ABS CAN Time-Out

DTC U0121: ABS CAN time-out





CAUTION:
^ If there is any problem in the CAN bus lines, an incorrect DTC may be set. Prior to this diagnosis, always diagnose the CAN bus lines.
^ Before replacing the ECU, ensure that the communication circuit is normal.

OPERATION
AWD-ECU receives the data (such as ABS sensor signal, ABS operation signal) via the CAN bus lines. If it cannot receive such data, it interrupts the system.

DTC SET CONDITIONS
If the following conditions are met when the system is activated, AWD-ECU gradually switches the control from AWD to FWD, flashes the AWD/LOCK indicators alternately, and sets the DTC No. U0121.
^ IG1 voltage: 10 V or more
^ ABS signal (such as wheel speed signal) cannot be received.

TROUBLESHOOTING HINTS

Current trouble
^ Wiring harness or connector failure between ASC-ECU and the stop lamp switch
^ Malfunction of the ABS sensor
^ Malfunction of the stop lamp switch
^ ASC-ECU malfunction
^ AWD-ECU malfunction

Past trouble
^ Refer to "How to treat past trouble" for proceeding the diagnostics.

DIAGNOSIS
^ MB991958: Scan Tool (M.U.T.-III Sub Assembly)
^ MB991824: Vehicle Communication Interface
^ MB991827: M.U.T.-III USB Cable
^ MB991910: M.U.T.-III Main Harness A

STEP 1. Scan tool MB991958 CAN bus diagnostics
Use scan tool MB991958 to diagnose the CAN bus lines.
Q: Is the check result normal?
YES: Go to Step 2.
NO: Repair the CAN bus line. Then go to Step 2.

STEP 2. Scan tool MB991958 data list
Check the following service data. Data List Reference Table
^ Item 07: CAN system (ABS/ASC)
Q: Is the check result normal?
YES: Go to Step 3.
NO: Troubleshoot the ABS system.

STEP 3. Check the following connector:
Check the connectors below for improper engagement, terminal damage or terminal drawn in the connector case.
^ C-128 AWD-ECU connector
^ C-313, C-317 ETACS-ECU connector
Q: Are the connectors and terminals in good condition?
YES: Go to Step 4.
NO: Repair the connector(s) or terminal(s). Then go to Step 6.

STEP 4. Wiring harness check
Check the wiring harness between the C-128 AWD-ECU connector and the C-313 ETACS-ECU connector for damage or other problem.
Q: Is the wiring harness in good condition?
YES: Go to Step 5.
NO: Repair damage or other problem in the wiring harness. Then go to Step 6.

STEP 5. Check whether the DTC is reset.
1. Erase the DTC.
2. Turn the ignition switch from the LOCK (OFF) position to the ON position.
3. Check if the DTC is set.
Q: Is the DTC No. U0121 set?
YES: Replace AWD-ECU. Then go to Step 6.
NO: This diagnosis is complete.

STEP 6. Check whether the DTC is reset.
Q: Is the DTC No. U0121 set?
YES: Diagnose again from Step 1.
NO: This diagnosis is complete.