3)Common Faults
（1）CAN HI Short Circuit to Power,is shown in Fig.2.10.
Fig.2.10 CAN High Short Circuit to Power
The inspection method is as follows:
①Turn on the ignition switch,namely,all the joints of the whole car are electrified.
②Adjust the universal meter to voltage range;
③Connect the red probe to the diagnostic interface pin CAN HI,connect the black probe to the ground(GND),and then test the voltage of CAN HI,If the voltage value is around 2.6 V,it is normal;and if the voltage value is above 5 V or abore the voltage of storage battery,it means that CAN HI is short-circuited with power.
（2）CAN LO Short Circuit to Power is shown in Fig.2.11.
Fig.2.11 CAN LO Short Circuit to Power
The inspection method is as follows:
①Turn on the ignition switch,namely,all the joints of the whole car are electrified.
②Adjust the universal meter to voltage range;
③Connect the red probe to the diagnostic interface pin CAN LO,connect the black probe to the ground(GND),and then test the voltage of CAN LO,If the voltage value is around 2.4 V,it is normal;and if the voltage value is above 5 V or above the voltage of storage battery,it means that CAN LO is short-circuited with power.
(3)CAN High Short Circuit to GND is shown in Fig.2.12.
Fig.2.12 CAN High Short Circuit to the Ground
The inspection method is as follows:
①Disconnect the positive pole of the battery,making power of the whole car be cut off.
②Adjust the universal meter to resistance range.
③Connect the red probe to the diagnostic interface pin CAN HI,connect the black probe to the ground(GND),and then test the resistance to ground of CAN HI;If the resistance is above 100 kΩ,it is normal,and if the resistance value is too small,it means that CAN HI is short-circuited with the ground.
④CAN LO Short Circuit to GND,is shown in Fig.2.13.
Fig.2.13 CAN Low Short Circuit to the Ground
The inspection method is as follows:(https://www.daowen.com)
①Disconnect the positive pole of the battery,making power of the whole car be cut off.
②Adjust the universal meter to resistance range;
③Connect the red probe to the diagnostic interface pin CAN LO,connect the black probe to the ground(GND),and then test the resistance to ground of CAN LO;If the resistance is above 100 kΩ,it is normal,and if the resistance value is too small,it means that CAN LO is short-circuited with the ground.
(5)Short Circuit of CAN High and CAN Low,is shown in Fig.2.14.
Fig.2.14 Short Circuit of CAN High and CAN Low
The inspection method is as follows:
①Disconnect the positive pole of the battery,making power of the whole car be cut off.
②Adjust the universal meter to resistance range;
③Connect the red probe and black probe to the end of diagnostic interface pin CAN_H and CAN_L,to test whether CAN_H and CAN_L are short-circuited or not.120 Ωresistance is installed on the both ends of CAN and CAN internet is connected,so the normal resistance between CAN High and CAN Low is around 60 Ω.If the result is abnormal,check whether short circuit or open circuit exists between CAN HI and CAN LO.
（6）Breakdown of CAN High or CAN Low,is shown in Fig.2.15.
Fig.2.15 Breakdown of CAN Low
The inspection method is as follows:
①Disconnect the positive pole of the battery,making power of the whole car be cut off.
②Adjust the universal meter to resistance range.
③Connect the red probe and black probe to the ends of diagnostic interface pin CAN HI and CAN LO,to test the resistance value between CAN HI and CAN LO and it whll be 60 Ω.If the resistance value is 120 Ω,there is a breakdown in CAN HI or CAN LO.
Note:
①Sometimes,CAN internet terminal resistance(the resistance inside the computer or external resistance)itself goes wrong,resulting in the resistance value between CAN HI or CAN LO not being around 60 Ω.
②In the maintenance process,the common fault“CAN Joint A Bus goes wrong”is not necessarily the fault of ECU itself,normally,it is caused by the abnormal voltage of CAN Internet of the whole car and other CAN control failures.
③If nothing abnormal appears in previous tests,“Bus Communication Failure”still appears on the panel,then according to the experience,the reasons may be as follows:
a.Power of certain joint is broken down,causing no message to be sent out.
b.Certain joint doesn’t comply with the communication rate of 250 kb/s(CAN wire harness is broken down)
c.The inside of certain joint itself goes wrong.