Automatic A/C Diagnosis

DTC U1120: Failure Information on ECM [M/T] or PCM [A/T] (Related to Engine)

CAUTION:
- If DTC U1120 is set in the A/C-ECU, diagnose the CAN main bus line.
- Whenever the ECU is replaced, ensure that the communication circuit is normal.

TROUBLE JUDGEMENT
The A/C-ECU receives the vehicle speed signal and the coolant temperature from the ECM [M/T] or PCM [A/T] via CAN bus lines. If failure information is sent to the vehicle speed signal and the coolant temperature, DTC U1120 will be set.

TROUBLESHOOTING HINTS
- Malfunction of connector.
- Malfunction of the harness (CAN line is open/shorted to ground)
- Malfunction of the A/C-ECU.
- Malfunction of the ECM [M/T] or PCM [A/T].

DIAGNOSIS

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

STEP 1. Using scan tool MB991958, diagnose the CAN bus line.







CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

1. Connect scan tool MB991958 to the data link connector.
2. Turn the ignition switch to the "ON" position.
3. Diagnose the CAN bus line.
4. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the CAN bus line found to be normal?

YES : Go to Step 2.

NO : Repair the CAN bus line.

STEP 2. Using scan tool MB991958, read the ECM [M/T] or PCM [A/T] diagnostic trouble code.

Check whether any engine or automatic transaxle DTCs are set or not.

1. Turn the ignition switch to the "ON" position.
2. Check for engine and automatic transaxle DTCs.
3. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Go to Step 3.

NO : Refer to Diagnostic Trouble Code Diagnostic Trouble Code Descriptions [3.8L ENGINE].

STEP 3. Using scan tool MB991958, check for any diagnostic trouble code.

Check if a DTC, which relates to CAN communication-linked systems below, is set.
- Combination meter
DTC indicating a time-out error related to the engine or automatic transaxle control system
- ETACS
DTC indicating a time-out error related to the engine or automatic transaxle control system

1. Turn the ignition switch to the "ON" position.
2. Check for a DTC related to the relevant system.
3. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Go to Step 5.

NO : Go to Step 4.

STEP 4. Replace the ECM [M/T] or PCM [A/T] and then recheck the DTC.

Check again if the DTC is set.

1. Turn the ignition switch to the "ON" position.
2. Check if the DTC is set.
3. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : The procedure is complete.

NO : Replace the A/C-ECU.

STEP 5. Replace the A/C-ECU and then recheck the DTC.

Check again if the DTC is set.

1. Connect scan tool MB991958 to the data link connector
2. Turn the ignition switch to the "ON" position.
3. Check if the DTC is set.
4. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : The procedure is complete.

NO : Go to Step 1.