Symptom Diagnostic Procedures

INSPECTION PROCEDURE 1

Communication with Scan Tool MB991502 is not Possible. (Communication 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

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

STEP 1. Check the ground line at the SRS-ECU connector C-44 by backprobing.
1. Do disconnect the connector C-44.




2. Check the continuity between terminal 3 and 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 wire between SRS-ECU connector C-44 and ground.

Q: Is the harness wire between SRS-ECU connector C-44 and ground in good condition?

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

STEP 3. Check the ignition switch (IG1) line at the SRS-ECU connector C44 by backprobing.
1. Do disconnect the connector C-44.
2. Turn the ignition switch to the "ON" position.




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

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

Q: Is the voltage between terminal 9, 12 and ground 9 volts 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-44.

NOTE: After inspecting intermediate connectors C-57, C-84 and C-87, inspect the wires. If intermediate connector C-57, C-84 or C-87 is damaged, repair or replace it. Refer to Harness Connector Inspection. Then go to Step 6. If harness wires are in good condition, go to Step 6. If any harness wires between the ignition switch (IG1) and SRS-ECU connector C-44 are damaged, repair them. Then go to Step 6.

Q: Are the harness wires between ignition switch (IG1) and SRS-ECU connector C44 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-44 and data link connector C-23.

NOTE: After inspecting intermediate connector C-57, inspect the wires. If intermediate connector C-57 is damaged, repair or replace it. Refer to Harness Connector Inspection.

Q: Are the harness wires between SRS-ECU connector C-44 and data link connector C-23 in good condition?

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

STEP 6. Check symptom.

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 How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunction.)
NO: There is no action to be taken.