Isuzu Amigo / Axiom / Trooper / Rodeo / VehiCross. Manual - part 1170

BRAKE CONTROL SYSTEM

5A–9

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 accumulators. 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.

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Legend

(1) Brake
(2) Master Cylinder

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

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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 CONTROL SYSTEM

5A–11

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.

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:

• Box Wrench

• Brake Fluid

• 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.

5A–12

BRAKE CONTROL SYSTEM

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.

• 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.

• The EHCU and valve block connectors should never

be connected or disconnected with the ignition “ON” .

• 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
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:

• Suspected harness damage.

• Poor mating of connector halves or terminals not fully

seated in the connector body (backed out).

• 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 Diagnostic .

NOTE:

• Be sure to give a test drive on a wide, even road with

a small traffic.

• 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.