EXPLANATION ABOUT THE M.U.T.-III CAN BUS DIAGNOSTICS


The M.U.T.-III CAN bus diagnostics carries out the three checks below automatically, and then displays current condition of the CAN bus lines according to the check results.

CAN Bus Line Diagnostic Flow




1.M.U.T.-III CAN bus diagnostics
The M.U.T.-III diagnoses CAN bus lines in accordance with the following strategy.
(1)
note When the M.U.T.-III carries out the voltage and resistance measurements, it will cease the communication between each of the ECU. If the M.U.T.-III can not cease the communication, it will carry out the voltage measurement.
Measuring voltage in CAN bus line
Diagnoses the power supply (such as wires of higher voltage than CAN communication line) and earthing (such as wires of lower voltage than CAN communication line) of CAN bus lines for short circuit by measuring the voltages between the CAN_L line or H line and body earth.
Terminal to be diagnosed
 Normal value
 Measurement value
 Trouble when the measurement value does not meet the normal value
 Note
Measuring the voltage between the CAN_L line and body earth
 1.0 V or more and 4.0 V or less
 Less than 1.0.
 Short to earth of the CAN_L line
 If the CAN_L or H line is shorted to earth or power supply, a diagnosis code may not be set.
More than 4.0.
 Short to power supply of the CAN_L line
Measuring the voltage between the CAN_H line and body earth
 1.0 V or more and 4.0 V or less
 Less than 1.0.
 Short to earth of the CAN_H line
More than 4.0.
 Short to power supply of the CAN_H line
(2) Measuring resistance in CAN bus line
Checks the terminator resistors (incorporated in the combination meter or the engine-ECU <M/T> or engine-A/T-ECU <A/T>), which are connected to each end of a CAN bus line, for breakage and a CAN bus main line for open circuit by measuring the resistance value between a CAN_L line and H line.
Normal value
 Measurement value
 Trouble when the measurement value does not meet the normal value
 Note
60 ± 10 Ω
 120 ± 20 Ω
 Trouble in a CAN main line or terminator resistor
 If only one terminator resistor is broken at either side, the CAN communication will continue although noise suppression is deteriorated.
If a CAN main bus line is open circuit, the CAN communication is suspended at that open circuit point.
No continuity
 Trouble in CAN main bus line or between the diagnosis connector and main bus line
 -
2 Ω or less
 CAN bus line (between CAN_L and H lines) is shorted
 If a CAN bus line is shorted, all ECUs cease communicating each other (This fail-safe function is called "Bus off").
Other than above
 Poorly engaged connector
 -
(3) Checking the communication condition of ECUs
The M.U.T.-III narrows down troubles in circuit by itself. Its strategy are as follows.


ECU which cannot communicate with the M.U.T.-III
 Possible trouble spot
 Logic for narrowing down trouble spot
ECU.
 CAN bus line (a) and power supply system to ECU.
 ECU A communicates with the M.U.T.-III via CAN bus lines (a) and (b). The M.U.T.-III 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.
 CAN bus line (g) and power supply system to ECU.
 ECU C communicates with the M.U.T.-III via CAN bus lines (b), (c), (d) and (g). The M.U.T.-III 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.
 Trouble in CAN bus line (d)
 ECUs C ad D communicate with the M.U.T.-III via CAN bus lines (b), (c), (d), (e) and (g). The M.U.T.-III 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 D. CAN bus line (d) is shared by ECUs C and D when they communicate with the M.U.T.-III, 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.
 CAN bus line (e) or (f) or power supply system to ECU B or.
 ECUs C ad D communicate with the M.U.T.-III via CAN bus lines (b), (c), (d), (e) and (g). The M.U.T.-III 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 D.

2.If diagnosis code related to CAN communication is set as past trouble, isolate opens as described below.
note If you pinpoint trouble spot according to diagnosis code, you should use time-out diagnosis code. diagnosis code related to failure information is set when the data to be set contains an error, so CAN bus line itself is probably normal.
note Time-out diagnosis code codes are stored in each ECU memory individually. Therefore, it is possible that these diagnosis code codes have not been set simultaneously. If the trouble spot can not be found when you diagnose by judging from multiple diagnosis code codes, check the communication lines between each ECU.
Diagnosis code to be set
 Possible trouble spot
 Logic for narrowing down trouble spot
Time-out diagnosis code associated with ECU D is stored in ECU A, ECU B and ECU.
 Trouble in CAN bus line (e) and power supply system to ECU.
 When time-out diagnosis code associated with ECU D is stored in ECU A, B and C, or time-out diagnosis code associated with ECUs A, B and C is stored in ECU D, or "bus off" diagnosis code is stored in ECU D, CAN bus line (e) is suspected. When diagnosis code is not stored in ECU D, the power supply to ECU D is suspected.

Time-out diagnosis code associated with ECUs A, B and C is stored in ECU.
"Bus off" diagnosis code is stored in ECU.
Time-out diagnosis code associated with ECU A is stored in ECUs B, C and.
 Trouble in CAN bus line (a) and power supply system to ECU.
 When time-out diagnosis code associated with ECU A is stored in ECUs B, C and D, or time-out diagnosis code associated with ECUs B, C and D is stored in ECU A, or "bus off" diagnosis code is stored in ECU A, CAN bus line (a) or (c) is suspected. When diagnosis code is not stored in ECU A, the power supply to ECU A is suspected.


Time-out diagnosis code associated with ECUs B, C and D is stored in ECU.
"Bus off" diagnosis code is stored in ECU.
Time-out diagnosis code codes associated with ECUs C and D are stored in ECU A and ECU.
 Trouble in CAN bus line (d)
 If time-out diagnosis code 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 diagnosis code codes associated with ECUs A and B are stored in ECU C and ECU.