DTC 42

IG1 Power Circuit System (Fuse 18 Circuit)

IG1 Power Supply Circuit (fuse No.18 Circuit):

CIRCUIT OPERATION
- The SRS-ECU is powered from the ignition switch (IG1).
- The SRS-ECU power is supplied from two circuits. Even if one circuit is shut off, the air bag can inflate.

DTC SET CONDITIONS
This DTC is set if the voltage between the IG1 terminals (fuse 18 circuit) and ground is lower than a predetermined value for a continuous period of five second or more. However, if the vehicle condition returns to normal, DTC 42 will be automatically erased, and the SRS warning light will switch off.

TROUBLESHOOTING HINTS
- Damaged wiring harnesses or connectors
- Malfunction of the SRS-ECU

DIAGNOSIS

Required Special Tools:
- MB991502: Scan Tool (MUT-II)
- MB991223 (MB991222): Harness set (Probe)




STEP 1. Check junction block multi-purpose fuse number 18.

Q: Is the fuse burned out?

YES: Go to Step 4.

NO: Go to Step 2.

STEP 2. Check the power supply circuit for open circuit at SRS-ECU connector C-44.

1. Disconnect the negative battery terminal.




2. Disconnect SRS-ECU connector C-44.
3. Connect the negative battery terminal.
4. Turn the ignition switch to the "ON" position.






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

5. Measure the voltage between C-44 harness connector terminal 9 and body ground. Voltage should measure 9 volts or more.

Q: Is the measured voltage within the specified range?

YES: Erase the DTC memory, and check for the diagnostic trouble code. If DTC 42 is set, replace the SRS-ECU. Then go to Step 8.

NO: Go to Step 3.













STEP 3. Check the harness for open circuit between SRS-ECU connector C-44 (terminal 9) and ignition switch connector C-58 (terminal 2).

NOTE: After inspecting intermediate connectors C-57, C-84 and C-87, inspect the wiring harness. If intermediate connectors are damaged, repair or replace them. Refer to Harness Connector Inspection. Then go to Step 8.

Q: Is the harness wire between SRS-ECU connector C-44 (terminal 9) and ignition switch connector C-58 (terminal 2) in good condition?

YES: Go to Step 8.

NO: Repair the harness wire between SRS-ECU connector C-44 and the ignition switch connector C-58.Then go to Step 8.

STEP 4. Check a burned-out fuse.

1. Replace the fuse.
2. Turn the ignition switch to the "ON" position, wait for at least one minute and then turn the ignition switch to the "LOCK" (OFF) position.
3. Check the fuse.

Q: Is the fuse in good condition?

YES: Go to Step 8.

NO: Go to Step 5.

STEP 5. Check the SRS-ECU power supply circuit for short circuit to ground at junction block connector C-84.




1. Disconnect junction block connector C-84, and measure at the harness connector.






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

2. Check for continuity between terminal 7 and body ground.
It should be open circuit.

Q: Does continuity exist?

NO: Check the other circuits, which flow through fuse number 18.

YES: Go to Step 6.

STEP 6. Check the power supply circuit for short circuit to ground at SRS-ECU connector C-44.




1. Disconnect SRS-ECU connector C-44, and measure at the harness connector.






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

2. Check for continuity between terminal 9 and body ground. It should be open circuit.

Q: Does continuity exist?

NO: Erase the DTC memory, and check for the diagnostic trouble code. If DTC 42 is set, replace the SRS-ECU. Then go to Step 8.

YES: Go to Step 7.










STEP 7. Check the harness for short circuit to ground between SRS-ECU connector C-44 (terminal 9) and junction block connector C-84 (terminal 7).

NOTE: After inspecting intermediate connector C-57, inspect the wiring harness. If intermediate connector is damaged, repair or replace them. Refer to Harness Connector Inspection. Then go to Step 8.

Q: Is the harness wire between SRS-ECU connector C-44 (terminal 9) and junction block connector C-84 (terminal 7) in good condition?

YES: Go to Step 8.

NO: Repair the harness wire between SRS-ECU connector C-44 and junction block connector C-84. Then go to Step 8.

STEP 8. Check for the diagnostic trouble code.

Q: Is DTC 42 set?

YES: Return to Step 1.

NO: The procedure 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).