CAN Bus feature Part One
Since the introduction of the CAN bus in production cars in 1991, this technology has grown from just the luxury models then to virtually all levels of vehicles today. In the past several years, aftermarket technicians are becoming more aware of this now mature technology. Used as a means of intercommunication between ECUs, the CAN bus has provided a simple means of connecting diagnostic tools to the vehicle via the standardised On Board Diagnostic (OBD) connector in all vehicles today.
Whilst there are tools readily available for diagnostic connection to the CAN bus, these tools do not always give the full picture of where problems may lie. Sometimes Diagnostic Trouble Codes (DTC) do not always get to the real source of the problem. This article shows some quick alternatives for checking for an unhealthy CAN bus.
Several incidents in the past have shown DTCs in the order of “Wrong ID from ECU_X”. ECU_X could be a body controller or instrument pack. Many times the DTC is not indicative of a problem with that ECU. It could be a problem that is reported from the actual faulty ECU. This is a typical problem encountered from a CAN bus problem.
One step is to check the CAN bus with an Oscilloscope. An over all bus check can be down via the OBD connector that is in most cars somewhere around the base of the steering wheel. Some imported cars will have the OBD connector in a similar area on the passenger side.
Figure 1a and 1b
You can view the CAN electrical signals by connecting a two channel oscilloscope to pins 6 (CAN_H) and 14 (CAN_L) of the OBD connector. Also ensure a ground connection is made at signal ground (pin 5). An OBD connector can be specially fabricated for this purpose. Figure 1a shows the pin connections on the vehicle side. Figure 1b shows a typical OBD connector case.
Figure 2 Oscilloscope view of healthy CAN bus showing many CAN frames
Healthy CAN signal waveforms will appear as shown in figure 2. This is a typical view showing bursts of many CAN frames on the bus. CAN_H is in red, and CAN_L is in blue.
A zoomed in view of a typical CAN frame and its data areas are illustrated in Figure 3, below. Here you can see a close up view of the differential CAN_H and CAN_L signals. 
Figure 3 Zoom in on a CAN frame
One case study reported by Pico Technology (www.picoauto.com/tutorials/corsa-no-start.htm) was on a 2004 Corsa that was showing DTCs P1615 WRONG VEHICLE ID FROM BODY CONTROLLER (BC) and P1616 WRONG VEHICLE ID FROM INSTRUMENT (INS). Immediately it was suspected that it was a CAN issue or an exchange of data between control units.
The Engine Control Module (ECM) is reporting a problem with both INS and BC, yet data exchange between INS and BC themselves appears OK (no DTCs) making the ECM a common denominator. Both INS and BC appear to be functioning fine with all their associated auxiliaries working, so a power supply issue is unlikely. Also there is good communication between the scan tool and ECM, allowing different self-activations and accurate live parameter readings, so again a power supply issue at the ECM is unlikely. The next step is to verify the data transmission line: the CAN bus.
PC based Oscilloscopes are used, as they are an inexpensive way to obtain an O-Scope if you already have a laptop computer. At first, a check at the OBD connector indicated a healthy CAN bus. It was then decided to check the three ECUs involved - ECM, BC and INS. The bus accessed directly at each ECU connector – not an easy task to access, but possible. At the BC and INS, the CAN bus appeared to be normal, but the result was different at the ECM. Figure 4 show the resulting wave form. Here it was found that this is far from the pair of mirror-image signals you would normally expect to see. CAN_H represents a good signal structure with well-defined dominant and recessive bits, but CAN_L displays no recognisable bit transmission and its line stability has drifted low. On further investigation, it was discovered that the CAN_L wire was badly chaffed and corroded.
Figure 4. CAN wave form at the Engine Control Module connector
If problems with the CAN bus are quite drastic and communication is very noisy, one quick check that can be made is to see if the terminators are present. Access can be had at the OBD connector pins 6 (CAN_H) and 14 (CAN_L). Ensure all the power is off the vehicle. Connect an ohmmeter to pins 6 and 14. Here you should read approximately 60 ohms. This is the equivalent of two 120 ohm resistors in parallel. A standard CAN bus will have a 120 ohm resistor termination at each end of the bus. This is required to avoid reflections that are caused be digital data communications. The resistors act an “electrical shock absorber”. If the resistors are not fitted correctly, the digital signals will “bounce” off the end of the data bus causing noisy reflections. This will have a significant effect on the performance of the CAN bus.
If your reading is 120 ohms, this means that one of the terminations is missing. In a vehicle Terminators always reside in the ECUs at each end of the CAN bus. If one is missing, this may indicate that one of the end ECUs is missing. If the resistance reading is a very high value (typically 10s of thousands of ohms), this indicates that there are no terminators fitted. This value is basically reading the input impedance of the ECUs. This is a drastic problem that will require the checking of connections at each end of the CAN bus.
Here it can be seen that the use of a couple of simple tools can give indications of problems with the CAN bus that Fault Code reader tools cannot always pinpoint. An inexpensive PC based O-Scope, and a simple multi-meter using the Ohmmeter option, can give valuable immediate information on the health of the CAN bus. In a later article, we plan to discuss how a simple Windows-based CAN analysis tool could be used with similar effect. This tool tells more than the typical Fault Code reader without the complication of complex analysis software.
Here we can see a physical CAN frame as seen on the twisted pair data bus. Note that during bus idle, the level is constant recessive (logic 1). When the CAN node transmits a frame, it starts with a dominant start of frame bit (logic 0). This informs the other CAN nodes that data is on the way. Of course, it can be seen that the rest of the CAN frame follows (Arbitration, Control, Data, CRC, ACK, EOF). Figure 2 shows the structure of the CAN frame.
