Common Faults and Repairs of CAN Bus Faults

CAN (Controller Area Network) is a multi-host local network serial communication protocol developed by the German Bosch company in the early 1980s to solve the problem of data exchange between numerous electronic control units (ECUs) in modern automobiles.
Developed jointly by Bosch and Intel in 1983;
1987 The first CAN controller chip (Intel);
1990 The first mass-produced car using CAN: Mercedes S-Class;
CAN 2.0 was released in 1991 (PART A and PART B);
In 1993, CAN became an ISO standard (ISO 11898);
Many students fainted when they saw this name. In fact, CAN communication can be simply understood as a conference call. When one person speaks, others listen (broadcasting). When multiple people speak at the same time, certain rules are used to decide who speaks first and who speaks last (arbitration). This is like you and the leader speaking at the same time in a meeting. You will definitely let the leader speak first.
But it is worth noting that in this meeting, the speaker will confirm whether the listener has successfully received the information. If the information delivered by the speaker is incorrect, the listener will point out the error in time.

Detection of CAN bus terminal resistance

Multimeter detection of CAN bus

CAN bus detection method:
① Before checking the data bus system, it must be ensured that all electronic control units connected to the data bus have no functional faults. Functional fault refers to a fault that does not directly affect the data bus system, but affects the functional process of a certain system. If the sensor is damaged, the consequence is that the transmitter signal cannot be passed through the data bus. This functional failure has an indirect effect on the data bus system, affecting communication between electronic control units that require signals from this sensor. If there is a functional fault, the fault should be eliminated first. Make a note of the fault and eliminate all fault codes from the electronic control unit.
② After eliminating all functional faults, if data transmission between electronic control units is still abnormal, the data bus system needs to be checked. When checking for data bus system faults, the following two possible situations must be distinguished: ■ Detection of a two-wire data bus system composed of 2 electronic control units. ■ Detection of two-wire data bus system consisting of 3 or more electronic control units.
③ If the cause of hardware damage cannot be found on the data bus, you should check whether a certain electronic control unit causes the fault. Disconnect all electronic control units that transmit data through the CAN bus, turn off the ignition switch, and connect one of the electronic control units. For example, for Volkswagen system models, connect the fault diagnosis instrument, turn on the ignition switch, and clear the fault code of the electronic control unit just connected. Use function 06 to end the output, turn off and on the ignition switch, turn on the ignition switch for 10 seconds and then use the fault diagnosis tool to read the contents of the fault memory of the electronic control unit that was just connected. If “hardware damage” is displayed, replace the electronic control unit just connected; if “hardware damage” is not displayed, connect the next electronic control unit and repeat the above process.

Common faults and causes of CAN bus:
(1) Common faults of CAN bus In the CAN bus electronic control unit, there may be two bus fault records: CAN communication fault and CAN bus line fault.
① CAN communication failure There are two situations of communication failure: ■ The electronic control unit is open circuit. ■ The electronic control unit is damaged.
② CAN bus line failure CAN bus line failure has the following situations:
■ CAN bus wire short circuit.
■ One wire of the CAN bus is broken.
■ CAN bus wire is grounded.
■ Break between CAN bus wires.
■ Cross-connection between CAN-Low line and CAN-High line.
■ The CAN-Low line is short-circuited to the positive terminal of the battery.
■ The CAN-High line is short-circuited to the positive terminal of the battery.
■ The CAN-Low line is short-circuited to the negative terminal of the battery.
■ The CAN-High line is short-circuited to the negative terminal of the battery.

(2) Causes of CAN bus failures The causes of communication failures on the CAN bus are as follows:
① The CAN-Low line or CAN-High line communication line is open or short-circuited. ② The plug connection is damaged, such as damaged contacts, 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 fails. ⑤ Power supply failure of an electronic control unit. When the battery is almost depleted, a slow drop in battery voltage may cause a fault record to be stored because not all electronic control units are shut down at the same time due to the voltage drop.
Short circuit of the CAN bus to the positive pole, short circuit to the ground, and short circuit of the wires to each other will not damage the electronic control unit, but in the most serious case, it will cause the bus system to malfunction. The bus system in the vehicle is not only subject to open circuit or short circuit faults. When water vapor invades the plugs in the bus system, contact resistance may appear between the ground, positive pole and CAN bus wires, causing the bus system to work abnormally. Multimeter detection method of CAN bus CAN bus can use a digital multimeter to test the voltage signal to roughly determine whether there is a fault in the signal transmission of the data bus. The detection method is shown in the figure below:

▲ Multimeter detection of CAN bus
When measuring frequency signals with a digital multimeter, the multimeter has the working characteristics of segmented acquisition and effective value calculation. 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 a digital multimeter is used to measure the signal voltage of the CAN bus, there is a corresponding relationship between the multimeter’s display value and the main signal voltage value of the CAN bus.
(1) Use a multimeter to measure the power CAN bus
The voltage of the CAN-High signal is approximately 2.5V when the bus is idle. When there is signal transmission on the bus, the voltage value fluctuates at high frequency between 2.5~3.5V, so the main body voltage of CAN-High should be 2.5V, so the measured value when measured with a multimeter is 2.5~3.5V, which is greater than 2.5V but close to 2.5V. Similarly, the voltage of the CAN-Low signal when the bus is idle is about 2.5V. When there is signal transmission on the bus, the voltage value fluctuates at high frequency between 1.5~2.5V, so the main voltage of CAN-High should be 2.5V, so the measured value when measured with a multimeter is 1.5~2.5V, which is less than 2.5V but close to 2.5V.
(2) Use a multimeter to measure the Comfort CAN bus
The signal of Comfort CAN has a voltage of approximately 0 when the bus is idle. When there is signal transmission on the bus, the voltage value fluctuates at high frequency between 0 and 5V. Therefore, the main body voltage of CAN-High should be 0, so the measured value when measured with a multimeter is about 0.35V. Similarly, the CAN-Low signal has a voltage of about 5V when the bus is idle. When there is signal transmission on the bus, the voltage value fluctuates at high frequency between 0 and 5V. Therefore, the main voltage of CAN-High should be 5V, so the measured value when measured with a multimeter is about 4.65V.
(3) Detection of CAN bus terminal resistance
In order to measure the total resistance of the two terminal resistors, you can use the multimeter function of the VAS5051 detector to perform the test as shown in the figure below.

▲Measure the total resistance of the two terminal resistors. The steps for measuring the terminal resistors are as follows:
① Remove the wires (cables) from the positive and negative terminals of the battery. ② Wait for about 5 minutes until all capacitors are fully discharged. ③ Connect the VAS5051 detector, call the multimeter function, connect the measurement leads, measure the total resistance of the terminal resistor and record it. ④ Unplug a wiring harness plug with a terminal resistor electronic control unit (such as an engine electronic control unit) and observe whether the total resistance of the terminal resistor changes. ⑤ Connect the wiring harness plug of the first electronic control unit (with terminal resistor, such as engine electronic control unit); then unplug the wiring harness plug of the second electronic control unit (such as ABS electronic control unit). Observe whether the total resistance of the terminal resistor changes. ⑥ Analyze the measurement results. What is set in the electronic control unit is not a terminal resistor with a fixed resistance, but a combination of many measured resistors. For example, on the 1.9TDI model with a pump nozzle unit, the engine electronic control unit is equipped with a 66Ω terminal resistor. The terminal resistor is designed according to the vehicle model, and the total resistance value depends on the vehicle’s bus structure. After measuring the total resistance, you need to unplug a plug with a terminal resistor electronic control unit and measure the individual resistance twice. When the measured resistance value changes after the electronic control unit is removed, it means that both 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 information CAN bus cannot be measured with a multimeter. For example, take the Audi A21.4 model as an example to analyze the total resistance of its drive CAN bus. The two electronic control units with terminal resistors are connected by the CAN bus harness, and the two terminal resistors are connected in parallel on the bus. The measurement result is that the resistance of each terminal resistor is approximately 120Ω, and the total resistance is approximately 60Ω. From this measurement data, it can be judged that the terminal resistance driving the CAN bus is normal. It should be noted that the resistance of a single terminal resistor is not necessarily about 120Ω, and its specific value varies depending on the bus structure.

Another example is analyzing the single resistance value of the Audi A21.4 model’s drive CAN bus. After measuring the total resistance, unplug the wiring harness of an electronic control unit with a terminating resistor, and then measure again. At this time, the resistance value displayed on the screen should change (this is the measurement of the terminal resistance value of an electronic control unit, and the actual measured resistance value of a single terminal resistor driving the CAN bus). If the measured resistance value does not change after unplugging the wiring harness plug of an electronic control unit with a terminating resistor, there is a problem in the system. It may be that the terminal resistor of the removed electronic control unit is damaged, or there is an open circuit in the CAN bus. If the displayed resistance becomes infinite after the electronic control unit is removed, then either the terminal resistor 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.