DTC 21

Driver's Air Bag Module (Squib) System Fault 1 (Short Circuit Between Terminal Of The Squib Circuit)

Driver's Air Bag Module (Squib) Circuit:

CIRCUIT OPERATION
- The SRS-ECU judges how severe a collision is by detecting signals from 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.

DTC SET CONDITIONS
- This DTC is set if there is abnormal resistance between the input terminals of the driver's side air bag module (squib). The most likely causes for this code to be set are shown in the table: However, if no DTC resets, the SRS warning light will be switched off (DTC will be retained).

TROUBLESHOOTING HINTS
- Improper engaged connector or defective short bar*
- 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" bar (which prevents the air bag from deploying unintentionally due to static electricity by shorting the positive wire to the ground wire in the squib circuit when the connectors are disconnected). Therefore, if the connector C-72, C-85 or C-81 is damaged or improperly engaged, the short bar may not be released when the connector is connected.

Required Special Tools:
- MB991502: Scan Tool (MUT-II)
- MB991613: SRS Check Harness
- MB991865: Dummy resister
- MB991866: Resister harness

STEP 1. Check the DTC.

Q: Is DTC 34 output?

YES: Go to Step 2.

NO: Go to Step 3.




STEP 2. Check the SRS-ECU connector C-72.

Q: Is the connector correctly engaged?

YES: Go to Step 3.

NO: Engage the connector correctly. Then go to Step 9.




STEP 3. Check the air bag module connector C-81.
Remove the air bag module mounting equipment and check air bag module connector C-81 .At this time, check that there is no gap at place B shown in the illustration.

Q: Is the connector correctly connected?

YES: Go to Step 4.

NO: Insert the connector to the place, where there remains no gap at place B shown in the illustration. Then go to STEP 9.

STEP 4. Check the SRS-ECU connector C-72 and clock spring connector C-85.

1. Disconnect the negative battery terminal.







2. Disconnection connectors C-72 and C-85 then reconnect them.
3. Connect the negative battery terminal.
4. Erase the DTC memory, and then check the DTC.

Q: Is DTC 21 output?

YES: Go to Step 5.

NO: The inspection is complete. (It is assumed that DTC 21 set as connector C-72 or C-85 was engaged improperly.)

STEP 5. Check the driver's air bag module line.

1. Disconnect the negative battery terminal.
2. Remove the driver's air bag module.




3. Connect connector
4. of special tool MB991613 to clock spring connector C-81. special tool MB991613 to special tool MB991866.
4. Connect connector (1) of special tool MB991613 to connector (2).
5. Connect terminals 22 to 24, and terminals 23 to 25 of special tool MB991613 to connector (5).
6. Connect the clock spring to the body wiring harness.
7. Connect the negative battery terminal.
8. Erase the DTC memory, and then recheck the DTC.

Q: Is DTC 21 output?

YES: Go to Step 6.

NO: Replace the driver's air bag module. Then go to Step 9.

STEP 6. Check the clock spring.

1. Disconnect the negative battery terminal.
2. Disconnect the clock spring connector C-85.




3. Connect special tool MB991865 to special tool MB991866.

CAUTION: Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.

4. Insert special tool MB991866 into clock spring harness side connector C-85 (terminal No.3 and 4) by backprobing.
5. Connect the negative battery terminal.
6. Erase the DTC memory, and then recheck the DTC.

Q: Is DTC 21 output?

YES: Go to Step 7.

NO: Replace the clock spring. Then go to Step 9.

STEP 7. Check harness between the SRS-ECU and the clock spring for short circuit.










1. Disconnect SRS-ECU connector C-72.

CAUTION: To release the short bar in the SRS-ECU connector, disconnect connector C-85 to short the squib circuit.

2. Disconnect the clock spring connector C-85.

CAUTION: Insert an insulator such as a cable tie to a depth of 4 mm (0.16 inch) or more, otherwise the short bar will not cable tie release.

3. Insert a cable tie [3 mm (0.12 inch) wide, 0.5 mm (0.02 inch) thick] between terminals 7, 8 and the short bar to release the short bar.
4. Measure at the wiring harness side.

CAUTION: Do not insert a test probe into the terminal from its front side directly as the connector contact pressure may be weakened.




5. Check the continuity between terminals 7 and 8.
- There should be open circuit.

Q: Is the continuity normal?

YES: Erase the DTC memory, and recheck if any DTC sets. If DTC 21 sets, replace the SRS-ECU. Then go to Step 9.

NO: Go to Step 8.







STEP 8. Check the harness for short circuit between SRS-ECU connector C-72 (terminal No.7 and 8) and clock spring connector C-85 (terminal No.3 and 4).

Q: Are harness wires between SRS-ECU connector C-72 (terminal No.7 and 8) and clock spring connector C-85 (terminal No.3 and 4) in good condition?

YES: Go to Step 9.

NO: Repair the harness wires between SRS-ECU connector C-72 and clock spring connector C-85. Then go to Step 9.

STEP 9. Check the DTC.

Q: Is DTC 21 output?

YES: Return to Step 1.

NO: This diagnosis is complete. (If no malfunctions are not found in all steps, an intermittent malfunction is suspected. Refer to How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunction.)