Manual A/C
DTC U1120: Failure Information on ECMCAUTION:
- 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.
- The engine control system-related DTC may be set when DTC U1120 is set. (For details refer to Intersystem Affiliated DTC Reference Table.) Diagnose the engine control system first when the engine control system-related DTC is set.
TROUBLE JUDGMENT
The A/C-ECU receives the vehicle speed signal and the coolant temperature from the ECM
TROUBLESHOOTING HINTS
- Malfunction of the A/C-ECU.
- Malfunction of the harness (CAN line is open/shorted to ground)
- Malfunction of connector
- Malfunction of the ECM
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 (Refer to Information Bus - Diagnosis).
STEP 2. Using scan tool MB991958, read the ECM
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 MFI - Diagnostic Trouble Code.
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
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 heater control.
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.