Isuzu Amigo / Axiom / Trooper / Rodeo / VehiCross. Manual - part 158

5A–10

BRAKE CONTROL SYSTEM

System Components

Electronic Hydraulic Control Unit (EHCU), three Wheel
Speed Sensors, Warning Light, and G-sensor.

Electronic Hydraulic Control Unit (EHCU)

The EHCU consists of ABS control circuits, fault detector,
and a fail-safe.  The signal received from each sensor
activates the hydraulic unit accordingly and cancels the
ABS to return to normal braking if a malfunction occurs in
the ABS system.
The EHCU has a self-diagnosing function which can
indicate faulty circuits during diagnosis.
The EHCU is mounted on the engine compartment rear
right side.  It consists of a Motor, Plunger Pump, Solenoid
Valves.
Solenoid Valves: Reduces or holds the caliper fluid
pressure for each front disc brake or both rear disc brakes
according to the signal sent from the EHCU.
Reservoir: Temporarily holds the brake fluid that returns
from the front and rear disc brake caliper so that pressure
of front disc brake caliper can be reduced smoothly.
Plunger Pump: Feeds the brake fluid held in the reservoir
to the master cylinder.
Motor: Drives the pump according to the signal from
EHCU.
Check Valve: Controls the brake fluid flow.

ABS Warning Light

821R200015

Vehicles equipped with the Anti-lock Brake System have
an amber “ABS” warning light in the instrument panel.
The “ABS” warning light will illuminate if a malfunction in
the Anti-lock Brake System is detected by the Electronic
Hydraulic Control Unit (EHCU).In case of an electronic
malfunction, the EHCU will turn “ON” the “ABS” warning
light and disable the Anti-lock braking function.
The “ABS” light will turn “ON” for approximately three
seconds after the ignition switch is turned to the “ON”
position.

If the “ABS” light stays “ON” after the ignition switch is
turned to the “ON” position, or comes “ON” and stays
“ON” while driving, the Anti-lock Brake System should be
inspected for a malfunction according to the diagnosis
procedure.

Wheel Speed Sensor

It consists of a sensor and a rotor.  The sensor is attached
to the knuckle on the front wheels and to the rear axle
case on the rear differential.
The front sensor rotor is attached to the each brake rotor
by bolts.
The rear rotor is press-fit in the differential case.
The magnetic flux generated from electrodes magnetized
by a magnet in the sensor varies due to rotation of the
rotor, and the electromagnetic induction generates
alternating voltage in the coil.  This voltage draws a “sine
curve” with the frequency proportional to rotor speed and
it allows detection of wheel speed.

G-Sensor

The G-sensor installed inside the EHCU detects the
vehicle deceleration speed and sends a signal to the
EHCU.  In 4WD operation, all four wheels may be
decelerated in almost the same phase, since all wheels
are connected mechanically.
This tendency is noticeable particularly on roads with low
friction coefficient, and the ABS control is adversely
affected.
The G-sensor judges whether the friction coefficient of
road surface is low or high, and changes the EHCU’s
operating system to ensure ABS control.

Normal and Anti-lock Braking

Under normal driving conditions, the Anti-lock Brake
System functions the same as a standard power assisted
brake system. However, with the detection of wheel
lock-up, a slight bump or kick-back will be felt in the brake
pedal. This pedal “bump” will be followed by a series of
short pedal pulsations which occurs in rapid succession.
The brake pedal pulsation will continue until there is no
longer a need for the anti-lock function or until the vehicle
is stopped. A slight ticking or popping noise may be heard
during brake applications when the Anti-lock features is
being used.
When the Anti-lock feature is being used, the brake pedal
may rise even as the brakes are being applied. This is
also normal.  Maintaining a constant force on the pedal
will provide the shortest stopping distance.

Brake Pedal Travel

Vehicles equipped with the Anti-lock Brake System may
be stopped by applying normal force to the brake pedal.
Although there is no need to push the pedal beyond the
point where it stops or holds the vehicle, by applying more
force the pedal will continue to travel toward the floor.
This extra brake pedal travel is normal.

5A–11

BRAKE CONTROL SYSTEM

Acronyms and Abbreviations

Several acronyms and abbreviations are commonly used
throughout this section:

ABS
Anti-lock Brake System

CIM
Coil Integrated Module

CKT
Circuit

DLC
Data Link Connector

EHCU
Electronic Hydraulic Control Unit

FL
Front Left

FR
Front Right

GEN
Generator

H/U
Hydraulic Unit

MV
Millivolts

RR
Rear

RPS
Revolution per Second

VDC
DC Volts

VAC
AC Volts

W/L
Warning Light

WSS
Wheel Speed Sensor

General Diagnosis

General Information

ABS troubles can be classified into two types, those
which can be detected by the ABS warning light and those
which can be detected as a vehicle abnormality by the
driver.
In either case, locate the fault in accordance with the
“BASIC DIAGNOSTIC FLOWCHART” and repair.
Please refer to 

Section 5C for the diagnosis of

mechanical troubles such as brake noise, brake judder
(brake pedal or vehicle vibration felt when braking),
uneven braking, and parking brake trouble.

ABS Service Precautions

Required Tools and Items:

D

Box Wrench

D

Brake Fluid

D

Special Tool

Some diagnosis procedures in this section require the
installation of a special tool.

J-39200 High Impedance Multimeter
When circuit measurements are requested, use a circuit
tester with high impedance.

Computer System Service Precautions

The Anti-lock Brake System interfaces directly with the
Electronic Hydraulic Control Unit (EHCU) which is a
control computer that is similar in some regards to the
Powertrain Control Module. These modules are designed
to withstand normal current draws associated with
vehicle operation. However, care must be taken to avoid
overloading any of the EHCU circuits. In testing for opens
or shorts, do not ground or apply voltage to any of the
circuits unless instructed to do so by the appropriate
diagnostic procedure. These circuits should only be
tested with a high impedance multimeter (J-39200) or
special tools as described in this section. Power should
never be removed or applied to any control module with
the ignition in the “ON” position.
Before removing or connecting battery cables, fuses or
connectors, always turn the ignition switch to the “OFF”
position.

General Service Precautions

The following are general precautions which should be
observed when servicing and diagnosing the Anti-lock
Brake System and/or other vehicle systems. Failure to
observe these precautions may result in Anti-lock Brake
System damage.

D

If welding work is to be performed on the vehicle using
an electric arc welder, the EHCU and valve block
connectors should be disconnected before the
welding operation begins.

D

The EHCU and valve block connectors should never
be connected or disconnected with the ignition “ON” .

D

If only the rear wheels are rotated using jacks or drum
tester, the system will diagnose a speed sensor
malfunction and the “ABS” warning light will
illuminate. But actually no trouble exists. After
inspection stop the engine once and re-start it, then
make sure that the “ABS” warning light does not
illuminate.

If the battery has been discharged
The engine may stall if the battery has been completely
discharged and the engine is started via jumper cables.
This is because the Anti-lock Brake System (ABS)
requires a large quantity of electricity.  In this case, wait
until the battery is recharged, or set the ABS to a
non-operative state by removing the fuse for the ABS
(50A). After the battery has been recharged, stop the
engine and install the ABS fuse. Start the engine again,
and confirm that the ABS warning light does not light.

Note on Intermittents

As with virtually any electronic system, it is difficult to
identify an intermittent failure. In such a case duplicating
the system malfunction during a test drive or a good
description of vehicle behavior from the customer may be
helpful in locating a “most likely” failed component or

5A–12

BRAKE CONTROL SYSTEM

circuit. The symptom diagnosis chart may also be useful
in isolating the failure. Most intermittent problems are
caused by faulty electrical connections or wiring. When
an intermittent failure is encountered, check suspected
circuits for damage:

D

Suspected harness damage.

D

Poor mating of connector halves or terminals not fully
seated in the connector body (backed out).

D

Improperly formed or damaged terminals.

Test Driving ABS Complaint Vehicles

In case of an intermittent ABS lamp illumination, see
“Diagnosis by ABS Warning Light illumination Pattern” or
go to 5A–37. In some cases, the vehicle may need to be
test driven by following the test procedure below.

1. Start the engine and make sure that the “ABS” W/L

goes OFF.  If the W/L remains ON, it means that the
Diagnostic Trouble Code (DTC) is stored.  Therefore,
read the code and locate the fault.

NOTE: The DTC cannot be cleared if the vehicle speed
does not exceed 12 km/h (8 mph) at DTC, even though
the repair operation is completed.

2. Start the vehicle and accelerate to about 30 km/h (19

mph) or more.

3. Slowly brake and stop the vehicle completely.
4. Then restart the vehicle and accelerate to about 40

km/h (25 mph) or more.

5. Brake at a time so as to actuate the ABS and stop the

vehicle.

6. Be cautious of abnormality during the test.  If the W/L

is actuated while driving, read the DTC and locate the
fault.

7. If the abnormality is not reproduced by the test, make

best efforts to reproduce the situation reported by the
customer.

8. If the abnormality has been detected, repair in

accordance with the “SYMPTOM DIAGNOSIS” .

NOTE:

D

Be sure to give a test drive on a wide, even road with a
small traffic.

D

If an abnormality is detected, be sure to suspend the
test and start trouble diagnosis at once.

“ABS” Warning Light

When ABS trouble occurs to actuate “ABS” warning light,
the trouble code corresponding to the trouble is stored in
the EHCU.  Only ordinary brake is available with ABS
being unactuated.  Even when “ABS” warning light is
actuated, if the starter switch is set ON after setting it OFF
once, the EHCU checks up on the entire system and, if
there is no abnormality, judges ABS to work currently and
the warning light is lit normally even though the trouble
code is stored.

NOTE: Illumination of the “ABS” warning light indicates
that anti-lock braking is no longer available. Power
assisted braking without anti-lock control is still available.

Normal Operation

“ABS” Warning Light
When the ignition is first moved from “OFF” to “RUN” , the
amber “ABS” warning light will turn “ON” . The “ABS”
warning light will turn “ON” during engine starting and will
usually stay “ON” for approximately three seconds after
the ignition switch is returned to the “ON” position. The
warning light should remain “OFF” at all other times.

5A–13

BRAKE CONTROL SYSTEM

Tech 2 Scan Tool

From 98 MY, Isuzu dealer service departments are
recommended to use Tech 2. Please refer to Tech 2 scan
tool user guide.

901RW257

Legend

(1) PCMCIA Card
(2) SAE 16/19 Adaptor

(3) DLC Cable
(4) Tech–2