The Controller Area Network (CAN) is a communication protocol that allows modules to share data across a twisted two-wire circuit. It’s a common network that virtually all manufacturers use to communicate powertrain, body, and now, the Advanced Driver Assistance System (ADAS) information. I’ve written CAN bus articles in the past, with the most recently published in the December 2021 issue entitled “Riding the CAN bus to school.” Thank you, Gears, for giving me such a catchy title! Please refer to that article, which is always available at gearsmagazine.com, to learn some basics on how the CAN bus operates and how it’s constructed. In that article, I cover the basics of CAN bus construction and diagnostics For example, I go through a no-start issue on a Silverado that had an intermittent faulty wiring harness. The Silverado had a really basic CAN bus system, and it was an excellent vehicle to cover what the CAN bus provides and how to do some simple diagnostic tests. I ended the article by mentioning that many vehicles use multiple CAN networks and that some of the diagnostics would be different on those vehicles, which leads to the purpose of this article.
The networking on late-model vehicles has grown tremendously and has become quite complicated. For example, if you research a fairly common vehicle, such as a Jeep Grand Cherokee, you’ll see that the late-model variations contain up to 38 modules across multiple networks (Figure 1).
Many vehicles have ethernet networks, a Local Interface Network (LIN), an Interior High-Speed CAN (CAN-IHS), a Chassis CAN (CAN-C), and various Flexible Data CANs (FDCAN). These networks allow communication between modules, but the individual networks also communicate with each other through a Secure Gateway Module (SGM) and a Body Control Module (BCM). Low-speed networks like the LIN and CAN-IHS share low-priority data, such as HVAC, Infotainment, and Passenger Controls. Whereas CAN-C, CAN-FD, and Ethernet share high-speed data for powertrain control, ADAS functions, and video transmission. Although these networks are complex, splitting them into smaller sub-networks gives us an advantage when diagnosing a failure.
Now I’m going to take about ten steps backward and share a diagnosis on a vehicle with multiple networks that is nowhere near as complicated as that Jeep. We had a Mitsubishi Outlander Sport with a no-start, and the dash was lit up like a Christmas tree (1st hint of a network issue), as shown in Figure 2. After attempting to retrieve Diagnostic Trouble Codes (DTCs) and module data, we were delivered the dreaded no-communication message (2nd hint of a network issue). The full vehicle scan did report modules online, and those modules had DTCs relating to communication problems with various powertrain modules. A wiring schematic proves invaluable in these situations because it shows how the vehicle’s network system is constructed.
In the Outlander’s case, there were three networks (Figure 3)
- A high-speed powertrain network connecting the engine control module (ECM), transmission control module (TCM), anti-lock brake system module (ABS), electronic power steering module (EPS), steering wheel sensor module (SWS), and the all-wheel drive (AWD) module.
- A medium-speed body network connecting modules such as airbag (SRS), occupant classification (OCC), radio, satellite, HVAC, instrument cluster, and keyless operation (KOS).
- A high-speed diagnostic CAN is a simple network connecting the scan tool to the Electronic Time and Alarm Control System module (ETACS-ECU), which serves as the gateway between these three networks.
If one of these networks fails, the communication between the modules on the affected network also fails, which results in no access to the modules on that network. The scan tool displays no communication because it relies on the network to provide access. It’s like someone cut the phone line! Assuming people still use phones with actual lines anymore.
From that December 2021 Gears article, we know that a high-speed CAN network uses two wires, CAN + and CAN –, to connect each module. We also know that CAN+ and CAN – are typically connected through two 120-ohm resistors, which puts the total CAN resistance at about 60 ohms. The Diagnostic Link Connector (DLC) pins 6 and 14 connect to the CAN network. On a simple CAN circuit, we could check the resistance between pins 6 and 14 to check the network’s total resistance, which should be around 60 ohms. In the case of the Outlander (and most late-model vehicles), after reviewing the wiring schematic in image three, this test would only check the connection between the DLC and the ETACS, which we know isn’t a problem since we are getting communication to body modules. This measurement would not check the powertrain or body network resistance because the wiring goes straight from the DLC to the ETACS, and the powertrain and body networks are independently connected to the ETACS. So, to measure the powertrain network total resistance, we will need to tap directly into a powertrain networked module and check the circuit through their respective CAN + and CAN- wiring or connections.
On this Outlander, the ECM and ABS are relatively easy to access. When measuring the powertrain CAN bus resistance, the result was about 100 ohms, which wasn’t quite expected (Figure 4). Using a scope to view the quality of the CAN bus voltage pattern, you can see that there is definitely an issue. The displayed voltages don’t exhibit the expected pattern where CAN + should shift cleanly between 2.5v to 3.5v, and CAN – should shift cleanly between 2.5v and 1.5v.
Since the ECM was easy to disconnect, we used the scope to measure CAN voltage with the ECM disconnected and obtained a very clean pattern with expected voltages (Figure 5). With the ECM disconnected, we repeated the resistance test between CAN+ and CAN-, which measured 120 ohms. This was expected because the ECM contained one of the terminating resistors, and by disconnecting it, we removed it from the circuit. Remember, when checking circuit resistance, the CAN bus cannot have any voltage, or it will compete with the voltage the DVOM uses to perform the ohm measurement. 
When performing the resistance test, you might have to disconnect the battery to ensure the network has no voltage.
The next step is to determine if the ECM is faulty or if the connection is bad. Before condemning a module, verify that the wiring and connector pass a power and ground load test. Also, verify that you have a good connection between the network terminals. This module has very thin terminals, measuring about .024” or 0.6mm (Figure 6). This was smaller than my smallest terminal assurance test probe, so to test terminal assurance, I used the smooth shank of a drill bit from a welding tip cleaner (Figure 7), or you could use the shank from a properly sized micro drill bit. The micro drill bit set or welding tip cleaner costs less than $10 on Amazon. This diagnostic check found the CAN – female terminal (as well as a couple of others) had very poor assurance, and replacing the terminal fixed this vehicle.
In automotive diagnostics, technicians are constantly challenged with every new problem that enters the shop. It’s a never-ending education, and the lessons learned from this diagnostic include:
- Wiring schematics are invaluable to determine how the networks are configured and help guide where it’s possible to test the network.
- Checking network resistance at the DLC doesn’t always work since the DLC might not directly connect to the network with a fault.
- An abnormal CAN scope pattern can result from a bad module connection, not just a module failure or an open or short circuit.
- Visual inspections aren’t good enough when it comes to terminal connections. You need to check for tight pin connections with an appropriate tool.
With electrical diagnostic labor times not very forgiving, it’s important to have a strategy when diagnosing no-communication faults. With more high-tech features on vehicles than ever before, the module count is getting higher, and the module networking is becoming more advanced. Now is the time to strengthen your knowledge of how these networks operate and practice the most effective ways to diagnose them. Keep learning, and just remember, if you put your mind to it, you CAN do it!
