Inspection Procedure 5: Brake Warning Light Does Not Illuminate When the Ign Switch Is Turned to on Position (Engine Stopped)

Inspection Procedure 5: Brake Warning Light does not Illuminate when the Ignition Switch is Turned to ON Position (Engine Stopped).

CAUTION
^ If there is any problem in the CAN bus lines, an incorrect DTC may be set. Prior to this diagnosis, diagnose the CAN bus lines.
^ Whenever ECU is replaced, ensure that the CAN bus lines are normal.
^ When the hydraulic unit (integrated with ASC-ECU) is replaced, always carry out the calibration of the steering wheel sensor, the G and yaw rate sensor and brake fluid pressure sensor.

OPERATION
^ ASC-ECU sends the illumination request signal of the break warning light to the combination meter through ETACS-ECU via the CAN communication.
^ ASC-ECU illuminates brake warning light via ETACS-ECU for approximately 3 seconds for bulb check with the ignition switch turned to the ON position.

COMMENTS ON TROUBLE SYMPTOM
This may be caused by faults in the CAN bus line, ETACS-ECU, combination meter or ASC-ECU.

PROBABLE CAUSES
^ Damaged wiring harness and connectors
^ Malfunction of ETACS-ECU
^ Malfunction of the combination meter
^ ASC-ECU malfunction

DIAGNOSIS

Required Special Tools:
^ MB991958: Scan Tool (M.U.T.-III Sub Assembly)
^ MB991824: Vehicle Communication Interface (V.C.I.)
^ MB991827: M.U.T.-III USB Cable
^ MB991910: M.U.T.-III Main Harness A

STEP 1. M.U.T.-III CAN bus diagnostics
Use scan tool to diagnose the CAN bus lines.

Q: Is the check result normal?
YES: Go to Step 2.
NO: Repair the CAN bus lines. Then go to Step 7.

STEP 2. M.U.T.-III DTC
Use scan tool to check the DTC for the ASC system.

Q: Is the DTC set?
YES: Carry out the diagnosis for the DTC.
NO: Go to Step 3.

STEP 3. Actuator test of combination meter
Perform the following actuator test by the combination meter system, and check if the brake warning light illuminates.
^ Item No.07: Indicator 1

Q: Is the check result normal?
YES: Go to Step 4.
NO: Diagnose the combination meter. Then go to Step 7.

STEP 4. M.U.T.-III other system DTC
Using scan tool, check that DTC U0141 is not set by the combination meter system.

Q: Is the DTC set?
YES: Diagnose the combination meter. Then go to Step 7.
NO: Go to Step 5.

STEP 5. M.U.T.-III other system DTC
Using scan tool, check that DTC U0121 is not set by the ETACS system.

Q: Is the DTC set?
YES: Perform troubleshooting on ETACS-ECU. Then go to Step 7.
NO: Go to Step 6.

STEP 6. Retest the system.

Q: Does the brake warning light turn ON and OFF
YES: Intermittent malfunction.
NO: Replace the hydraulic unit (ASC-ECU), and then go to Step 7.

STEP 7. Retest the system.

Q: Does the brake warning light turn ON and OFF normally?
YES: The procedure is complete.
NO: Return to Step 1.