Automotive electronics CAN bus principle and fault diagnosis method Chengdu Panfeng Technology

Automotive electronic bus principles and fault diagnosis methods
． Node failure
． Failure mechanism
The node is the electronic control module in the automobile multi-channel information transmission system, so node failure is the failure of the electronic control module. It includes software faults, that is, transmission protocols or software programs are defective or conflicting, causing communication confusion or inability to work in the automotive multiplex information transmission system. Such faults generally occur in batches and cannot be repaired. Hardware failures are generally caused by failures in communication chips or integrated circuits that cause the automobile's multi-channel information transmission system to fail to work properly. For automobile multi-channel information transmission systems that use low-version information transmission protocols and return to point-to-point information transmission protocols, if there is a node failure, the entire automobile multi-channel information transmission system will not work.
． Fault example
． ． Fault phenomenon
A Shanghai Passat car experienced an engine oil pressure warning light and an airbag failure indicator light while in use, and the engine tachometer failed to operate.
． ． Fault Detection
Use a fault reader to read the fault codes of the engine control system and find two sporadic fault codes: / No signal output from the airbag control unit / Error in instrument data output. Use a fault reader to read the fault code of the instrument system: No communication from the engine control unit to the airbag control unit.
． ． Fault Analysis and Troubleshooting
By reading the fault code, it can be initially determined that the fault lies in the automobile's multi-channel information transmission system. By analyzing the automotive electrical wiring, the probability of a fault caused by the power system is very small. The fault is likely to be a node or link failure. The fault of the airbag control unit can be eliminated by trying the replacement method.
． Link failure
． Fault Mechanism
When the link (or communication line) of the automobile multi-channel information transmission system fails, such as short circuit, open circuit of the communication line, and attenuation or distortion of the communication signal caused by the physical properties of the line, it will cause multiple electronic control units to fail. Work or electronic control system malfunction. When determining whether it is a link failure, an oscilloscope or a special automotive optical fiber diagnostic instrument is generally used to observe whether the communication data signal is consistent with the standard communication data signal.
． Fault example
． ． Fault phenomenon
In the electronically controlled automatic air conditioning system of an Audi car, the blower can work when the switch is turned on, but the air conditioning system does not cool.
． ． Fault Detection
Through observation, it was found that the electromagnetic clutch of the air conditioning compressor did not engage but the engine was working normally. Check the resistance value of the electromagnetic clutch circuit. The resistance value meets the specified value. Check that there is no output signal at the output end of the air conditioning control unit. At this time, use a fault reader to read the fault codes of the engine control system and air conditioning control system, and there is no fault code. Use the fault reader to read the data stream of the air conditioning control unit. The engine speed data is zero. Since the engine is working normally, the engine speed signal received by the engine control unit should be normal. Check the communication line between the engine control unit and the air-conditioning control unit. It is found that the pins of the special-speed communication line between the two are deformed, causing a link break. After repairing the connector, troubleshooting.
． Summary
Through the analysis of the above three types of automobile multi-channel information transmission system faults, it can be concluded that the general diagnostic steps of the system are:
. Understand the characteristics of the automobile multi-channel transmission system of this model (including: transmission media, several subnets and the structural form of the automobile multi-channel information transmission system, etc.).
． The functions of the automobile multi-channel information transmission system include: whether there is a wake-up function and a sleep function, etc.
． Check whether there are faults in the car's power supply system, such as whether the output waveform of the alternator is normal (if it is abnormal, it will cause signal interference and other faults), etc.
． Check whether there is a fault in the link of the automobile multi-channel information transmission system by using the replacement method or the cross-line method.
． If it is a node failure, it can only be detected using the replacement method.