ISUZU TF SERIES 2004 Workshop Manual

BRAKE CONTROL SYSTEM 5A-5

General Description
EHCU has controlled ABS (Anti-lock Brake System)
and EBD (Electronic Brake-force Distribution System).
ABS works on all four wheels. EBD system works on
rear 2 wheels. A combination of wheel speed sensor
and Electronic Hydraulic Control Unit (EHCU) can
determine when a wheel is about to stop turning and
adjust brake pressure to maintain best braking. This system helps the driver maintain greater control of
the vehicle under heavy braking conditions.

Note: The Electronic Hydraulic Control Unit (EHCU)
comprises the Hydraulic Unit (H/U) and Control Unit.













This illustration is based on RHD model
C05L300010
Legend

(1) Electronic (6) Front Left Wheel Speed Sensor
(2) Hydraulic (7) Rear Right Wheel Speed Sensor
(3) Hydraulic Unit (H/U) (8) Rear Left Wheel Speed Sensor
(4) Control Unit (9) G-sensor (4WD only)
(5) Front Right Wheel Speed Sensor (10) 2-4WD Control Unit (4WD only)

5A-6 BRAKE CONTROL SYSTEM

EHCU, Brake Pipe Diagram








This illustration is based on RHD model

C05L300011

Legend

(1) Front Right Brake Port (out)
(4) Front Brake Port (in)
(2) Front Left Brake Port (out)
(5) Rear Brake Port (in)
(3) Rear Brake Port (out)

BRAKE CONTROL SYSTEM 5A-7

Hydraulic Unit (H/U)


C05L300004
Legend

(1) Brake Pedal (5) Dump Valve
(2) Master Cylinder
(6) Buffer Chamber
(3) Motor and Pump (7)Brake
(4) Isolation Valve

5A-8 BRAKE CONTROL SYSTEM

Normal Braking
During normal (non anti-lock) braking, the solenoid
valve has current flow. The dump valve is closed and
the isolation valve is opened due to spring force.
Brake fluid travels through the center of the isolation
valve (normally open) around the dump valve (normally
closed) then to the brake pistons.
C05L300012

BRAKE CONTROL SYSTEM 5A-9

Pressure Isolation (Pressure Maintain)
The electronic-hydraulic control unit is activated when
the brakes are applied.
If the information from the wheel speed sensors
indicates excessive wheel deceleration (imminent
lockup), the first step in the anti-lock sequence is to
isolate the brake pressure being applied by the brake
pedal.
The microprocessor in the Control Unit sends a voltage
to the coil to energize and close the isolation valve.
This prevents any additional fluid pressure applied by
the brake pedal from reaching the wheel. With the
isolation valves closed, unnecessary increase in the
brake pressure is prevented.
C05L300013

5A-10 BRAKE CONTROL SYSTEM

Pressure Reduction
Once the brake pressure is isolated, it must be reduced
to allow the wheels to unlock. This is accomplished by
dumping a portion of the brake fluid pressure into a
buffer chamber.
The microprocessor activates the normally closed dump
valve to open, allowing fluid from the wheels to be
dumped into the buffer chamber. This is done with very
short activation pulses opening and closing the dump valve passageway. Brake pressure is reduced at the
wheel and allows the wheel to begin rotating again. The
fluid from the brake piston is stored in the buffer
chamber against spring pressure and a portion of this
fluid also primes the pump.
The dump valves are operated independently to control
the deceleration of the wheel.
C05L300014

BRAKE CONTROL SYSTEM 5A-11

Brake Release
At the end of the anti-lock stop, when the brake pedal is
released, the pump will remain running for a short time
to help drain any fluid from the buffer chamber. As this
fluid returns into the system, the spring forces the piston back to its original position.
The isolation valve opens and fluid may return to the
master cylinder. Conventional braking is then resumed.
C05L300015

5A-12 BRAKE CONTROL SYSTEM

Circuit Diagram


RTW45AXF000101

BRAKE CONTROL SYSTEM 5A-13

Connector List
No. Connector face No. Connector face
B-23
Green
Meter-A C-44
WhiteStop Light Switch
B-24
Green
Meter-B C-53
OrangeABS Sensor Front-RH
B-48
Black
G-sensor C-54
OrangeABS Sensor Front-LH
B-54
White
I2 C-67
BlackEHCU
B-56
White
J/B I4 C-107
WhiteJ/B E2
B-58
Black
Data Link Connector C-108
WhiteJ/B E1
B-62
White
Ignition Switch (IGSUB : G1) C-109
SilverBody-LH ; Ground
B-63
White
Ignition Switch (IGSUB : G2) F-4
Gray ABS Sensor Rear
C-2
Silver
Engine Room-RH ; ground H-6
WhiteEngine Room ~ INST
C-36
Silver
Engine Room-LH ; Ground H-7
WhiteEngine Room ~ INST

5A-14 BRAKE CONTROL SYSTEM


No. Connector face No. Connector face
H-15
White
Engine Room ~ Chassis


H-18
White
Engine Room ~ INST


P-2
Silver
Relay & Fuse Box


P-6
Silver
Body Earth (Ground)


P-10
Silver
Engine Ground


R-15
Black
2-4WD Control Unit