Common CAN bus Problems: A Troubleshooting Guide

CAN bus systems are essentially high-speed freeways for digital information exchange within a tightly-packed and complex electrical system such as within an automobile. These tight spaces can’t fit a computer so Controller Area Networks (CAN) help bridge the gap to allow microcontrollers and other devices to “talk” to one another. But what happens when there’s an issue with a CAN bus system? Can you work on a CAN bus cable? How do you diagnose CAN bus problems?

To answer these questions and more, today’s LiveWire spotlight deep-dives into the complex world of CAN bus systems. We’ll cover common CAN bus problems and walk through troubleshooting and diagnostics with these intricate systems.

CAN Bus Wire & Cable Systems 101

CAN bus systems are vital components of the communication system within a vehicle, medical device, industrial control system, or other highly-complex yet space-constrained application. Within a vehicle, this allows form the different control modules to all talk to one another at the same time. Vital safety systems like the anti-lock brakes communicating with the engine’s own module, parts of the transmission, and more are then all displaying real-time information to the driver via the instrument cluster. This type of sophisticated communication requires cables and wires to be specifically engineered for the rigorous demands they’ll undergo. 

When CAN bus systems are working correctly, they’ll send and receive the data they are transmitting within a set of frames. Even with a closed system like a vehicle, the data is sent via a code that helps control modules prioritize the data that’s being sent. Within the code that’s coming from a control module, the code being sent has certain values that can designate the priority of a given message. This helps control the flow of data that’s going through the CAN bus system so that really important data, such as something related to a safety system, can be sent and received ahead of data that’s operationally generic. 

Within the code is also all of the data that needs to be transmitted in that instance from one control module to another. Think of your vehicle driving through snow or water and your vehicle automatically rotating tires at different speeds to compensate. This action may seem mundane when you drive everyday, but the intensely-complex actions to get there were all thanks to the CAN bus system crunching the data frames to communicate the proper action to take in the given situation. 

When the data is transmitted from one control module, the receiving module should always run a validity test on the incoming data to make sure it’s valid. This process is called a checksum and acts as a sort of checks and balances for the module sending the data. When the receiving module validates the data, it responds with acknowledgement of the validity to the transmitter and the system continues to operate as it was intended. When a checksum is not returned valid, a host of issues can develop which we’ll cover in the troubleshooting section below.

Different Types of CAN Bus Subsystems

There’s three types of CAN bus subsystems that help make it possible for different data transmission requirements. When performing within their specifications, these systems facilitate different types of communication between control modules simultaneously and without interference. 

The three types of CAN bus subsystems include:

  1. High-Speed CAN bus subsystems: used for speeds all the way up to 125 KB per second. The construction of the CAN bus high-speed wires creates an effect of opposing voltages. These properties are useful to help cancel out RMF.
  1. Medium-Speed CAN bus subsystems: will handle anywhere from 10 KB to 125 KB per second. These systems are usually made with a single wire that can be shielded to combat interference.
  2. Low-Speed CAN bus subsystems: for handling speeds that are generally 10 KB per second and lower via a single wire set-up. For these systems, wire engineers typically create a centralized control module that then controls other modules in the network.

When there’s an issue within the CAN bus system, a myriad of different concerns can occur. With a vehicle, for example, this may mean the vehicle ceases to run at all. Figuring out what you’re dealing with in a CAN bus system failure can be difficult but we’ll cover some common things to look for that may give you a hint at the issue. 

Spotting Issues Within a CAN Bus System

CAN bus systems have a lot of data flowing through their subsystems at any given moment. If the CAN bus wiring components aren’t performing up to spec, there can be major issues at play within the system. Many of the issues related to wiring can include poor-quality wiring, terminations done incorrectly, or using several different frequencies within a single CAN bus. 

Here’s a few different things you can look for as you diagnose a CAN bus system:

  • Make sure to have the perfect termination resistance. You can use a multimeter to measure this resistance present between the CAN low and high systems. An unpowered system typically runs at 60 Ohms and has two separate termination resistors. 
  • Any form of termination resistors must be located at the endline of the CAN bus system. These termination resistors will perform their best when they are located with precision. 
  • In general terms, the CAN bus system network resembles that of a tree. Believe it or not, the dimensions of the “tree” matter and even a slight variation can wreak havoc on the data integrity. This also goes for the lengths of several different CAN bus wires.
  • The majority of CAN bus wires should be twisted pairs. Twisted pair cables will take two different insulated wires and twist them together so as to run parallel to one another. This is used to combat the interference we discussed above.
  • When using a shielded cable on a CAN bus system, it’s important than only one side be grounded. Otherwise a situation called a grounded ground loop can occur and cause interference. 
  • To that, all the different network devices need to share the same ground so that ground loops don’t occur.

These are just some of the many different avenues you can take when trying to diagnose your CAN bus wire issues as the system is complex and subject to a lot of variables. For example, when the electric system has experienced a surge, it can wipe memory and cause malfunctions in the control modules. 

If you have issues that are above your head, contact our team. Our engineers can help diagnose your CAN bus system and suggest the best fix for your unique situation. 

Fixing Common CAN Bus Problems 

Running through the above can help you find the area of emphasis needed to repair CAN bus system issues. Fixing three of the more common problems that can befall a CAN bus system include:

  • Check over the CAN bus wiring

Ensure everything is solidly connected. All the wire junctions should have a strong solder and splices that are secure. You never want to see a junction that has a simple twist or wire nuts keeping it secure. You also don’t want to see things like terminal blocks, as they can end up distorting the bus wires’ signals. 

  • Pay attention to any stripped sections.

Stripping wires on a CAN bus system can easily lead to damaging sensitive components like the insulation. Without the insulation there to perform its job, interference can occur. Look over the system for any signs of junctions or sections that have been stripped and inspect the quality of the work. 

  • Test the termination, voltage, ground, and resistance levels.

The specifications for each of these can vary depending on the manufacturer so always test according to specifications. Within each of these fundamental tests you should find your CAN bus system performing in line. The smallest variations can add up to major sources of interference.

Seems simple but these complex systems rely just as much on quality components from the very first strand of wire onto the final wire harness that organizes the system in its intended environment. Meridian’s engineers sweat the details like these so that our customers don’t have to.

Using an Oscilloscope to Pinpoint CAN Bus Issues

When analysing waveforms with the use of an oscilloscope, you can actually spot the spikes where an error is occurring. An oscilloscope is an electric test that simply displays the system’s signal voltages. This is usually for more than one signal and presented as a function of time. The waveform has properties every wire engineer knows and loves like amplitude, frequency, and interval time. 

Using tools like an oscilloscope can help you spot differences between the mirror images of frequency and amplitude. Certain issues like a termination resistor failure or simply a communication malfunction present between control modules creates characteristic waveforms that a trained eye can easily spot.  

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