Classic CAN bus field failure

CAN L is short-circuited to the power supply (positive pole)

When a fault such as CAN L short-circuiting the power supply (positive pole) occurs, depending on the fault-tolerant characteristics of the CAN bus, the entire CAN network may be unable to communicate or a related fault code may be generated.

Since CAN L is short-circuited to the power supply, the CAN H voltage is also set to 12 V. The bus waveform when CAN L is short-circuited to the power supply is shown in the figure below.

Actually measure the voltage of the CAN wire. If the voltage of CAN L and CAN H wires are both about 12V, it means that this type of fault has occurred.

Cause of the fault: If the fault is not caused by the short circuit of the CAN L wire to the external power supply, then this fault may be caused by the damage of the CAN transceiver inside the control module. The fault finding method is the same as above.

CAN H open circuit

When the CAN H wire of a control module is broken, the control module will be unable to communicate, but other control modules can still communicate. The fault code of this faulty module may be read in other control modules. If the CAN H wires of multiple control modules are broken, the communication functions of these control modules will be affected.

If the faulty control module has a terminal resistor, the resistance measurement method can be used to determine. Measure
the resistance between CAN H and CAN L of the diagnostic interface. If it becomes 120Ω, it means that one of the terminal resistors is broken. If the faulty control module does not have a terminal resistor, then it is necessary to measure the conductivity of the CAN wire of the control module.

By replacing the control module involved in the fault code, you can quickly determine whether the fault is caused by the control module itself.

In addition, the network diagram should be used to find the breakpoints, because the corresponding bus hubs will be set up in the entire network. Different breakpoints will affect different components, and will also determine the control modules that the diagnostic instrument can diagnose. The distribution of CAN network and hubs is shown in the figure below.