming out.
6. Release the accelerator pedal and observe that the brake pedal moves downward as the engine
returns to idle speed.
Brake Booster Functional Test
Inspect all hoses and connections. All unused
vacuum connectors should be capped. Hoses and
their connections should be correctly secured and
in good condition with no holes and no collapsed
areas. Inspect the check valve on the brake booster
for damage.
Brake Booster Operation Check
1. Check the hydraulic brake system for leaks orlow fluid.
2. With the transaxle in the NEUTRAL position, stop the engine and apply the parking brake.
Apply the brake pedal several times to exhaust
all the vacuum in the system.
3. With the engine turned off and the vacuum in the system exhausted, apply the brake pedal
and hold it down. Start the engine. If the vacuum
system is operating, the brake pedal will tend
to move downward under constant foot
pressure. If no motion is felt, the vacuum
booster system is not functioning.
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4. Remove the vacuum hose from the brakebooster. Manifold vacuum should be available
at the brake booster end of the hose with the
engine at idle speed and the transaxle in the
NEUTRAL position. Make sure that all unused
vacuum outlets are correctly capped, hose
connectors are correctly secured and vacuum
hoses are in good condition. When it is
established that manifold vacuum is available
to the brake booster, connect the vacuum hose
to the brake booster and repeat Step 3. If no
downward movement of the brake pedal is felt,
install a new brake booster.
5. Operate the engine a minimum of 10 seconds at approximately 1200 rpm. Stop the engine and
let the vehicle stand for 10 minutes. Then, apply
the brake pedal with approximately 89 N (20 lb)
force. The pedal feel (brake application) should
be the same as that noted with the engine
operating. If the brake pedal feels hard (no
power assist), install a new vacuum check valve
and then repeat the test. If the brake pedal still
feels hard, install a new brake booster. If the
brake pedal movement feels spongy, bleed the
brake system. REFER to: (206-00 Brake System
- General Information)
Brake System Bleeding (General Procedures),
Brake System Pressure Bleeding (General
Procedures),
Component Bleeding (General Procedures).
Brake Master Cylinder
Usually, the first and strongest indicator of anything
wrong with the braking system is a feeling through
the brake pedal. In diagnosing the condition of the
brake master cylinder, check pedal feel as
evidence of a brake concern. Check for the red
brake warning indicator illumination and the fluid
level in the master cylinder reservoir.
Normal Conditions
The following conditions are considered normal
and are not indications that the brake master
cylinder is in need of service.
– Modern brake systems are not designed to
produce as hard a pedal effort as in the past.
Complaints of light pedal efforts should be compared to pedal efforts of another vehicle, of
the same model and year.
– During normal operation of the brake pedal, the fluid level in the reservoir will rise during brake
pedal application and fall during release. The
net fluid level (i.e., after brake pedal application
and release) will remain unchanged.
– A trace of brake fluid will exists on the brake booster shell below the master cylinder
mounting flange. This results from the normal
lubricating action of the master cylinder bore
end seal.
– The fluid level will fall with brake shoe and lining wear.
Abnormal Conditions
NOTE: Prior to performing any diagnosis, make
sure the brake system warning indicator is
functional.
Changes in brake pedal feel or travel are indicators
that something could be wrong with the braking
system. The diagnostic procedure and techniques
use brake pedal feel, warning indicator illumination
and low brake fluid level as indicators in diagnosing
braking system concerns. The following conditions
are considered abnormal and indicate that the
brake master cylinder is in need of service.
– The brake pedal goes down fast. This could be caused by an external or internal leak.
– The brake pedal eases down slowly. This could be caused by an external or internal leak.
– The brake pedal is low and or feels spongy. This condition may be caused by no fluid in the brake
master cylinder reservoir, reservoir cap vent
holes clogged or air in the hydraulic system.
– The brake pedal effort is excessive. This may be caused by a bind or obstruction in the pedal
or linkage, clogged fluid control valve or
insufficient booster vacuum.
– The rear brakes lock up during light pedal force. This may be caused by incorrect tire pressures,
grease or fluid on the brake shoes and linings,
damaged brake shoes and linings, incorrectly
adjusted parking brake, or damaged or
contaminated brake pressure control valves.
– The brake pedal effort is erratic. This condition could be caused by a brake booster malfunction,
extreme caliper piston knock back or incorrectly
installed brake shoes and linings.
– The red brake warning indicator is ON. This may be caused by low fluid level, ignition wire routing
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Anti-Lock Control - Stability Assist – System Operation andComponent Description
System Diagram
E82694
M
P
UP
U
P
U
P
U
P
U
1
10
22
23
456
7
8
9
11
1213
1415
1617
18
2324
13
11
10
19
2526
2120
181818
18
Description
Item
Brake booster
1
Primary hydraulic circuit
2
Secondary hydraulic circuit
3
Pulsation damper
4Description
Item
Pulsation damper
5
engine control module
6
Damping chamber
7
Dual circuit hydraulic pump
8
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Description
Item
Battery
1
Battery junction box (BJB) in the engine
compartment
2
Generic electronic module (GEM)
3
Powertrain Control Module (PCM)
4
Instrument cluster
5
Data link connector (DLC)
6
Steering Wheel Rotation Sensor
7
ABS/ESP module or hydraulic control unit
(HCU)
8Description
Item
Combined yaw rate sensor and lateral
acceleration sensor / longitudinal
acceleration sensor
9
Front wheel sensor
10
Rear wheel sensor
11
Rear wheel sensor
12
Front wheel sensor
13
ESP switch
14
Rear brakes
15
Front brake
16
The ABS monitors the different wheel speeds of
the vehicle with the aid of wheel speed sensors.
Using the data from all of the wheel speed sensors,
the ABS module calculates the so-called reference
speed, which is a measure of the actual road
speed. The ABS module compares the individual
circumferential wheel speeds with this reference
speed when the driver initiates braking. If one or
more of the circumferential wheel speeds deviates
too far from the reference speed, this means that
slip at the affected wheels is so great that steering
stability of the vehicle is no longer ensured. The
ABS module actuates electro-mechanical valves
which influence the brake pressure at the relevant
wheels.
Like the traction control system (TCS), the ESP
system uses a large proportion of the ABS
components. In addition, there are sensors which
pick up the steering angle, the acceleration forces
acting on the vehicle and the yaw rate or yaw
moment. The sensors transmit these signals to the
combined ABS/ESP module. Using the wheel
speed and steering angle data, the ABS/ESP
module calculates the direction of travel planned
by the driver and determines the corresponding
speed-dependent lateral acceleration and yaw
moment. These values are compared with those
actual measured. If the actual lateral acceleration
and the yaw moment deviate excessively from the
target values (unstable driving characteristics), the
ABS/ESP module actuates individual brakes
selectively via the HCU (hydraulic control unit). In
addition, the engine speed is reduced by
intervention in the engine management system.
How the system works for understeer: In the
event of understeer, brake intervention occurs at
the wheels on the inside of the curve. The rear
wheel is braked heavily, so that a high amount of slip is caused. In this way, the cornering force of
the rear axle is heavily reduced and the centrifugal
force that now becomes effective turns the rear of
the vehicle back into the curve. The front wheel is
not braked as hard. The braking force that is
transmitted via the front wheel to the road surface
generates a torque with the aid of the lever arm
(vertical tire force to the vehicle's centre of gravity),
which supports the yaw moment of the vehicle.
Both measures together result in the vehicle
reverting back to the curved path intended by the
driver.
How the system works for oversteer:
In the
event of oversteer the wheels on the outside of the
curve are braked. This time, the front wheel is
subjected to a high level of slip so that the
cornering force at the front axle is reduced. The
rear wheel is not braked as heavily and, together
with the effective lever arm, results in a reduction
in the vehicle yaw moment. Both measures
together result in the vehicle being stabilized and
reverting back to the curved path intended by the
driver.
If ESP control occurs, possible ABS interventions
will be overridden as the ESP works at higher slip
rates than the ABS.
Emergency brake assist (EBA): The emergency
brake assist helps drivers in emergency braking
situations by automatically applying the brakes with
the maximum possible braking force.
If the brake pedal is pressed very suddenly, the
ABS module increases the hydraulic pressure to
all of the brakes until the threshold for ABS
intervention is reached. This applies the maximum
braking effort for the available traction. The ABS
control unit monitors inputs from the brake pedal
switch and from the pressure sensor within the
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