Isuzu N-Series. Service manual — part 1189

SRS CONTROL SYSTEM 9C-3

Damaged SRS Connector (Case and Terminal)
If the connector or terminal of SRS harness (except lead
wire) is damaged, replace it.

Damaged Lead wire of SRS Part
If the lead wire of parts for SRS airbag, combination
switch assembly (including SRS coil assembly), etc. is
damaged, replace all the parts (including lead wire).

Procedure to stop the function of SRS

Removal
Turn the starter switch to “LOCK”, and remove the key.

1. Disconnect the SRS fuse from the fuse box on the

upper right portion of dashboard or disconnect ter-
minal of battery.

2. Remove the 2-pole connector (yellow) located in

the lower portion of steering column. (for SRS
airbag)

Caution:

When removing SRS fuse and turn starter switch to ON,
SRS warning lamp remains on. This is normal condition.

Procedure to Resume the Function of SRS

Installation

WARNING:

DO NOT USE SRS AIRBAG OF OTHER VEHICLE OR
MODEL. BE SURE TO CONFIRM THE PART NUM-
BER TO USE APPROPRIATE SRS AIRBAG.

Turn the starter switch to “LOCK”, and remove the key.

1. Connect the 2-pole connector (yellow) located in

the lower portion of steering column and confirm
that it is connected securely.

2. Install the “SRS fuse” to fuse box on the upper right

portion of dashboard.

Turn the starter switch to “ON”, and confirm that the
“SRS” warning lamp goes off after it blinks seven times.
If the “SRS” warning lamp does not operate in that way,
perform “Function check: SRS system check”.

Precautions on Handling/Installation/Diagnosis

1. Do not expose SRS airbag to temperature of 93

°C

(199

°F) or more.

2. If SRS airbag and SRS control unit are dropped

down with some sort of reason, do not use them.

3. If disconnecting connectors related to SRS airbag

and SRS control unit during repair, be sure to make
sure that they are connected completely during re-
assembly.

4. When replacing SRS control unit, install it to

mounting surface correctly. Be sure to make sure
there is no dust or other foreign matter on mounting
surface of SRS control unit.

5. When installing SRS airbag parts, be sure to tight-

en with specified tightening torque.

6. Do not energize the system unless all SRS system

parts are connected, or diagnosis chart requires
doing so. If energizing the system, unnecessary di-
agnostic trouble code will be set in SRS control
unit.

7. Never diagnose the circuit using other than speci-

fied tester.

8. At first, perform “SRS system check”.

“SRS system check” checks whether “SRS” warn-
ing lamp operates properly and proceeds to appro-
priate chart to judge the function fault of system.
Failure to perform this procedure leads to longer di-
agnosis period, wrong diagnosis or replacement of
wrong part.

About Terms

SRS airbag
This is expansion-type cloth cushion designed to be de-
ployed on frontal collision above a certain impact. SRS
airbag reduces an impact on occupant’s head and trunk
and supplements the protection by seat belts.

B+
Battery voltage (B+)

Warning lamp bulb check
When no function fault is detected after starter switch is
turned to “ON” position, “SRS” warning lamp blinks sev-
en times and goes off.

“Constant diagnosis”
Fault diagnosis is constantly performed by SRS control
unit as long as “Ignition A” voltage is within the range of
specified operating voltage of the unit.

Diagnostic connector (16-pin)
This is a connector to communicate with Tech 2.

Deployment
This means expansion of airbag.

Deployment circuit
This is a circuit which sends current to SRS airbag as-
sembly to deploy SRS airbag.

Diagnostic trouble code (DTC)
This is number used by SRS control unit to identify the
trouble in SRS function. (Self-diagnosis code)

Current supply for driver’s seat
This is output of SRS control unit to send current to driv-
er’s airbag assembly circuit during “Ignition device resis-
tance test”.

EEPROM
This is an electrically erasable programmable read only
memory. This is a memory which can store contents
without power on SRS control unit.

9C-4 SRS CONTROL SYSTEM

Ignition device
This is electric ignition device to combust gas generating
agent of inflator.

Specified operating voltage range
This is voltage range from 18 to 32 V measured between
“Ignition A” terminal and “Ground” terminal of SRS con-
trol unit.

Tech 2
This is diagnostic equipment to read diagnostic informa-
tion from SRS control unit via diagnostic connector.

SRS control unit
This supplies backup current to deployment circuit and
deploy airbag if necessary. Also it diagnoses all the SRS
parts.

Serial data
This is Tech 2 information which indicates SRS operat-
ing condition.

SRS
This means supplemental restraint system.

SRS coil assembly
This is device to maintain successive continuity on de-
ployment circuit for SRS airbag and allow steering wheel
to rotate.

SRS harness
These are harnesses and connectors to connect SRS
parts electrically.

Initial diagnosis
These are tests for SRS, performed by SRS control unit
on the following conditions: after starter switch is “ON”,
immediately after “Ignition A” voltage is energized by
SRS control unit, or before “Constant diagnosis”. (per-
formed within 6 seconds after starter switch is “ON”)

Description of Function and Operation

SRS Airbag System
SRS airbag supplements the driver’s seat belt protec-
tion by deploying airbag at the top center of the steering
column.
SRS airbag deploys in case of front collision over certain
degree of impact.

Legend

1. Deployed airbag
2. Seat belt

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SRS CONTROL SYSTEM 9C-5

This system consists of SRS control unit, driver’s SRS
airbag, SRS coil assembly, exclusive wire harness, and
“SRS” warning lamp in the meter and forms a deploy-
ment circuit.
A sensor is equipped to this circuit to convert the impact
into an electrical signal in case of front collision over cer-
tain degree of impact. The electrical signal generated at
this time will be processed in the SRS control unit to
compare with the value stored in the memory.
When the generated signal exceeds the setting value,
SRS control unit sends sufficient current to SRS airbag
for deployment.

SRS Control Unit

WARNING:

DURING SERVICE WORK, BE CAREFUL NOT TO
TAP OR SHAKE THE SRS CONTROL UNIT. ALSO,
FAILURE TO INSTALL THE SRS CONTROL UNIT TO
VEHICLE PROPERLY MAY LEAD TO MALFUNC-
TION OF THE UNIT WHEN THE STARTER SWITCH IS
TURNED TO ON. THIS MAY RESULT IN INJURY DUE
TO SUDDEN DEPLOYMENT OF SRS AIRBAG.

Legend

1. SRS control unit

B

ACC

ON

ST

STARTER SWITCH

METER

AIRBAG

SRS CONTROL UNIT

DATA LINK CONNECTOR

16 15 14 13 12 11 10 9

8 7 6 5 4 3 2 1

0.5

B

0.5

GY

0.5

SB

0.5

GY

0.5

SB

16

4

19

15

18

7

8

20

6

5

0.85B

0.85B

0.5

B/R

0.5

B/R

0.5

W/R

0.5

B/R

0.5

B/R

0.5

W/R

0.5

B

1.25

B

2L

3
B/Y

2

14

3W/L

8B

3W/L

40A
KEY SW

10A
SRS

AIRBAG

SRS COIL

ASSEMBLY

1

2

2

1

0.5

Y

0.5

Y/B

3

10A
METER

0.5
B/Y

18

OFF

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9C-6 SRS CONTROL SYSTEM

Following functions are included in the SRS control unit.

1. Energy back-up — SRS control unit builds up the

energy necessary to deploy airbag when it lose the
ignition voltage during front collision.

2. Front collision detection — SRS control unit moni-

tors the impact and detects the front collision for
which airbag must be deployed.

3. SRS airbag deployment — SRS control unit sends

sufficient current to the SRS airbag assembly for
airbag deployment, when it detects the front colli-
sion over certain degree of impact.

4. Function fault detection — SRS control unit diag-

noses electrical parts of this system. It set the diag-
nostic trouble code once it detects function fault.

5. Front collision detection — SRS control unit

records information regarding operating conditions
of this system during the event of front collision.

6. Function fault diagnostics — SRS control unit dis-

plays diagnostic trouble codes (DTC) and operat-
ing conditions of the system using Tech 2.

7. Warning to the driver — SRS control unit warns

driver of function fault of this system by turning
“SRS” warning lamp on.

SRS control unit is connected to SRS harness with a 21-
pole connector. This harness connector has shorting
clip in the contact surface of particular terminal.

“SRS” Warning Lamp

When the starter switch is turned “ON” or “START”, SRS
control unit controls “SRS” warning lamp as follows.

1. When the starter switch is turned to “ON”, it checks

“SRS” warning lamp and SRS control unit opera-
tions while blinking the “SRS” warning lamp seven
times.

2. It warns the driver of function fault in the electrical

system of this system. Those function faults may
cause undeployment of SRS airbag in the event of
front collision, or unnecessary deployment.

“SRS” warning lamp tells the driver of function fault
of this system. Refer to “SRS system check” for
proper operation of warning lamp.

SRS Coil Assembly

Legend

1. Lock pin
2. Neutral mark

SRS coil assembly is installed to the steering column. It
maintains successive continuity on deployment circuit
for SRS airbag while allowing steering wheel to rotate.

It has shorting clip in the 2-pole connector (yellow) to be
connected to the SRS body harness located in the lower
portion of steering column.

Upon removal of 2-pole connector (yellow), the shorting
clip will short the SRS airbag. This prevents unneces-
sary deployment of SRS airbag when servicing the
steering column or other SRS system parts.

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