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EXPLANATION ABOUT THE SCAN TOOL (M.U.T.-III) CAN BUS DIAGNOSTICS
 
 
Scan tool MB991958 CAN bus diagnostics carries out the two checks below automatically, and then displays current condition of the CAN bus lines according to the check results.
 
 
CAN BUS LINE DIAGNOSTIC FLOW
1. Scan tool CAN bus diagnostics
Scan tool MB991958 diagnoses CAN bus lines in accordance with the following strategy.
noteAfter you determine whether the CAN-C lines are in good condition, then determine whether the CAN-B lines are in good condition. Then confirm each judgment result on the scan tool screen.
(1) Check that the ETACS-ECU sets a diagnostic trouble code.
You can narrow down the points to be diagnosed by confirming an ETACS-ECU diagnostic trouble code.
(2) Checking the communication condition of ECUs
Scan tool MB991958 narrows down troubles in circuit by itself. Its strategies are as follows.
ECU which cannot communicate with the scan toolPossible trouble spotLogic for narrowing down trouble spot
ETACS-ECU and all ECUsCAN bus line (h) and power supply system to ETACS-ECUThe ETACS-ECU and the other ECUs use the CAN bus line (h) when they communicate with scan tool MB991958. Since none of the ETACS-ECU and the other ECUs can communicate with scan tool MB991958, CAN bus line (h) or the power supply circuit to the ETACS-ECU may be faulty.
ECU ACAN bus line (a) and power supply system to ECU AECU A communicates with the scan tool MB991958 via CAN bus lines (a) and (b). Scan tool MB991958 judges that CAN bus line (b) is normal, because it can communicate with other ECUs. Possible trouble may be present in CAN bus line (a) or the power supply system to ECU A.
ECU CCAN bus line (g) and power supply system to ECU CThe ECU C communicates with scan tool MB991958 via CAN bus lines (b), (c), (d) and (g). Scan tool MB991958 judges that CAN bus lines (b), (c) and (d) are normal, because it can communicate with ECUs B and D. Possible trouble may be present in CAN bus line (g) or the power supply system to ECU C.
ECU C and ECU DTrouble in CAN bus line (d)ECUs C and D communicate with scan tool MB991958 via CAN bus lines (b), (c), (d), (e) and (g). Scan tool MB991958 judges that CAN bus lines (b) and (c) are normal, because it can communicate with ECU B. Possible trouble may be present in CAN bus line (d), (e) or (g) or the power supply system to ECU C and ECU D. CAN bus line (d) is shared by ECUs C and D when they communicate with scan tool MB991958, so CAN bus line (d) is suspected as ultimate cause. CAN bus line (g) or (e) and power supply systems to ECU C or D are also suspected as second cause.
ECU B and ECU DCAN bus line (e) or (f) or power supply system to ECU B or DECUs B and D communicate with scan tool MB991958 via CAN bus lines (b), (c), (d), (e) and (f). Scan tool MB991958 judges that CAN bus lines (b), (c) and (d) are normal, because it can communicate with ECU C. Possible trouble may be present in CAN bus line (f) or (e) or the power supply system to ECU B or ECU D.
All ECU (except ETACS-ECU)CAN bus line (b)The other ECUs except the ETACS-ECU use CAN bus lines (b) and (h) when they communicate with scan tool MB991958. It must be assumed that CAN bus line (b) is defective since the ETACS-ECU can communicate with scan tool MB991958.
2. Pinpoint possible trouble spot according to diagnostic trouble code
If diagnostic trouble code related to CAN communication is set as past trouble, isolate opens as described below.
noteIf you pinpoint trouble spot according to diagnostic trouble code, you should use time-out diagnostic trouble code. Diagnostic trouble code related to failure information is set when the data to be set contains an error, so CAN bus line itself is probably normal.
noteTime-out diagnostic trouble codes are stored in each ECU memory individually. Therefore, it is possible that these diagnostic trouble codes have not been set simultaneously. If the trouble spot cannot be found when you diagnose by judging from multiple diagnostic trouble codes, check the communication lines between each ECU.
Diagnostic trouble code to be setPossible trouble spotLogic for narrowing down trouble spot
Time-out diagnostic trouble code associated with ECU D is stored in ECU A, ECU B and ECU C.Trouble in CAN bus line (e) and power supply system to ECU DWhen time-out diagnostic trouble code associated with ECU D is stored in ECU A, B and C, or time-out diagnostic trouble code associated with ECUs A, B and C is stored in ECU D, or "bus off" diagnostic trouble code is stored in ECU D, CAN bus line (e) is suspected. When diagnostic trouble code is not stored in ECU D, the power supply to ECU D is suspected.
Time-out diagnostic trouble code associated with ECUs A, B and C is stored in ECU D.
"Bus off" diagnostic trouble code is stored in ECU D.
Time-out diagnostic trouble code associated with ECU A is stored in ECUs B, C and D.Trouble in CAN bus line (a) or (c) and power supply system to ECU A.When time-out diagnostic trouble code associated with ECU A is stored in ECUs B, C and D, or time-out diagnostic trouble code associated with ECUs B, C and D is stored in ECU A, or "bus off" diagnostic trouble code is stored in ECU A, CAN bus line (a) or (c) is suspected. When diagnostic trouble code is not stored in ECU A, the power supply to ECU A is suspected.
Time-out diagnostic trouble code associated with ECUs B, C and D is stored in ECU A.
"Bus off" diagnostic trouble code is stored in ECU A.
Time-out diagnostic trouble codes associated with ECUs C and D are stored in ECU A and ECU B.Trouble in CAN bus line (d)If time-out diagnostic trouble codes associated with ECUs C and D are stored in ECUs A and B, or time-out codes associated with ECUs A and B are stored in ECUs C and D, CAN bus line (d) is suspected. CAN bus line (g) or (e) and power supply systems to ECU C or D are also suspected as second cause.
Time-out diagnostic trouble codes associated with ECUs A and B are stored in ECU C and ECU D.
Time-out diagnostic trouble codes associated with ECUs A, B, C and D are stored in ETACS-ECU.Trouble in CAN bus line (b)It must be assumed that a fault was present in CAN bus line (b) when the ETACS-ECU has set a time-out diagnostic trouble code for ECU A, B, C or D.
Time-out diagnostic trouble codes associated with ETACS-ECU is stored in ECU A, B, C and ECU D.