Symptom Diagnostic Procedures

Communication with scan tool MB991958 is not possible (Communication is not possible with SRS).

SRS-ECU Power Supply Circuit Part 1:

SRS-ECU Power Supply Circuit Part 2:

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.
- The SRS system diagnosis can be done by connecting scan tool MB991958 to the data link connector.

TECHNICAL DESCRIPTION (COMMENT)
If communication is not possible with the SRS only, the cause is probably an open circuit in the on-board diagnostic output circuit of the SRS or in the power circuit (including ground circuit).

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

DIAGNOSIS

STEP 1. Check that the scan tool can communicate with the other systems.

Q: Can the scan tool communicate with the other systems?

YES: Go to Step 2.

NO: Refer to MFI Diagnosis in Computer and Control Systems.

STEP 2. Check the communication line between the SRS-ECU and the scan tool.




1. Disconnect SRS-ECU connector C-12 and data link connector C-14 and measure at the wiring harness side.

CAUTION: Do not insert a test probe into the terminal of the SRS-ECU connector C-12 from its front side directly as the connector contact pressure may be weakened.







2. Check for continuity between the following terminals. SRS-ECU connector C-12 (terminal No.20) and date ling connector C-14 (terminal No.7) It should be less than 2 ohms.

Q: Does continuity exist?

YES: Go to Step 3.

NO: Go to Step 5.

STEP 3. Check the ground circuit to the SRS-ECU.




1. Disconnect SRS-ECU connector C-12, and 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.




2. Check for continuity between terminal 7 and body ground. It should be less than 2 ohms.

Q: Does continuity exist?

YES: Go to Step 4.

NO: Go to Step 6.

STEP 4. Check the power supply circuit to the SRS-ECU.




1. Disconnect the negative battery terminal.
2. Disconnect SRS-ECU connector C-12, and measure at the wiring harness side.
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 terminals 13, 16 and body ground.
- Voltage should measure 9 volts or more.

Q: Is the measured voltage within the specified range?

YES: Recheck the trouble symptom. If it is not solved, replace the SRS-ECU. Then go to Step 8.

NO: Go to Step 7.










STEP 5. Check the harness wires between SRS-ECU connector C-12 (terminal No.20) and data link connector C-14 (terminal No.7).

NOTE: After inspecting intermediate connector C-101, inspect the wiring harness.

If the intermediate connector C-101, is damaged, repair or replace it. Refer to Harness Connector Inspection.

Q: Are the harness wires between SRS-ECU connector C-12 (terminal No.20) and data link connector C-14 (terminal No.7) in good condition?

YES: Go to step 8.

NO: Repair the harness wires between SRS-ECU connector C-12 and data link connector C-14. Then go to Step 8.







STEP 6. Check the harness wire between SRS-ECU connector C-12 (terminal No.7) and ground.

Q: Are the harness wire between SRS-ECU connector C-12 (terminal No.7) and ground in good condition?

YES: Go to Step 8.

NO: Repair the harness wire between SRS-ECU connector C-12 and ground. Then go to Step 8.










STEP 7. Check the harness wires between SRS-ECU connector C-12 (terminal No.13 and 16) and ignition switch connector C-208 (terminal No.2).

NOTE: After inspecting intermediate connector C-214 and C-211, inspect the wiring harness.

If the intermediate connector C-214 or C-211 is damaged, repair or replace it. Refer to Harness Connector Inspection.

Q: Are the harness wires between SRS-ECU connector C-12 (terminal No.13 and 16) and ignition switch connector C-208 (terminal No.2) in good condition?

YES: Go to Step 8.

NO: Repair the harness wires between SRS-ECU connector C-12 and ignition switch connector C-208. Then go to Step 8.

STEP 8. Retest the system.

Q: Does the scan tool communicate normally with the SRS system?

YES: The procedure is complete (If no malfunctions are found in all steps, an intermittent malfunction is suspected. Refer to How to Use Troubleshooting/INSPECTION Service Points - How to Cope with Intermittent Malfunction).

NO: There is no action to be taken.