Code No.21: Driver’s
air bag module (squib) system (short-circuited between terminals of
the squib circuit)
OPERATION
- The SRS-ECU judges how severe a collision is by detecting
signals from the
front impact sensors and the front air bag analog G-sensor. If
the impact is over a predetermined
level, the SRS-ECU outputs an ignition signal. At this time, if
the front air bag safing G-sensor
is on, the SRS air bag will inflate.
- The ignition signal is input to the air bag module via the clock
spring to inflate
the air bag.
DIAGNOSIS CODE SET CONDITIONS
This diagnosis code is set if
one driver’s air bag squib wire shorted to the
other. However, if no diagnosis code resets, the SRS warning lamp will
be switched off (diagnosis
code will be retained).
PROBABLE
CAUSES
- Improper engaged connector or defective short
spring*
- Short circuit in the clock spring
- Short circuit between the driver’s air bag module (squib)
circuit terminals
- Damaged connector(s)
- Malfunction of the SRS-ECU
| note |
*: The squib circuit connectors
integrate a "short" spring (which prevents the
air bag from deploying unintentionally due to static
electricity by shorting the positive wire
to the earth wire in the squib circuit when the connectors
are disconnected). Therefore, if
connector C-147, C-309 or C-312 is damaged or improperly
engaged, the short spring may not be
released when the connector is connected.
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STEP 1.M.U.T.-III diagnosis code
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Q.
Is diagnosis code 34 set?
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 Go to Step
2.
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 Go to Step
3.
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STEP 2. Connector lock check: SRS-ECU connector C-147
(M.U.T.-III diagnosis
code).
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(1)Disconnect the negative battery
terminal.
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(2)Disconnect connectors C-147 and
then reconnect them.
(3)Connect the negative battery
terminal.
(4)Erase the diagnosis code memory, and check
the diagnosis code.
Q.
Is diagnosis code 21 set?
Go to Step
4.
The
procedure is complete. It is assumed that diagnosis code 21
set as connector
C-147 was engaged improperly.
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STEP 3. Connector lock check: SRS-ECU connector C-147,
clock spring
connector C-309 and driver’s air bag module
connector C-312 (M.U.T.-III diagnosis
code).
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(1)Disconnect the negative battery
terminal.
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(2)Disconnect connectors C-147,
C-309 and C-312, and then reconnect them.
(3)Connect the
negative battery terminal.
(4)Erase the diagnosis code
memory, and check the diagnosis code.
Q.
Is diagnosis code 21 set?
Go to Step
4.
The
procedure is complete. It is assumed that diagnosis code 21
set as connector
C-147, C-309 or C-312 was engaged improperly.
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STEP 4. Check the diagnosis code by connecting a dummy
resistor (M.U.T.-III
diagnosis code).
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(1)Disconnect the negative battery
terminal.
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(2)By sliding the A section (in the
figure) of air bag module connector C-312 in the arrow
direction, disconnect the connector.
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(3)Connect special tool dummy
resistor (MB991865) to special tool resistor harness
(MB991866).
(4)
| caution |
Do not insert a test probe
into the
terminal from its front side directly as the
connector contact pressure may be
weakened.
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Insert special tool (MB991866) into clock spring
side air bag module connector C-312 by
backprobing.
(5)Connect the negative battery
terminal.
(6)Erase the diagnosis code memory, and check
the diagnosis code.
Q.
Is diagnosis code 21 set?
Go to Step
5.
Replace
the driver’s air bag module (Refer to  ).
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STEP 5. Check the diagnosis code by connecting a dummy
resistor (M.U.T.-III
diagnosis code).
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(1)Disconnect the negative battery
terminal.
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(2)Disconnect the clock spring
connector C-309.
(3)Connect special tool dummy resistor
(MB991865) to special tool resistor harness
(MB991866).
(4)
| caution |
Do not insert a test probe
into the
terminal from its front side directly as the
connector contact pressure may be
weakened.
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Insert special tool (MB991866) into clock spring
harness side connector C-309 (terminal
No.3 and 4) by backprobing.
(5)Connect the negative
battery terminal.
(6)Erase the diagnosis code memory, and
check the diagnosis code.
Q.
Is diagnosis code 21 set?
Go to Step
6.
Replace
the clock spring (Refer to ).
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STEP 6. Resistance measurement at the SRS-ECU connector
C-147.
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(1)Disconnect SRS-ECU connector
C-147.
| danger |
To prevents the air bag from
deploying unintentionally, disconnect
the clock spring connector C-309 to short the
squib circuit.
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(2)Disconnect the clock spring
connector C-309.
| caution |
Insert an insulator such as a
cable tie to a depth of 4 mm or more, otherwise
the short
spring will not be released.
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(3)Insert a cable tie [3 mm wide, 0.5 mm
thick]
between terminals
11, 12 and the short spring to release the short
spring.
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(4)
| caution |
Do not insert a test probe
into the terminal from its front side directly
as the connector
contact pressure may be weakened.
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Resistance measurement between C-147 harness side
connector terminals 11 and 12.
OK: Open
circuit
Q.
Is the check result normal?
Go to Step
7.
Repair
the harness wires between SRS-ECU connector C-147 (terminal
No.11 and 12)
and clock spring connector C-309 (terminal No.3 and 4).
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STEP 7. Check whether the diagnosis code is
reset.
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Q.
Is diagnosis code 21 set?
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Replace the SRS-ECU
(Refer to ).
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An intermittent
malfunction is suspected (Refer to GROUP 00, How to Use
Troubleshooting/Inspection Service
Points ).
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