Yamaha YP250A 1999 Manual

FOREWORD
ThisSupplementaryServiceManualhasbeenpreparedtointroducenewserviceanddatafortheYP250A
’99.ForcompleteserviceinformationproceduresitisnecessarytousethisSupplementaryServiceManual
together with the following manual.
YP250 SERVICE MANUAL: 4UC-AE1
YP250 (J) SUPPLEMENTARY SERVICE MANUAL: 4UC-AE2
YP250D SUPPLEMENTARY SERVICE MANUAL: 5DF-AE1
YP250A ’99
SUPPLEMENTARY
SERVICE MANUAL
1998 by Yamaha Motor Co., Ltd.
First Edition, July 1998
Any reproduction or unauthorized use
without the written permission of
Yamaha Motor Co., Ltd.
is expressly prohibited.

NOTE:
WARNING
CAUTION:
EB001000 NOTICE
This manual was produced by the Yamaha Motor Company primarily for use by Yamaha dealers and their
qualified mechanics. It is not possible to include all the knowledge of a mechanic in one manual, so it is
assumed that anyone who uses this book to perform maintenance and repairs on Yamaha scooter has a
basic understanding of the mechanical ideas and the procedures of scooter repair. Repairs attempted by
anyone without this knowledge are likely to render the scooter unsafe and unfit for use.
Yamaha Motor Company, Ltd. is continually striving to improve all its models. Modifications and significant
changesinspecificationsor procedureswillbeforwardedtoallauthorizedYamaha dealersandwillappear
in future editions of this manual where applicable.
Designs and specifications are subject to change without notice.
IMPORTANT INFORMATION
Particularly important information is distinguished in this manual by the following notations.
The Safety Alert Symbol means ATTENTION! BECOME ALERT! YOUR SAFETY
IS INVOLVED!
FailuretofollowWARNINGinstructionscouldresult insevereinjury ordeathtothe
scooter operator, a bystander or a person inspecting or repairing the scooter.
A CAUTION indicates special precautions that must be taken to avoid damage to
the scooter.
NOTE: A NOTE provides key information to make procedures easier or clearer.

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243
5
6
7
6
8
YP002000 HOW TO USE THIS MANUAL
MANUAL ORGANIZATION
This manual consists of chapters for the main categories of subjects. (See “Illustrated symbols”)
1st title 1 : This is the title of the chapter with its symbol on the upper right corner of each page.
2nd title 2 : Thistitle indicates the section of thechapterandonlyappearsonthefirst page of each sec-
tion. It is located in the upper left corner of the page.
3rd title 3 : This title indicates a sub-section that is followed by step-by-step procedures accompanied
by corresponding illustrations.
EXPLODED DIAGRAMS
Tohepsidentifypartsandclarifyproceduresteps,thereareexplodeddiagramsatstartofeachremovaland
disassembly section.
1. An easy-to-see exploded diagram 4 is provided for disassembly and assembly jobs.
2. Numbers 5 aregivenintheorderof jobsintheexplodeddiagram.Anumberthat isenclosedbyacircle
indicates a disassembly step.
3. Anexplanationofjobsandnotesispresentedinaneasy-to-readwaybytheuseofsymbolmarks 6 .The
meanings of the symbol marks are given on the next page.
4. A job instruction chart 7 accompanies the exploded diagram, providing the order of jobs, names of
parts, notes in jobs, etc.
5. Forjobsrequiringmoreinformation, the step-by-stepformatsupplements 8 aregiven in additiontothe
exploded diagram and the job instruction chart.

22
1
3
5
7
9
2
4
8
6
24 25
2321
19 2018
16 1715
1413
11 12
10
GEN
INFO SPEC
ENG
CARB
ELECCHAS
COOL
INSP
ADJ
TRBL
SHTG
EB003000
ILLUSTRATED SYMBOLS
Illustrated symbols 1 to 9 are designed as
thumb tabs to indicate the chapter’s number and
content.
1General information
2Specifications
3Periodic inspection and adjustment
4Engine
5Cooling system
6Carburetion
7Chassis
8Electrical
9Troubleshooting
Illustrated symbols 10 to 17 are used to identify
the specifications appearing in the text.
10 Possible to maintain with engine mounted
11 Filling fluid
12 Lubricant
13 Special tool
14 Tightening
15 Wear limit, clearance
16 Engine speed
17 Ω, V, A
Illustrated symbols 18 to 23 in the exploded dia-
gramsindicate the types of lubricants and lubrica-
tion points.
18 Apply engine oil
19 Apply gear oil
20 Apply molybdenum disulfide oil
21 Apply wheel bearing grease
22 Apply lightweight lithium-soap base grease
23 Apply molybdenum disulfide grease
Illustrated symbols 24 to 25 in the exploded dia-
grams indicate the where to apply locking agent
24 and when to install new parts 25 .
24 Apply locking agent (LOCTITE)
25 Use new one

CONTENTS
GENERAL INFORMATION 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ABS OUTLINE 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HU OPERATION TEST WHILE ABS IS WORKING 1. . . . . . . . . . . . .
YAMAHA ABS PROVIDES 3 MAIN ADVANTAGES 3. . . . . . . . . . . . .
SYSTEM LAYOUT 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TOTAL SYSTEM BLOCK DIAGRAM 4. . . . . . . . . . . . . . . . . . . . . . . . . .
COMPOSITION OF PARTS 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COMPOSITION AND OPERATION OF PARTS 5. . . . . . . . . . . . . . . .
SYSTEM OPERATION 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CAUTION FOR OPERATION 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SPECIAL TOOLS 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SPECIFICATIONS 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GENERAL SPECIFICATIONS 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MAINTENANCE SPECIFICATIONS 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ENGINE 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHASSIS 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ELECTRICAL 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CABLE ROUTING 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHASSIS 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ANTI-LOOK BRAKE SYSTEM 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
TERMS EXPLANATION 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BRAKE FORCE AND THE VEHICLE STABILITY 28. . . . . . . . . . . . . . .
WHEEL SLIP AND THE HYDRAULIC CONTROL 29. . . . . . . . . . . . . .
ABS OPERATION AND THE VEHICLE CONTROL 29. . . . . . . . . . . . .
ELECTRONICS ABS FEATURE 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ABS COMPONENTS FUNCTION 31. . . . . . . . . . . . . . . . . . . . . . . . . . . .
HYDRAULIC UNIT (HU) 34. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OTHER ABS MECHANISM COMPONENTS 35. . . . . . . . . . . . . . . . . . .
ABS TROUBLESHOOTING OUTLINE 37. . . . . . . . . . . . . . . . . . . . . . . .
ABS PARTS LOCATION CHART 39. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ABS CONNECTOR LOCATION CHART 40. . . . . . . . . . . . . . . . . . . . . .
ABS CIRCUIT DIAGRAM 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
BASIC INSTRUCTION FOR TROUBLESHOOTING 42. . . . . . . . . . . .
BASIC PROCESS FOR TROUBLESHOOTING 43. . . . . . . . . . . . . . . .
ABS TROUBLESHOOTING 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[A] MALFUNCTION CHECK BY THE ABS WARNING LIGHT 44. . . .
[B] DETAIL CHECK OF MALFUNCTION 44. . . . . . . . . . . . . . . . . . . . . .
[B-1] WARNING LIGHT DOES NOT GO ON 44. . . . . . . . . . . . . . . . . . .
[B-2] WARNING LIGHT REMAINS GOING ON 44. . . . . . . . . . . . . . . . .
[B-3] WARNING LIGHT KEEPS FLASHING 44. . . . . . . . . . . . . . . . . . .
[B-4] MALFUNCTION CHECK BY THE ABS SELF DIAGNOSIS
(PAST MALFUNCTION) 45. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[B-5] MALFUNCTION CHECK BY THE ABS SELF DIAGNOSIS
(PRESENT MALFUNCTION) 46. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[C] SUPPOSING THE MALFUNCTION CAUSE AND POSITION 47.

[C-1] ONLY ABS WARNING LIGHT DOES NOT GO ON WHEN
THE MAIN SWITCH IS TURNED ON 47. . . . . . . . . . . . . . . . . . . . . . . . .
[C-2] ABS WARNING LIGHT AND ALL OTHER INDICATORS
DO NOT GOES ON 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[C-3] ABS WARNING LIGHT KEEPS FLASHING 48. . . . . . . . . . . . . . .
[C-4] ABS WARNING LIGHT FLASHES EVERY 0.5 SECOND 49. . .
[C-5] DIAGNOSIS BY THE MALFUNCTION CODE 50. . . . . . . . . . . . .
ECU AND FAIL SAFE RELAY 60. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-1] MAINTENANCE FOR ECU 61. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-2] MAINTENANCE OF THE ABS FAIL-SAFE RELAY 62. . . . . . . . .
FRONT WHEEL SENSOR AND SENSOR ROTOR 64. . . . . . . . . . . . .
[D-3] MAINTENANCE OF THE FRONT WHEEL SENSOR AND
SENSOR ROTOR 65. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REAR WHEEL SENSOR AND SENSOR ROTOR 69. . . . . . . . . . . . . .
[D-4] MAINTENANCE OF THE REAR WHEEL SENSOR AND
SENSOR ROTOR 70. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HYDRAULIC UNIT (HU) 72. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-5] MAINTENANCE OF THE HYDRAULIC UNIT (HU) 73. . . . . . . . .
AIR BLEEDING 78. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CHECK THE OPERATION OF HYDRAULIC UNIT
ACCORDING TO THE DECOMPRESSION 78. . . . . . . . . . . . . . . . . . .
[D-6] FINAL CHECK 78. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-6-1] CHECKING THE RESERVOIR TANK FLUID LEVEL 79. . . . .
[D-6-2] RECHECKING THE ASSEMBLY STATUS OF
WHEEL SENSORS 79. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-6-3] HU OPERATION TEST 2 80. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[D-6-4] DELETING THE MALFUNCTION CODE 82. . . . . . . . . . . . . . .
[D-6-5] TRIAL RUN 83. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REAR SHOCK ABSORBER AND SWING ARM 84. . . . . . . . . . . . . . . . . . .
YP250A ’99 WIRING DIAGRAM

1
ABS OUTLINE GEN
INFO
NOTE:
GENERAL INFORMATION
ABS OUTLINE
Introduction
YP250A is equipped with the ABS (Anti-lock Brake System).
HU OPERATION TEST WHILE ABS IS WORKING
With the scooter stopped, YP250A allows a driver to test the performance of brake lever reaction force
which is generated when the ABS operates.
There are two methods for the HU operation test, but this section describes only about the HU operation test 1.
HU operation test 1: Equal reaction force to the time of ABS operation is generated on the lever.
HU operation test 2: Mainly performs when checking the function after the ABS system has been disas-
sembled, adjusted and serviced. Refer to [D-6-3] HU operation test 2.
HU operation test 1
1. Turn the main switch off.
2. Remove the protective cap and insert the test
coupler adapter 1 into the test connector 2.
Test coupler adapter
90890-03149
To access the testcoupler,remove the lower pan-
el of windscreen.
3. Turn the engine stop switch 1 off.

2
ABS OUTLINE GEN
INFO
NOTE:
CAUTION:
4. Turn the main switch on.
WaituntiltheABSwarninglightgoesoffafterturn-
ing the main switch on.(2 seconds)
5. Turn on the starter switch 1 for longer than 4
seconds.
Do not grip the brake lever in this case.
6. After releasing the starter switch (OFF), apply
the front brake lever.
Reaction force is generated on the front brake
lever 0.5 second later and lasts about 2 se-
conds.
7. If the reaction force of front brake lever is lost,
apply the rear brake lever while applying the
front brake lever.
Reaction force is generated on the front brake
lever 0.5 second later and lasts about 2 se-
conds.
8. Turn the main switch off.
9. Remove the test coupler adapter from the test
connector.
10. Turn the main switch on.
11. Set the engine stop switch to RUN position.

3
ABS OUTLINE GEN
INFO
1ABS warning light
2Front/Rear master cylinders
Front/Rear brake levers
Front/Rear brake switches
3Fail-safe relay
4Brake light relay
5Rear brake caliper
6Rear wheel sensor
7Electronic control unit
8Rear disc rotor
9Hydraulic unit
10 Front brake caliper
11 Front wheel sensor
12 Front disc rotor
YAMAHA ABS PROVIDES 3 MAIN ADVANTAGES
1. Securely controls the wheel locking at urgent braking under various conditions of road surface and
weather, and gives full play to tires’ original performance.
For higher reliability, the electronic control unit (ECU) is composed of the main and sub processors and
equipped with the high level self diagnostic function.
2. Smooth braking can be also performed when the ABS system is activated. Change of chassis behavior
such as pitching is few and this system will not damage the rider’s comfortable drive feeling.
When the ABS works, the system informs the brake fluid pressure change to a rider via brake levers.
3. Weightofthehydraulicunit(HU)whichisthemainofsystemis2.2kgforYAMAHAABS.Wedesignedto
gather the mass by locating this unit at the central portion of scooter. Since the wheel assembly (wheel
sensor) also has extremely lightweight structure, the vehicle’s weight gives no damage to the mobility
which YP250A provides such as an excellent safety operation.
SYSTEM LAYOUT

4
ABS OUTLINE GEN
INFO
1Rear master cylinder
2Hydraulic unit
3Silencer chamber
4Hydraulic pump
5Electric motor
6Buffer chamber
7Hydraulic control valve
8Front master cylinder
9Rear brake caliper
10 Front brake caliper
11 Electronic control unit
12 Rear wheel sensor
13 Front wheel sensor
14 ABS warning light
TOTAL SYSTEM BLOCK DIAGRAM
COMPOSITION OF PARTS
Component Function
Wheel sensor Detects each wheel rotation pulse of front and rear wheels
and inputs them to the electronic control unit.
ABS warning light Warns the ABS system failure to the rider.
Fail-safe relay Supplies the power to the solenoid valve and motor of hy-
draulic unit.
Hydraulic unit (HU) Controlsthebrake fluid pressureoffront and rearwheelsac-
cording to the output signal sent from the electronic control
unit.
Electronic control unit (ECU)
Outputstheoperationsignaltothehydraulicunitsothatthe
unit controls matching the road conditions according to the
wheel rotation pulses transmitted from front and rear wheel
sensors.
In case of ABS system failure, the unit activates the ABS
warning light and shuts off the power to hydraulic unit and
deactivates the ABS operation.

5
ABS OUTLINE GEN
INFO
CAUTION:
COMPOSITION AND OPERATION OF PARTS
1. Wheel sensor and sensor ring
A wheel sensor is composed of magnet and coil
andinstalledwithfrontandrearwheels’housings.
The sensor ring attached to front and rear cast
wheels have 36 serration each. The front end of
wheel sensor is close to the serration.
When the sensor rotor with serration rotates, the
flux generation from the wheel sensor magnet
changesto generate the alternatingvoltagein the
coil. The electronic control unit calculates the
wheel speed by detecting the repeating times per
unit period since this alternating voltage changes
the frequency proportioning to the wheel rotation
speed.
1At high speed
2At low speed
3Wheel sensor
4Sensor rotor
2. ABS warning light
Ifthe failure occurred in theABS system, theABS
warning light alerts the rider by going on the ABS
warning light.
When the main switch is turned on, the system
goes on the light for about 2 seconds to check the
disconnection of ABS warning light and system
function, then it goes off.
It goes on while the starter switch is pushed to
check the disconnection.
Iftherearwheelisracedbyliftingwiththecen-
terstand or other means, the system may
judgeas the malfunction to flashes or goes on
the ABS warning light.
In this case, turn the main switch off once and
restart the engine.
The system is normal if the ABS warning light
goes on and off in about 2 seconds.
1ABS warning light

6
ABS OUTLINE GEN
INFO
1To rear brake master cylinder
2Silencer chamber
3Hydraulic pump
4Electric motor
5Buffer chamber
6Hydraulic control valve
7To front brake master cylinder
8To the rear brake caliper
9To the electronic control unit
10 To the front brake caliper
3. Hydraulic unit
Hydraulic unit is composed of two pieces of hy-
draulic control valve (solenoid valve, flow control
valve), buffer chamber, pump and silencer cham-
ber, and one motor. It adjusts the front/rear wheel
brake fluid pressure to control the wheel rotation
condition according to the signal sent from the
electronic control unit.

7
ABS OUTLINE GEN
INFO
Hydraulic control valve
Hydraulic control valve is composed of the sole-
noid valve and flow control valve.
1) Whenthenormalbrakeisapplied,thesolenoid
valve closes 2 , the spool 3 of flow control
valve does not move and the hydraulic line be-
tween the master cylinder and brake caliper is
opened.
2) While the ABS works, the solenoid valve 2 is
suppliedthepowertoopenwhentheelectronic
control unit signals the pressure reduction and
the spool 3 of flow control valve is moved to-
ward to the solenoid valve.
3) When the electronic control unit stops the sig-
nal of pressure reduction, the solenoid valve
2 closes and pressurizing starts again. Re-
pressurizing while the ABS works controls the
flowofbrakefluidbythe movementofspool 3
of flow control valve, and provides slow pres-
sure rising.
1Orifice
2Solenoid valve
3Spool
4Flow control valve
Buffer chamber
Accumulatesthedecompressedbrakefluidtem-
porarily while the ABS works.
1Buffer chamber (in normal state)
2Buffer chamber (decompressing)
3Raise the piston

8
ABS OUTLINE GEN
INFO
Front brake
switch signal
Power
Rear brake
switch signal
Stop lamp
signal
Starter
switch
signal
Starter switch
Front
wheel sensor
Rear
wheel sensor
Voltage
regulator
Front brake
switch
circuit
Rear brake
switch
circuit
Brake
light circuit
Starter
switch
circuit
Digital
conversion
circuit
Main CPU
(Main microcomputer)
Sub CPU
(Sub microcomputer)
Memory
Electronic control unit (ECU)
Motor monitor
circuit
Solenoid
motor
Solenoid drive
circuit
Fail-safe relay
drive circuit
Motor relay
drive circuit
Warning light
drive circuit
Operation mode
switching circuit
Fault code
output circuit
Solenoid valve Motor
Hydraulic
unit (HU)
Solenoid Motor relay Fail-safe
relay
ABS warning light
Checker
Speedometer
Front wheel speed
output circuit
4. Electronic control unit
Relation of the input signal and output signal is shown in the diagram.
The circuit receives the signals from wheel sensors, processes them according to the programs in the mi-
crocomputer and outputs the control signal to the hydraulic unit and ABS warning light.

9
ABS OUTLINE GEN
INFO
Composition and operation
Fail-safe relay is composed of the solenoid relay and motor relay .
Solenoidrelayiscontinuedwhentheelectroniccontrolunitoutputsthesystemstartsignal.Accordingly,the
solenoid valve is activated when the electronic control unit signals the decompression.
In case of malfunction, the solenoid relay is turned off and the solenoid valve is deactivated.
Thismakesitimpossibletoreducethehydraulicpressureandresumesnormal braking. Motor relay is also
continued by the signal of electronic control unit and starts the motor simultaneously when the ABS pres-
sure reduction begins.
When the solenoid is turned off, the motor relay is also turned off, therefore the motor stops its operation in
case of malfunction.
1 2
3
11 Power
12 Fail-safe relay
13 Hydraulic unit
1Solenoid relay
2Motor relay
3Solenoid valve
4Electric control unit
5Pomp motor relay coil
6Pump motor monitor
7ABS warning light
8Fail-safe relay coil
9Front solenoid
10 Rear solenoid
5. Fail-safe relay
Fail-saferelaycontrolsthepowerofhydraulicunit
and it is located inside of the tail cover.
1Fail safe relay

10
1Brake master
cylinder
2Brake switch
3Silencer chamber
4Motor
5Pump
6Buffer chamber
7Flow control valve
8Port A
9Spool
10 Port B
11 Orifice
12 Port D
13 Solenoid valve
14 Port C
15 Brake caliper
16 Electronic control
unit
17 ABS warning light
18 Brake fluid pressure
19 Time
20 Repressurizing
ABS OUTLINE GEN
INFO
SYSTEM OPERATION
ABSsystem’sfluidpressurecircuithasthe2linesoffrontwheelandrearwheelsystems.Inthissection,we
describe about one system of a front wheel.
Normal brake (ABS not activated)
PortD 12 ofthesolenoidvalveisclosedsincethecontrolsignalfromtheelectroniccontrolunitisnotsentto
the hydraulic unit. A 8 and B 10 ports of flow control valve spool are opened.
Accordingly, when arider apply thebrake lever and the mastercylinder hydraulic pressurerises, the brake
fluid is directed in the line from Port A 8 to Port B 10 then to brake caliper.
Atthistime,inletandoutletcheckvalvesofpumpclosethelinesandbrakefluidisnotsent.Therefore,mas-
ter cylinder directly pressurizes the brake caliper while braking normally.
Whenthebrakeleverisreleasednext,thebrakefluidinthebrakecaliperreturnsthentothemastercylinder
via Port B 10 , Port A 8.

11
1Brake master
cylinder
2Brake switch
3Silencer chamber
4Motor
5Pump
6Buffer chamber
7Flow control valve
8Port A
9Spool
10 Port B
11 Orifice
12 Port D
13 Solenoid valve
14 Port C
15 Brake caliper
16 Electronic control
unit
17 ABS warning light
18 Brake fluid pressure
19 Time
20 Repressurizing
ABS OUTLINE GEN
INFO
At the time of urgent braking (When ABS works)
1) Decompressed state
When the wheel is almost locked, the solenoid valve’s port D 12 opens by the “decompression” signal
from the electronic control unit. At this time, the spool 9 of flow control valve compresses the return
valve to close the port B 10 .
Brake fluid which has entered through port A 8 is restricted by the orifice 11 .
Accordingly, the brake fluid from the brake caliper is sent in the line of Port C 14 , Port D 12 then to the
buffer chamber 6.
The brake caliper fluid pressure is now reduced.
Brake fluid which has been stored in the buffer chamber is pumped by the fluid pressure pump inter-
locked with motor and returned to the master cylinder.

12
1Brake master
cylinder
2Brake switch
3Silencer chamber
4Motor
5Pump
6Buffer chamber
7Flow control valve
8Port A
9Spool
10 Port B
11 Orifice
12 Port D
13 Solenoid valve
14 Port C
15 Brake caliper
16 Electronic control
unit
17 ABS warning light
18 Brake fluid pressure
19 Time
20 Repressurizing
21 Chamber A
22 Chamber B
ABS OUTLINE GEN
INFO
2) Pressurized state
PortD 12 ofsolenoidvalveclosesaccordingtothe“increasepressure”signalfromtheelectroniccontrol
unit. Also, spool 9 of flow control valve compresses the return spring to close the port B 10 .
Brake fluid entered through port A 8 is further restricted by the orifice 11 .
Accordingly, the brake fluid in master cylinder is sent in the line of PortA 8 , Port C 14 then to the brake
caliper. However, the brake is pressurized at a constant speed regardless of the brake fluid pressure
levelsincethe port A’s 8 restrictionchangesso that the constant pressure differenceismaintainedbe-
tween A 21 and B 22 chambers of flow control valve.