Symptom Diagnostic Procedures
INSPECTION PROCEDURE 1Communication 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 Tool:
MB991223 (MB991222): Harness Set (Probe)
STEP 1. Check the ground line at the SRS-ECU connector C-72 by backprobing.
1. Do disconnect the connector C-72.
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-72 and ground.
Q: Is the harness wire between SRS-ECU connector C-72 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 C-72 by backprobing.
1. Do disconnect the connector C-72.
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 (EGI) and SRS-ECU connector C-72.
NOTE: After inspecting intermediate connectors C-108 and C-111, inspect the wires. If intermediate connector C-108 or C-111 is damaged, repair or replace it. Refer to Harness Connector Inspection. Then go to Step 6.
Q: Are the harness wires between ignition switch (IG1) and SRS-ECU connector C-72 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-72 and data link connector C-29.
NOTE: After inspecting intermediate connector C-77, inspect the wires. If intermediate connector C-77 is damaged, repair or replace it. Refer to Harness Connector Inspection. Then go to step 6.
Q: Are the harness wires between SRS-ECU connector C-72 and data link connector C-29 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.