Isuzu N-Series. Manual - part 235

ANTI-LOCK BRAKE SYSTEM (ABS) 5A4-5

When the Anti-lock feature is being used, the brake ped-
al may rise even as the brakes are being applied.
This is also normal. Maintaining a consist 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.

Acronyms and Abbreviations

Several acronyms and abbreviations are commonly
used throughout this section:
ABS
Anti-lock Brake System
CKT
Circuit
DLC
Data Link Connector
DTC
Diagnostic Trouble Code
DVM
Digital Volt Meter (High Impedance Multimeter)
EHCU
Electronic Hydraulic Control Unit
FL
Front Left
FR
Front Right
GEN
Generator
HU
Hydraulic Unit
MV
Millivolts
RL
Rear Left
RR
Rear Right
RPS
Revolution per Second
SW
Switch
VDC
Volts DC
VAC
Volts AC
W/L
Warning Light
WSS
Wheel Speed Sensor

5A4-6 ANTI-LOCK BRAKE SYSTEM (ABS)

GENERAL DIAGNOSIS

General Information

ABS malfunction 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 5A HYDRAULIC BRAKES 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 trou-
ble.

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.
High Impedance Multimeter: 5-8840-0285-0 (J-39200)
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
Engine Control Module. These modules are designed to
withstand normal current draws associated with vehicle
operation. However, care must be taken to avoid over-
loading any of the EHCU circuits. In testing for opens or
shorts, do not ground or apply voltage to any of the cir-
cuits unless instructed to do so by the appropriate diag-
nostic procedure. These circuits should only be tested
with a high impedance multimeter 5-8840-0285-0 (J-
39200) or special tools as described in this section.
Power should never be removed or applied to any con-
trol module with the ignition in the “ON” position. Before
removing or connecting battery cables, fuses or connec-
tors, always turn the ignition switch to the “OFF” posi-
tion.

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 connector
should be disconnected before the welding opera-
tion begins.

• The EHCU connector should never be connected

or disconnected with the ignition “ON”.

• The EHCU is not separately serviceable and must

be replaced as assemblies. Do not disassemble
any component which is designated as non-ser-
viceable in this Section.

• If only rear wheels are rotated using jacks or drum

tester, the system will diagnose a speed sensor
malfunction and the “ABS” warning light will illumi-
nate. 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 illumi-
nate.

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 de-
scription of vehicle behavior from the customer may be
helpful in locating a “most likely” failed component or cir-
cuit. 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 suspect cir-
cuits for:

• 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 that there has been an malfunction in the lighting
pattern of “ABS” warning light, the fault can be located
in accordance with the “DIAGNOSIS BY “ABS” WARN-
ING LIGHT ILLUMINATION PATTERN”. In case of such
trouble as can be detected by the driver as a vehicle
symptom, however, it is necessary to give a test drive
following the test procedure mentioned below, thereby
reproducing the symptom for trouble diagnosis on a
symptom basis:

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.

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 lo-
cate the fault.

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

make best efforts to reproduce the situation report-
ed by the customer.

ANTI-LOCK BRAKE SYSTEM (ABS) 5A4-7

8. If the abnormality has been detected, repair in ac-

cordance with the “SYMPTOM DIAGNOSIS”.

Notice:

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

with little 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 and actuates when possible
the “ABS” warning light, the trouble code corresponding
to the trouble is stored in the EHCU.
Only the ordinary brake system is available when the
ABS is turned OFF. When the “ABS” warning light is ac-
tuated, 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 works normally even though the
trouble code is stored.

Notice:
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 “ON”, the
amber “ABS” warning light will turn “ON” for 2 seconds
and will turn “OFF”.

Basic Diagnostic Flow Chart

Step

Action

Value(s)

YES

1. Customer complaint.
2. Questioning to customer.
3. Basic inspection (Refer to “Basic inspec-

tion procedure”).

Using Tech 2?

—