In the electric vehicle, the CAN bus is like the “nervous system” of the vehicle, which is responsible for the information transmission between the various electronic control units. Once this “nerve” goes wrong, the communication of the vehicle will fall into chaos, leading to all kinds of inexplicable failures.
When you step on the brake, the ABS system does not receive the signal. When you turn the steering wheel, the ESP system is slow to respond-these are typical manifestations of CAN bus failure.
Then, how to effectively detect and diagnose the CAN bus?
We will give you a detailed analysis of can bus detection methods, common faults and their causes, as well as how to use a multimeter for preliminary electrical testing.
① Before checking the data bus system, ensure that all electronic control units connected to the data bus have no functional failure. A functional fault is a fault that does not directly affect the data bus system, but does affect the functional flow of a system.
If the sensor is damaged, the result is that the transmitter signal cannot be transmitted through the data bus.
This functional failure has an indirect effect on the data bus system, affecting the communication between the electronic control units that need the sensor signal. If there is a functional fault, remove the fault first. Write down the fault and eliminate the fault codes of all electronic control units.
② After eliminating all functional faults, if the data transmission between electronic control units is still abnormal, check the data bus system.
When checking for data bus system faults, the following two possible cases must be distinguished:
Detection of two-wire data bus system composed of two electronic control units.
Detection of two-wire data bus system composed of three or more electronic control units.
③ If the cause of hardware damage cannot be found on the data bus, check whether the fault is caused by an electronic control unit.
Disconnect all electronic control units that transmit data through CAN bus, turn off the ignition switch, and connect one of the electronic control units.
Common Faults and Causes of CAN Bus
(1) Common faults of CAN bus There may be two bus fault records of CAN communication fault and CAN bus line fault in the CAN bus electronic control unit.
① CAN communication fault There are two types of communication faults:
Open circuit of electronic control unit.
The electronic control unit is damaged.
② CAN bus line fault CAN bus line fault has the following conditions:
Short circuit in CAN bus lead.
Open circuit in one of the CAN bus wires.
The CAN bus wire is grounded.
Open circuit between CAN bus wires.
Cross connection between the CAN-Low line and the CAN-High line. ■ The CAN-Low line is shorted to the battery positive. ■ The CAN-High line is shorted to the battery positive. ■ The CAN-Low line is short-circuited to the battery cathode. ■ The CAN-High line is short-circuited to the battery cathode.
(2) Causes of CAN bus failure The causes of CAN bus communication failure are as follows:
① Open circuit or short circuit of communication line of CAN-Low line or CAN-High line.
② The plug connection is damaged, such as contact damage, dirt and rust.
③ Voltage failure in the vehicle power system, such as caused by a damaged ignition coil or ground connection.
④ The communication component in an electronic control unit is faulty.
⑤ Power supply failure of an electronic control unit. When the battery is running out of charge, a slow drop in battery voltage may cause a fault record to be stored, because not all electronic control units are switched off at the same time due to the voltage drop. Short circuit of CAN bus to positive pole, short circuit to grounding and short circuit of wires to each other will not damage the electronic control unit, but will cause bus system failure in the most serious case. The bus system in the vehicle will not only have an open circuit or short circuit fault, but when water vapor intrudes into the plug in the bus system, contact resistance may occur between the ground, positive and CAN bus wires, causing the bus system to operate abnormally.
Multimeter Detection Method of CAN-bus
A digital multimeter can be used to test the voltage signal of the CAN bus to roughly determine whether there is a fault in the signal transmission of the data bus.
When the digital multimeter for CAN bus multimeter detection measures the frequency signal, the multimeter has the working characteristics of segmented acquisition and effective value operation. Therefore, the display value of the digital multimeter can only reflect the main signal voltage value of the measured signal, and cannot display every detail of the measured signal.
It can be seen that when the digital multimeter is used to measure the signal voltage of the CAN bus, the displayed value of the multimeter corresponds to the main signal voltage value of the CAN bus.
(1) Use a multimeter to measure the voltage of the CAN-High signal of the power CAN bus. When the bus is idle, the voltage is about 2.5V. When there is signal transmission on the bus, the voltage fluctuates between 2.5 ~ 3.5V. Therefore, the main voltage of the CAN-High should be 2.5V. Therefore, the measured value is 2.5 ~ 3.5V when measured by a multimeter. Greater than 2.5V but close to 2.5V. Similarly, the voltage of CAN-Low signal is about 2.5V when the bus is idle, and the voltage value fluctuates between 1.5 ~ 2.5V with high frequency when there is signal transmission on the bus. Therefore, the main voltage of CAN-High should be 2.5V, so the measured value is 1.5 ~ 2.5V when measured with a multimeter, which is less than 2.5V but close to 2.5V.
(2) Use a multimeter to measure the comfortable CAN bus. The voltage of the comfortable CAN signal is about 0 when the bus is idle. When there is signal transmission on the bus, the voltage fluctuates between 0 and 5V. Therefore, the main voltage of CAN-High should be 0. Therefore, the measured value is about 0.35 V when measured with a multimeter. Similarly, the voltage of CAN-Low signal is about 5V when the bus is idle, and the voltage value fluctuates between 0 ~ 5V with high frequency when there is signal transmission on the bus. Therefore, the main voltage of CAN-High should be 5V, so the measured value is about 4.65 V when measured with a multimeter.
(3) Detection of CAN bus terminal resistance In order to measure the total resistance of the two terminal resistors, the multimeter function of the VAS5051 tester can be used for detection as shown in the following figure.
Measure the total resistance of the two terminal resistors.
The steps for measuring the terminal resistance are as follows:
① Remove the wires (cables) from the positive and negative terminals of the battery.
② Wait for about 5min until all capacitors are fully discharged.
③ Connect the VAS5051 tester, call the multimeter function, connect the measuring wire, measure the total resistance of the terminal resistor and make a record.
④ Pull out the harness plug of an electronic control unit (such as engine electronic control unit) with a terminal resistor, and observe whether the total resistance of the terminal resistor changes.
⑤ Connect the harness plug of the first electronic control unit (with terminal resistor, such as engine electronic control unit), and then unplug the harness plug of the second electronic control unit (with terminal resistor, such as ABS electronic control unit), and observe whether the total resistance of the terminal resistor changes.
⑥ Analyze the measurement results. It is not a terminal resistor with a fixed resistance value that is set in the electronic control unit, but a combination of many measured resistors.
After measuring the total resistance, it is necessary to pull out the plug of an electronic control unit with a terminal resistor and measure the single resistance twice. When the measured resistance value changes after the electronic control unit is pulled out, it indicates that the two resistance values are normal.
The terminal resistance installed on the drive CAN bus can be measured with a multimeter, but the terminal resistance installed on the comfort CAN bus and the information CAN bus cannot be measured with a multimeter.
From this measurement data, it can be judged that the termination resistance of the drive CAN bus is normal. It is important to note that the value of a single termination resistor is not necessarily about 120 Ω, but varies depending on the bus structure.
If the measured resistance value does not change after unplugging the harness plug of an electronic control unit with a terminal resistor, it indicates that there is a problem in the system. It is possible that the terminal resistor of the removed electronic control unit is damaged, or there is an open circuit on the CAN bus.
If the displayed resistance value becomes infinite after the electronic control unit is removed, either the terminal resistance of the electronic control unit that has not been removed is damaged, or the CAN bus wire to the electronic control unit has an open circuit fault.
The detection of CAN bus is like a “neurological physical examination” for a car, which requires patience, careful and systematic thinking.
From troubleshooting functions to checking circuit hardware, from measuring voltage signals to detecting termination resistors, every step is interlinked.
In actual maintenance, many technicians tend to “skip” the first step to check the bus directly, only to find that a sensor is broken and affects communication.
Remember: the most efficient diagnostic process is to first troubleshoot the function, then use a multimeter for preliminary judgment, and finally use a special tool for accurate measurement.
Of course, CAN-bus technology is also evolving, from classic CAN to CAN FD (Flexible Data Rate) to future CAN XL, with increasing transmission speeds and bandwidths. As a practitioner or enthusiast, only by continuously learning new technology trends can we cope with the increasingly complex automotive electronic systems.
