Explanation About The Scan Tool (M.U.T.-III) Can Bus Diagnostics

Scan tool MB991958 CAN bus diagnostics carries out the three checks below automatically, and then displays the current condition of the CAN bus lines according to the check results.

Fig 1: Scan Tool MB991958 CAN Bus Diagnostic Chart

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Scan tool CAN bus diagnostics

Scan tool MB991958 diagnoses CAN bus lines in accordance with the following strategy.

Measuring voltage in CAN bus line Diagnoses the power supply (such as wires of higher voltage than CAN communication line) and grounding (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 ground.

VOLTAGE REFERENCE CHART

MEASURE THE VOLTAGE	NORMAL VALUE	MEASUREMENT VALUE	TROUBLE WHEN THE MEASUREMENT VALUE DOES NOT MEET THE NORMAL VALUE	NOTE
Between the CAN_L line and body ground	1.0 V or more and 4.0 V or less	Less than 1.0 V	Short to ground of the CAN_L line	If the CAN_L or H line is shorted to ground or power supply, a DTC may not be set.
Between the CAN_L line and body ground	1.0 V or more and 4.0 V or less	More than 4.0 V	A short to the power supply of the CAN_L line
Between the CAN_H line and body ground	1.0 V or more and 4.0 V or less	Less than 1.0 V	Short to ground of the CAN_H line
Between the CAN_H line and body ground	1.0 V or more and 4.0 V or less	More than 4.0 V	A short to the power supply of the CAN_H line

Measuring resistance in CAN bus line Checks the terminator resistors (incorporated in the combination meter, the engine control module (ECM) <M/T> or the powertrain control module (PCM) <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.

RESISTANCE REFERENCE CHART

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 bus 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 data link connector and main bus line	-
	2 ohms 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	-

Checking the communication condition of ECUs

Scan tool MB991958 narrows down troubles in circuit by itself. Its strategy is as follows:

REFERENCE CIRCUIT

Fig 2: ECUs Communication Diagram

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COMMUNICATION CONDITION OF ECUS

ECU WHICH CAN NOT COMMUNICATE WITH THE SCAN TOOL	POSSIBLE TROUBLE SPOT	LOGIC FOR DETERMINING DOWN TROUBLE SPOT
ECU A	CAN bus line (a) and power supply system to ECU A	ECU A communicates with 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.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
ECU C	CAN bus line (g) and power supply system to ECU C	ECU C communicates with scan tool MB991958 via CAN bus lines (b), (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.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
ECU C and ECU D	Trouble 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 D or C. 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.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
ECU B and ECU D	CAN bus line (e) or (f) or power supply system to ECU B or D	ECUs 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 D.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

If DTC code related to CAN communication is set as past trouble, isolate opens as described below.

NOTE: If you pinpoint trouble spot according to DTC code, you should use time-out DTC code. DTC 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 DTC codes are stored in each ECU memory individually. Therefore, it is possible that these DTC codes have not been set simultaneously. If the trouble spot cannot be found when you diagnose by judging from multiple DTC codes, check the communication lines between each ECU.

ECU COMMUNICATION REFERENCE CHART

DTC CODE TO BE SET	POSSIBLE TROUBLE SPOT	LOGIC FOR DETERMINING TROUBLE SPOT
Time-out DTC 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 D	When time-out DTC code associated with ECU D is stored in ECU A, B and C, or time-out DTC code associated with ECUs A, B and C is stored in ECU D, or "bus off" DTC code is stored in ECU D, CAN bus line (e) is suspected. When DTC code is not stored in ECU D, the power supply to ECU D is suspected.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Time-out DTC code associated with ECUs A, B and C is stored in ECU D
"Bus off" DTC code is stored in ECU D
Time-out DTC 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 DTC code associated with ECU A is stored in ECUs B, C and D, or time-out DTC code associated with ECUs B, C and D is stored in ECU A, or "bus off" DTC code is stored in ECU A, CAN bus line (a) or (c) is suspected. When DTC code is not stored in ECU A, the power supply to ECU A is suspected.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Time-out DTC code associated with ECUs B, C and D is stored in ECU A
"Bus off" DTC code is stored in ECU A
Time-out DTC codes associated with ECUs C and D are stored in ECU A and ECU B	Trouble in CAN bus line (d)	If time-out DTC 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.	Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Time-out DTC codes associated with ECUs A and B are stored in ECU C and ECU D	Trouble in CAN bus line (d)		Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.