Lexus SC300 / Lexus SC400. Service manual — part 60

TROUBLESHOOTING

AB–23

SRS

AIRBAG – Troubleshooting

Description

The SRS (Supplemental Restraint System) airbag, together with
the seat belt, is designed to help protect the driver. In a collision,
the airbag sensors detect the shock, and if the front–to–rear shock
is greater than a specified value, an airbag stored in the steering
wheel pad is inflated instantaneously to help reduce the shock to
the driver.

LOCATION OF COMPONENTS

AB–24

SRS

AIRBAG – Troubleshooting

How To Proceed With Troubleshooting

Malfunction symptoms of the airbag system are difficult to confirm, so the diagnostic codes become the most
important source of information when troubleshooting. Perform troubleshooting of the airbag system in accor-
dance with the following procedure:
HINT: Do not disconnect the battery negative (–) terminal cable until step [ 3 ], Diagnostic Code Check and Re-
cording, has been completed.

1.

CUSTOMER PROBLEM ANALYSIS
Using the CUSTOMER PROBLEM ANALYSIS CHECK SHEET (See page

AB–29

) for reference, ask the

customer in as much detail as possible about the problem.

2.

WARNING LIGHT CHECK
Check the airbag warning light. If the light remains on, a malfunction is stored in the center airbag sensor,
assembly, so proceed to step [ 3 ]. If the airbag warning light is not on, a malfunction has occurred in the
airbag warning light circuit, so perform troubleshooting for code 22.
HINT: Code 22 is recorded when a malfunction occurs in the airbag warning light system.
If an open malfunction occurs in the airbag warning light system, the airbag warning light does not light up,
so that until the malfunction is repaired, the diagnostic codes (including code 22) cannot be confirmed.

3.

DIAGNOSTIC CODE CHECK AND RECORDING
Check the diagnostic codes and make a note of any malfunction codes which are output. If a normal code
is output, an abnormality in the power source circuit may have occurred, so perform troubleshooting for
source voltage in step [ 8 ].
If code 22 is output, skip steps [ 4 ] and [ 5 ] and proceed to step [ 7 ].

4.

CLEARING OF MALFUNCTION CODE (EXCEPT CODE 41)
Clear the malfunction code.
HINT: The malfunction code output in step [ 3 ] indicates that a malfunction has occurred in the circuit desig-
nated by the malfunction code, but does not indicate whether the malfunction is still occurring or whether
it was in the past.
Accordingly, it is necessary to find out the present condition of the malfunction occurrence by clearing the
malfunction code and performing the diagnostic code check again. If this operation is neglected and trou-
bleshooting is performed using only the malfunction code confirmed in step [ 3 ], isolating the problem com-
ponent becomes difficult and invites mistaken diagnosis.

5.

DIAGNOSTIC CODE CHECK AND RECORDING

[

6 ] SYMPTOM SIMULATION

After repeating ignition switch ON–OFF operation (ON: wait 20 secs., OFF: wait 20 secs.) 5 times, check
the diagnostic codes. If any code other than code 41 is output, the malfunction is still occurring, so proceed
to step [ 7 ].
If code 41 only is output, the following three cases are possible:

•

Intermittent trouble occurred previously, but it is now normal.

•

The problem has been corrected, but clearing of code 41 has been forgotten.

•

There is a malfunction in the circuit for code 41.

Focusing on the circuit of the malfunction code stored in step 3, use the simulation method in step [ 6 ]
in order to simulate the malfunction. If the malfunction occurs, proceed to step [ 7 ]; if not, proceed to step.
[ 12 ].

AB–25

SRS

AIRBAG – Troubleshooting

NOTICE: When connecting the battery after clearing the malfunction code, always do it with the igni-
tion switch in LOCK position. When the battery has been reconnected, turn the ignition switch to
ACC or ON position after at least 2 seconds have elapsed.
If the battery is reconnected with the ignition switch in ACC or ON position, or the ignition switch
is turned to ACC or ON within 2 seconds of connecting the battery, it is possible that the diagnosis
system will not operate normally.
HINT: Determine the malfunction in the airbag system in step [ 6 ] by whether or not a malfunction code
other than code 41 is output.

7.

DIAGNOSTIC CODE CHART
Proceed to the appropriate flow chart in step [ 8 ] in accordance with the malfunction code found in step
[ 5 ] or [ 6 ].

8.

CIRCUIT INSPECTION

[

9 ] REPAIR

Find out if the problem lies in a sensor, actuator or wire harness and connector, and repair the problem.
After the problem part is repaired, reinstall the disassembled parts. Do not start work until at least 20 se-
conds after the ignition switch is turned to the LOCK position and the negative (–) terminal cable is discon-
nected.
CAUTION: If incorrect procedure is used, a malfunction may occur in the system or there is the dan-
ger that airbag may be accidentally activated during the repair operation. Carefully read the GEN-
ERAL DESCRIPTION (See page

AB–2

) and the cautions for each operation, and perform repairs in

the correct order using the correct methods.
HINT: The following illustration for the CIRCUIT INSPECTION shows each connector for the circuit from
the center airbag sensor assembly to the steering wheel pad (squib).

10

CLEARING OF MALFUNCTION CODE (EXCEPT CODE 41)
When all the malfunction codes found in steps [ 5 ] and [ 6 ] have been repaired, clear the malfunction
codes.

AB–26

SRS

AIRBAG – Troubleshooting