Inspection Procedure 2

Communication with Scan Tool MB991958 is not Possible with the SRS System Only.

Scan Tool Communication Circuit Part 1:

Scan Tool Communication Circuit Part 2:

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 front 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:
MB991222: Probe

STEP 1. Check the ground line at the SRS-ECU connector C-24.




1. Disconnect SRS-ECU connector C-24 and measure at the harness side (rear side).




2. Measure the resistance between terminal 7 and ground.
- It should be less than 2 Ohms.

Q: Does continuity exist?

YES: Go to Step 3.

NO: Go to Step 2.







STEP 2. Check the harness wires between SRS-ECU connector C-24 (terminal No.7) and ground.

NOTE: After inspecting intermediate connector D-13, inspect the wiring harness.

If intermediate connector D-13 is damaged, repair or replace it. Refer to Harness Connector Inspection.

Then go to Step 6.

Q: Is the harness wires between SRS-ECU connector C-24 (terminal No.7) and ground in good condition?

YES: Go to Step 6.

NO: Repair or replace the harness wire. Then go to Step 6.

STEP 3. Measure the ignition switch (IG1) line voltage at the SRS-ECU connector C-24.




1. Disconnect SRS-ECU connector C-24 and measure at the harness side (rear side).
2. Turn the ignition switch to the "ON position."




3. Measure the voltage between terminal 13 or 16 and ground.
- Voltage should measure 9 volts or more

Q: Is the voltage between terminal 13 or 16 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-24 (terminal No.13 and 16).

NOTE: After inspecting intermediate connectors D-08, D-13, D-14, inspect the wiring harness.

If intermediate connectors D-08, D-13, D-14 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-24 (terminal No.13 and 16) in good condition?

YES: Go to Step 6.

NO: Repair or replace the harness wire. Then go to Step 6.







STEP 5. Check the harness wires between SRS-ECU connector C-24 (terminal No.20) and data link connector C-69 (terminal No.7).

Q: Are the harness wires between SRS-ECU connector C-24 (terminal No.20) and data link connector C-69 (terminal No.7) in good condition?

YES: Replace the SRS-ECU. Then go to Step 6.

NO: Repair or replace the harness wire. Then go to Step 6.

STEP 6. Retest the system.

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

YES: The procedure is complete.(If no malfunctions are 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.