Mitsubishi 380. Manual — part 192
AUTO A/C DIAGNOSIS
HEATER, AIR CONDITIONING AND VENTILATION
55-64
DTC U1120: Failure Information on Electronic control unit (Related to Engine)
CAUTION
If DTC U1120 is set in the A/C-ECU, diagnose the
CAN main bus line.
CAUTION
Whenever the ECU is replaced, ensure that the
communication circuit is normal.
CAUTION
The engine control system- related DTC may be
set when DTC U1120 is set. (For details refer to
GROUP 00, Intersystem Affiliated DTC Reference
Table
.) Diagnose the engine control sys-
tem first when the engine control system- related
DTC is set.
.
TROUBLE JUDGMENT
The A/C-ECU receives engine control sys-
tem-related signal from the Electronic control unit by
the CAN bus lines. If a fail-safe related data is con-
tained in the signal from the Electronic control unit,
DTC U1120 will be stored.
.
TECHNICAL DESCRIPTION (COMMENT)
Current trouble
• The wiring harness wire or connectors may have
loose, corroded, or damage terminals, or termi-
nals pushed back in the connector, the Electronic
control unit, or the A/C-ECU may be defective.
Past trouble
• If DTC U1120 is stored as a past trouble, carry
out diagnosis with particular emphasis on wiring
and connector(s) in the CAN bus line between
the A/C-ECU and the Electronic control unit, and
the power supply system to the Electronic control
unit. If the connectors and wiring are normal, and
obviously the ECU is the cause of the trouble,
replace the ECU. If in doubt, do not replace the
ECU.
NOTE: For a past trouble, you cannot find it by
the diagnostic tool CAN bus diagnostics even if
there is a failure in CAN bus lines. In this case,
refer to GROUP 00, How to Use Troubleshoot-
ing/Inspection Service Points
− How to Cope with
Intermittent Malfunctions
.) and check the
CAN bus lines. You can narrow down the possi-
ble cause of the trouble by referring to the DTC,
which is set regarding the CAN communica-
tion-linked ECUs (Refer to GROUP 54C
− Expla-
nation about the diagnostic tool CAN bus
diagnostics
.
TROUBLESHOOTING HINTS
• Malfunction of the A/C-ECU.
• Malfunction of the Electronic control unit.
• Refer to component locations GROUP-
• Refer to configuration diagrams GROUP-
• Refer to circuit diagrams GROUP-
DIAGNOSIS
Required Special Tool:
• : Diagnostic Tool (MUT-III Sub Assembly)
• MB991824: Vehicle Communication Interface (V.C.I.)
• MB991827: MUT-III USB Cable
• MB991910: MUT-III Main Harness A (Vehicles with CAN
communication system)
AUTO A/C DIAGNOSIS
HEATER, AIR CONDITIONING AND VENTILATION
55-65
STEP 1. Using diagnostic tool , diagnose the CAN bus line.
CAUTION
To prevent damage to diagnostic tool (MUT-III), always turn
the ignition switch to the "LOCK" (OFF) position before
connecting or disconnecting diagnostic tool (MUT-III).
(1) Connect diagnostic tool 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 GROUP 54C,
).
STEP 2. Using diagnostic tool , read the Electronic control
unit diagnostic trouble code.
Check whether any engine or automatic transmission DTCs are
set or not.
(1) Turn the ignition switch to the "ON" position.
(2) Check for engine and automatic transmission 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 GROUP 13A, Diagnostic Trouble Code
00DB076A
MB991910
DATA LINK
CONNECTOR
MB991824
MB991827
00DB076A
MB991910
DATA LINK
CONNECTOR
MB991824
MB991827
AUTO A/C DIAGNOSIS
HEATER, AIR CONDITIONING AND VENTILATION
55-66
STEP 3. Using diagnostic tool , 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 transmission control system
• ETACS
DTC indicating a time-out error related to the engine or
automatic transmission control system
• Multi-center display unit
DTC indicating a time-out error related to the engine or
automatic transmission 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. Recheck for diagnostic trouble code.
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 : Replace the Electronic control unit. On completion,
check that the DTC is not reset.
NO : There is an intermittent malfunction such as poor
engaged connector(s) or open circuit (Refer to
GROUP 00, How to Use Troubleshooting/Inspection
Service Points
− How to Cope with Intermittent
Malfunctions
).
00DB076A
MB991910
DATA LINK
CONNECTOR
MB991824
MB991827
00DB076A
MB991910
DATA LINK
CONNECTOR
MB991824
MB991827
AUTO A/C DIAGNOSIS
HEATER, AIR CONDITIONING AND VENTILATION
55-67
STEP 5. Recheck for diagnostic trouble code.
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 : A poor connection, open circuit or other intermittent
malfunction is present in the lines between the
Electronic control unit and the A/C-ECU (Refer to
GROUP 00, How to Use Troubleshooting/Inspection
Service Points
− How to Cope with Intermittent
Malfunctions
.)
NO : Replace the A/C-ECU. On completion, check that the
DTC is not reset.
00DB076A
MB991910
DATA LINK
CONNECTOR
MB991824
MB991827
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