Service Manuals, User Guides, Schematic Diagrams or docs for : . Car Manuals Subaru Impreza 2001-2002.2004-2007 Approved Subaru Impreza 2004 WRX STi 08. MECHANISM AND FUNCTION 43. ABS 01. Anti-lock Brake System (ABS)
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W1860BE.book Page 2 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
1. Anti-lock Brake System (ABS)
A: FEATURE
The 5.3i type ABS used in the Impreza has a hydraulic control unit, an ABS control module, a
valve relay and a motor relay integrated into a single unit (called "ABSCM & H/U") for circuit simplic-
ity and reduced weight.
The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from
locking during braking on slippery road surfaces, thereby enabling the driver to maintain the direc-
tional control.
If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking
performance as with a conventional brake system. In that case, the warning light comes on to indi-
cate that the ABS is malfunctioning.
The ABS is a 4-sensor, 4-channel system. The front wheel system is an independent control de-
sign*1, while the rear wheel system is a select-low control design*2.
Vehicles with Sports ABS have an additional function which independently controls*1 the rear
wheels according to the value sent from the lateral G sensor. (STi models)
*1: A system which controls the left and right wheel brakes individually.
*2: A system which applies the same fluid pressure to both wheels if either of the left or right wheels
starts to lock.The pressure is determined based on the lower of the frictional coefficients of both
wheels.
ABS-2
W1860BE.book Page 3 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
(9)
(7)
(4)
(13)
(6)
(5)
(3)
(2)
(1)
(9)
(7)
(8)
(8)
(14)
(10)
(8) (11)
(12)
(7) (9)
(9)
(7)
(8)
ABS00329
(1) ABS control module and hydraulic (6) Transmission control module (AT (11) Stop light switch
control unit (ABSCM & H/U) models only)
(2) Two-way connector (7) Tone wheels (12) Master cylinder
(3) Diagnosis connector (8) ABS wheel speed sensor (13) Brake & EBD warning light
(4) ABS warning light (9) Wheel cylinder (14) Lateral G sensor (STi)
(5) Data link connector (for SUBARU (10) G sensor
select monitor)
ABS-3
W1860BE.book Page 4 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
B: FUNCTIONS OF SENSORS AND ACTUATORS
Name Function
ABS control module and ABSCM section It determines the conditions of the wheels and the vehicle body from the
hydraulic control unit (AB- wheel speed data and controls the hydraulic unit depending on the result.
SCM & H/U) When the ABS is active, the ABSCM provides the automatic transmission
control module with control signals which are used by the module for
cooperative control of the vehicle with the ABSCM.
Whenever the ignition switch is placed at ON, the module performs a self
diagnosis sequence. If anything wrong is detected, the module cuts off the
system.
It communicates with the SUBARU select monitor.
H/U section When the ABS is active, the H/U changes fluid passages to the wheel
cylinders in response to commands from the ABSCM.
It constitutes the brake fluid passage from the master cylinder to the wheel
cylinders together with the piping.
Valve relay section It serves as a power switch for the solenoid valves and motor relay coil. It op-
erates in response to a command from the ABSCM.
Motor relay section It serves as a power switch for the pump motor. It operates in response to a
command from the ABSCM.
ABS wheel speed sensors These sensors detect the wheel speed in terms of a change in the density of
the magnetic flux passing through them and convert it into an electrical sig-
nal. The electrical signal is sent to the ABSCM.
Tone wheels They give a change in the magnetic flux density by the teeth around them-
selves to let the ABS wheel speed sensors generate electrical signals.
G sensor It detects a change in acceleration in the longitudinal direction of the vehicle
and outputs it to the ABSCM as a voltage signal.
Lateral G sensor (STi) Converts the variation in lateral G on the vehicle into a change in capacity of
the sensor condenser, and outputs a varying voltage to the ABS ECU.
Stop light signal It provides information on whether the brake pedal is depressed or not to the
ABSCM. The ABSCM uses it to determine ABS operation.
ABS warning light It alerts the driver to an ABS fault. When the diagnosis connector and diag-
nosis terminal are connected, the light flashes to indicate a diagnostic trouble
code stored in the ABSCM.
Automatic transmission control module It provides gear controls (fixing the speed at 3rd or changing power transmis-
sion to front and rear wheels) in response to control signals from the ABSCM.
Brake warning light It alerts the driver to an EBD fault. This warning light is also used for parking
brake warning and brake fluid level warning.
ABS-4
W1860BE.book Page 5 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
(1)
(14)
(3)
(15)
(26)
(16)
(25)
(4)
(17)
(27)
(5)
(29) (28)
(2)
(18)
(19)
(20)
(30)
(6)
(31) (7)
(8)
(9)
(10)
(21) (11)
(22)
(12)
(23) (13)
(24)
ABS00334
ABS-5
W1860BE.book Page 6 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
(1) ABS control module and hydraulic (12) Rear right inlet solenoid valve (23) Rear left ABS wheel speed sensor
control unit
(2) ABS control module section (13) Rear right outlet solenoid valve (24) Rear right ABS wheel speed sen-
sor
(3) Valve relay (14) Automatic transmission control (25) IGN
module
(4) Motor relay (15) Diagnosis connector (26) Battery
(5) Motor (16) Data link connector (27) Brake warning light
(6) Front left inlet solenoid valve (17) ABS warning light (28) Parking brake warning light
(7) Front left outlet solenoid valve (18) Stop light switch (29) Brake fluid level switch
(8) Front right inlet solenoid valve (19) Stop light (30) Lateral G sensor (STi)
(9) Front right outlet solenoid valve (20) G sensor (31) Driver-controllable center differen-
tial control unit
(10) Rear left inlet solenoid valve (21) Front left ABS wheel speed sensor
(11) Rear left outlet solenoid valve (22) Front right ABS wheel speed sen-
sor
ABS-6
W1860BE.book Page 7 Tuesday, January 28, 2003 11:01 PM
ANTI-LOCK BRAKE SYSTEM (ABS)
ABS
C: PRINCIPLE OF ABS CONTROL
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle
speed does as well. The decrease in the vehicle speed, however, is not always proportional to the
decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle
speed is called "slip" and the magnitude of the slip is expressed by the "slip ratio" which is defined
as follows:
Slip ratio = (Vehicle speed ◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo