Can Bus Line Diagnostic Flow

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

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.

   Reference circuit

   Fig 2: CAN Reference Circuit Diagram

   Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

ECU which cannot communicate with the scan tool	Possible trouble spot	Logic for narrowing down trouble spot
ETACS-ECU and all ECUs	CAN bus line (h) and power supply system to ETACS-ECU	The 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 A	CAN bus line (a) and power supply system to ECU A	ECU 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 C	CAN bus line (g) and power supply system to ECU C	The 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 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 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 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 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.

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