2. Communication With Scan Tool Impossible (SRS 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-44 by backprobing.
1. Do disconnect the connector C-44.
2. Check the continuity between terminal 3 and the ground by backprobing.

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

YES: Go to Step 3.

NO: Go to Step 2.




STEP 2. Check the harness wires between SRS-ECU connector C-44 and ground.

Q: Is the harness wires between SRS-ECU connector C-44 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-44 by backprobing.
1. Do disconnect the connector C-44.
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.

Connector - C-73, C-77:

STEP 4. Check the harness wires between ignition switch (IG1) and SRS-ECU connector C-44.

NOTE: After inspecting intermediate connectors C-73, C-77 inspect the wiring harness. If intermediate connectors C-73, C-77 are damaged, repair or replace them. 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 C-44 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.

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 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 How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunction.)

NO: There is no action to be taken.