2. Communication With Scan Tool Impossible (SRS Only)

Communication with scan tool MB991502 is not possible with the SRS system only

Inspection Procedure 2 - SRS-ECU Power Supply Circuit:

Connector - C-23, C-44, C-73, C-77:

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-44
1. Disconnect SRS-ECU connector C-44 and use special tool MB991223 (MB991222) to measure at the harness side.
2. Check the continuity between terminal 3 and the ground.
Should be less than 2 ohm

If open circuit, go to Step 2.
If less than 2 ohms, go to Step 3.




STEP 2. Check the harness wires between SRS-ECU connector C44 and ground
If harness wires are in good condition, go to Step 5. If any harness wires between SRS-ECU connector C-44 and the ground are damaged, repair them. Then go to Step 5.




STEP 3. Check the ignition switch (IG1) line at the SRS-ECU connector C-44
1. Turn the ignition switch "ON."
2. Use special tool MB991223 (MB991222) to measure the voltage between terminal 9 and the ground.
Voltage should be 9 volts or more
3. Measure the voltage between terminal 12 and the ground.
Voltage should be 9 volts or more

If less than 9 volts, go to Step 4.
If 9 volts or more, go to Step 5.




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

Connector - C-73, C-77:

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 General Information, 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.







STEP 5. Check the harness wires between SRS-ECU connector C-44 and data link connector C-23
If the harness wires are in good condition, replace the SRS-ECU. Then go to Step 6.
If any harness wires between SRS-ECU connector C-44 and data link connector C-23 are damaged, repair them. Then go to Step 6.

STEP 6. Check trouble symptoms
Confirm that the scan tool communicates normally with the SRS system.