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Locate faults in the CAN network with these tips and techniques

Friday, February 1, 2019 - 09:00
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Once a module is suspected, the electrical circuit will need to be tested. This will need to be done with an oscilloscope. In order for a module to communicate it will only need the powers, grounds and communication wires. The testing will need to be at the suspected module connector and will check the power source at the module, the ground source at the module and the communication wiring at the module. It is important to know what the communication network waveform you are working on should look like. When you are scoping a high-speed Controller Area Network (CAN) system, the waveform is recessive (idle) at 2.5 volts and dominant (active) at 3.4 volts CAN-H and 2.4 volts CAN-L. Figure 4 shows the CAN high speed waveform.

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Figure 4

CAN High Speed is a Carrier Sense Multiple Access with Collision Resolution (CSMA/CR) communication network system and uses two opposing voltages to reduce noise emissions. These voltages are carried on a two-wire medium referred to as a balanced signal scheme. Each wire carries a voltage signal that occurs at the same time at two different voltage levels. By having one voltage level rise and the other voltage level fall, they will cancel each other’s noise emissions. CAN High Speed uses a twisted pair of wires; CAN high line (CAN-H) and CAN low line (CAN-L). These wires carry differential signal transmissions. The twisted wires reduce Radio Frequency (RF) both received and transmitted. RF is any of the electromagnetic wave frequencies that lie in the range extending from around 20 kHz to300 GHz, roughly the frequencies used in radio communication.

Figure 5

Another common CAN system used in vehicles is CAN medium speed Single wire. CAN Medium Speed uses voltage that is carried on a one-wire medium. The voltage is recessive (idle) at low voltage and dominant (active) at high voltage, as shown in Figure 5. The CAN medium speed Single wire system is a carrier Sense Multiple Access with Collision Resolution (CSMA/CR) communication network.   

The most common communication network systems used in the modern vehicle are: CAN high speed, CAN medium speed and LIN low speed. (For more information on CAN read “Understanding Control Area Network,” May 2016). All of these network systems use voltage changes over time to communicate their messages. Since the messages are based on voltage changes it is important to use an oscilloscope to check the basic voltage patterns produced. When using the oscilloscope it will not be necessary to check the message packets to the bitwise format. The bitwise format is the length of time each bit is recessive or dominant. These changes over time indicate the message to other modules on the network. These time intervals, for each bit, can be different for each system. Additionally they can change from manufacture to manufacture due to the CAN transceiver being programmable for different bit times. Thus, each time interval indicating a bit can be changed from system to system. Therefore these message packets are proprietary to the manufacture and are not shared in the light-duty market. To follow the bits within a data frame and have any understanding of what the message packets are communicating would be impossible if you did not have the code that was being used. One such example would be if you were testing a telegraph system with an oscilloscope. You would be able to see the voltage changes over time, but without having the code (e.g. Morse) that was being transmitted you would not understand the message that was produced. Therefore, it will not be necessary to read the message to the bitwise resolution, but to check the basic voltage patterns produced. The modules on the network are programed to understand the bitwise resolution of each data frame, so utilize the other modules to help diagnose the network under test. 

Now that we have knowledge of what to expect when analyzing these network waveforms, let us analyze several other waveforms that you will encounter when working on CAN high-speed networks. These are the basic waveforms that you will need to know.   

Figure 6

The first Can high-speed network waveform, shown in Figure 6, is produced when the ignition switch is turned to the accessory position “Accessory Mode” (on some systems) or when the system did not fully wake up. This can also be caused by power or ground issues. In this example, the waveform does not move in opposite direction from 2.5 volts as shown in Figure 4, but instead moves from 1.8 volts to 3.6 volts. This accessory mode waveform is one where you may not be able to communicate with the high-speed bus using a scan tool. In this mode there is still data transmission contained in each frame. In some cases, you may be able to communicate with just one module on the bus such as the transmission control unit.     

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