Symptom Diagnostic Procedures
INSPECTION PROCEDURE 1SRS-ECU Power Supply Circuit:
Connectors:
Communication with scan tool MB991502 is not possible with the SRS system.
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.
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
- Incorrect scan tool (MUT-II) ROM pack
DIAGNOSIS
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 Computers/Testing.
STEP 2. Check the communication line between the SRS-ECU and the scan tool.
1. Disconnect SRS-ECU connector C-12 and data link connector C-14 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 the continuity between the following terminals.
C-12 connector, terminal 20
C-14 connector, Terminal 7
- Should be less than 2 ohm.
Q: Is the continuity normal?
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-12, 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 the continuity between terminals 7 and body ground.
- Should be less than 2 ohm.
Q: Is the continuity normal?
YES: Go to Step 4.
NO: Got to Step 6.
STEP 4. Check the power supply circuit to the SRS-ECU.
1. Disconnect SRS-ECU connector C-12, and measure at the wiring harness side.
2. Connect the negative battery terminal.
3. 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.
4. Measure the voltage between terminals 13,16 and body ground
- Voltage should be 9 volts or more.
Q: Is the voltage normal?
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-12 and data link connector C-14.
NOTE: After inspecting intermediate connector C-19 inspect the wiring harness.
If the intermediate connector C-19 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-12 and data link connector C-14 in good condition?
YES: Go to Step 8.
NO: Repair the harness wires between SRS-ECU connector C-12 and data link connector C-14. Then go to Step 8.
STEP 6. Check the harness wires between SRS-ECU connector C-12 and ground.
Q: Are the harness wires between SRS-ECU connector C-12 and ground in good condition?
YES: Go to Step 8.
NO: Repair the harness wires between SRS-ECU connector C-12 and ground. Then go to Step 8.
STEP 7. Check the harness wires between SRS-ECU connector C-12 and ignition switch (IG1).
NOTE: After inspecting intermediate connector C-214, C-211 inspect the wiring harness.
If the intermediate connector C-214, C-211 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-12 and ignition switch (IG1) in good condition?
YES: Go to Step 8.
NO: Repair the harness wires between SRS-ECU connector C-12 and ignition switch (IG1). Then go to step 8.
STEP 8. 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.