Symptom Diagnostic Procedures

Communication with scan tool MB991958 is not possible with the SRS system.

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.
- The SRS-ECU judges how severe a collision is by detecting signals from the left and right side impact sensors and the analog G-sensor. If the impact is over a predetermined level, the SRS-ECU outputs an ignition signal. At this time, if the safing G-sensor is on, the SRS air bag will inflate.

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

Required Special Tools:
- MB991223 (MB991222): Harness set (Probe)

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.

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







1. Disconnect SRS-ECU connector C-72 and data link connector C-29 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 the following terminals.
- SRS-ECU connector C-72 (terminal No.16) and data link connector C-29 (terminal No.7)
- 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-72, 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 terminals 3 and body ground.
- 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-72, 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 9, 12 and body ground
- Voltage should measure 9 volts or more.

Q: Is the measured voltage within the specified range?

YES: Replace SRS-ECU. Then go to Step 8.

NO: Go to Step 7.










STEP 5. Check the harness wires between SRS-ECU connector C-72 (terminal No.16) and data link connector C-29 (terminal No.7).

NOTE: After inspecting intermediate connector C-77 inspect the wiring harness. If the intermediate connector C-77 is damaged, repair or replace it. Refer to Harness Connector Inspection. Than go to Step 8.

Q: Are the harness wires between SRS-ECU connector C-72 (terminal No.16) and data link connector C-29 (terminal No.7) in good condition?

YES: Go to step 8.

NO: Repair the harness wires between SRS-ECU connector C-72 and data link connector C-29. Then go to Step 8.




STEP 6. Check the harness wires between SRS-ECU connector C-72 (terminal No.3) and ground.

Q: Are the harness wires between SRS-ECU connector C-72 (terminal No.3) and ground in good condition?

YES: Go to Step 8.

NO: Repair the harness wires between SRS-ECU connector C-72 and ground. Then go to Step 8.







STEP 7. Check the harness wires between SRS-ECU connector C-72 (terminal No.9 and 12) and ignition switch (IG1).

NOTE: After inspecting junction block connectors C-108, C-111, inspect the wiring harness.
If the intermediate connectors C-108, C-111 is damaged, repair or replace it. Refer to Harness Connector Inspection. Go to Step 8.

Q: Are the harness wires between SRS-ECU connector C-72 (terminal No.9 and 12) and ignition switch (IG1) in good condition?

YES: Go to Step 8.

NO: Repair the harness wires between SRS-ECU connector C-72 and ignition switch (IG1). 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 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).

NO: There is no action to be taken.