Symptom Procedures

Communication with scan tool MB991502 is not possible with the SRS system only

SRS-ECU Power Supply Circuit:

Connectors:

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 MB991502 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

STEP 1. Check the ground line at the SRS-ECU connector C-137 by backprobing.
1. Do disconnect the connector C-137.




2. Check the continuity between terminal 3 and the ground by backprobing.

Q: Is the resistance between terminal 3 and ground less than 2 ohm?

YES: Go to Step 3.
NO: Go to Step 2.




STEP 2. Check the harness wires between SRS-ECU connector C-137 and ground.

Q: Is the harness wires between SRS-ECU connector C-137 and ground in good condition?

YES: Go to Step 5.
NO: Repair them. Then go to Step 5.

STEP 3. Check the ignition switch (IG1) line at the SRS- ECU connector C-137 by backprobing.
1. Do disconnect the connector C-137.
2. Turn the ignition switch to "ON" position.




3. Measure the voltage between terminal 9 and the ground.
- Voltage should be 9 volts or more

4. Measure the voltage between terminal 12 and the ground by backprobing.
- Voltage should be 9 volts or more

Q: Is the voltage between terminal 9,12 and the ground 9 V or more?

YES: Go to Step 5.
NO: Go to Step 4.







STEP 4. Check the harness wires between ignition switch (IG1) and SRS-ECU connector C-137.

NOTE: After inspecting intermediate connectors C-301, C-303 inspect the wiring harness. If intermediate connectors C-301, C-303 are damaged, repair or replace them. Refer to Harness Connector Inspection. Then go to Step 6.

Q: Are the harness wires between ignition switch (IG1) and SRS-ECU connector C-137 in good condition?

YES: Go to Step 6.
NO: Repair them. Then go to Step 6.







STEP 5. Check the harness wires between SRS-ECU connector C-137 and data link connector C-26.

Q: Are the harness wires between SRS-ECU connector C-137 and data link connector C-26 in good condition?

YES: Replace the SRS-ECU. Then go to Step 6.
NO: Repair them. Then go to Step 6.

STEP 6. Check symptoms

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

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