For quite some time we have had CAN bus communication systems on cars and trucks that carry information from module to module. By this I mean each module for each system on the bus would communicate with each other. Signals could be processed and outputs would be created accordingly. Simple, right? Well, manufacturers have now put in place CAN sub-systems, adding a deeper layer of complexity.
An overview of the CAN bus components
For example, as the title implies, Cummins uses a separate CAN network for its exhaust aftertreatment system. Cummins has two CAN bus links on it. These systems are not as simple as one may think. On this aftertreatment system are mutiple modules that communicate with the Cummins Computer, these include:- outlet NOx sensor
- exhaust gas temp module (which holds the temp sensors for the diesel oxidation catalyst)
- diesel particulate filter
- selective catalyst reduction chamber
- aftertreatment particulate matter sensor module
- DEF quality and temp sensor module
- intake NOx sensor
- variable geometry turbocharger
Intake and outlet NOx sensors give NOx information to the computer so it can adjust the timing of fuel injection, VGT rate, and how much EGR flow is needed to manage NOx output. On top of that, the computer also uses this information to control diesel exhaust fluid dosing (DEF) into the selective catalyst reduction (SCR) chamber.
The exhaust gas temp sensors should indicate the heat production created during proper diesel oxidation catalyst operation (DOC) and the diesel particulate filter (DPF) operation. The computer looks at these temperatures (in conjunction with the differential pressure sensor [DPF]) to infer the level of restriction in the system. These devices look at the pressure drop across the DPF. It has two ports on the sensor that look at the potential difference in pressure. So if the inlet side of the sensor has a higher pressure than the outlet, it confirms a restriction is starting to take place in the DPF.
The input from the temperature sensors infers that the DOC can light off to create the high heat needed so the soot can be turned into ash. The DPF will then catch the ash so it does not exist in the tailpipe and pollute the atmosphere.
The SCR temperature sensors monitor the heat production to make sure the resulting chemical reaction taking place inside the chamber is occurring properly. If the temperature was inadequate, it could indicate a restriction or that something is not correct with the DEF doser or doser pump (lacking chemical reaction). Incorrect DEF quality could also cause an issue. This would be reported to the DEF quality sensor module.
Last is the aftertreatment particulate matter sensor. This device measures how much soot is coming out of the entire system. The computer watches the sensor to see how long it takes to burn off the soot and then knows the condition of the DPF. It displays its reading in microamps on Cummins Insite scan tool. In short, that’s a lot of modules to consider.
When analyzing CAN bus network issues of today you must make sure you are analyzing the correct communication bus. Always view a wiring schematic to verify. On heavy-duty trucks and buses as well as many other new vehicles, there are “repeaters” or “gateway modules.” These link the scan tools to the CAN networks. Scope testing through these devices will not allow direct connection to the bus. When testing the network with a scope, be sure you connect your scope to the actual network you are testing, by bypassing the gateway module.
Another thing to keep in mind when using your scan tool is the fact that the data being viewed is processed. What I mean is that the information is sent over the CAN bus network and it is not a live reading of the data. The only way to verify these signals are proper is to measure with a scope. So now that one can understand what I was dealing with, we can continue on this journey and show that it can be easy diagnosing a CAN bus system fault. Having a proper understanding is what it takes to be successful.
The subject vehicle
A 2017 New Flyer Excelsior Transit — a 40 ft. bus XDE40 — with 129,000-plus miles on it came in with a complaint of the check engine light coming on at initial key on. The vehicle had the following DTCs stored (Figure 1):- 3232 “Aftertreatment Intake NOx Sensor Abnormal Update Rate 2 counts Inactive”
- 2771 “Aftertreatment Outlet NOx Sensor Abnormal Update Rate 2 counts inactive”
- 6688 “Aftertreatment 1 Particulate Matter Sensor Abnormal Update Rate 2 counts”
- 4151 “Aftertreatment Diesel Particulate Filter Temp Sensor Module Abnormal Update Rate”
Viewing these DTCs offers an indication that a communication issue exists between these modules. All of them show abnormal update rates. This means that messages were not sent at all or were not received on time. Some questions come to mind:
- What could cause all these modules to code?
- Common voltage and/or a ground issue?
- Common open in the CAN network circuits?
- A malfunctioning sensor bringing the network down?
I checked the freeze frame for all the DTCs and the only useful data I got was that this issue occurred at a key-on event. So, I’m assuming the vehicle was operating at a relatively low temperature. That could be somewhat helpful but I didn't know for sure, yet. Where should I start?
The first thing we should do again is to gather a wiring diagram to develop a plan. I looked at the schematics to see if I could locate a suspect area for a common problem with the four modules. That’s a logical approach, considering what is known so far. Unfortunately, there is no common point shared by all four of the modules. There was also not a common voltage feed, ground, or CAN bus link. The extension harness connection was only showing a common point for three of the modules flagging DTCs (Figure 2).
I decided that the next easy thing to do was a backbone resistance test of the system. A backbone resistance test is taking a resistance measurement of the CAN network with everything intact. Think of the backbone as the spinal cord in the human body. It goes from your brain (one module), down to all the other muscles in your body (other modules), connected by your nerve endings (bus network). So, if you have something wrong with the spinal cord at a certain point, those messages from your brain will not be received by the muscles.
I measured 58 ohms at the 3-pin service tool connector. This is anticipated because of the two 120 ohm terminating resistors in parallel. Ohm’s Law states the resistance reading in parallel will always show a resistance less than that of the smallest resistance in the circuit. Aord of warning, however, when doing this resistance test on the backbone: When you split the system at the 14-pin connector and check resistance in both directions, neither side will show 120 ohms individually. This is because the modules are bringing down the resistance. So refer to the service manual for the correct readings when you do this procedure.
I don’t like this test very much because it brings variables into the equation. The connected modules affect the result of the resistance test, and you don't know if the backbone is intact. Therefore, unless the measurement is way out of spec, this test can be misleading. I recommend just doing the main backbone test with everything plugged in and make sure you measure close to 60 ohms. These systems usually measure 58 to 60 ohms.
This is one of the reasons why I solely use a scope for CAN bus testing. I can see the working voltages in real time (the actual data that is being transmitted). This is much more accurate than a simple resistance test.
Basic tests lead the way to more involved testing
So I have performed some basic checks and also a visual inspection. I see no obvious damage at all. The codes are inactive and the only useful information available (so far) is that the wiring appears to be intact, and the fault occurred at a key-on event. This was determined by viewing the freeze frame data.We must now bring out the digital storage oscilloscope (DSO). I chose to use a 4-channel lab scope. I sampled from the CAN-Hi and CAN-Low at the service tool connector. Unfortunately, I did not gather any useful information from the first capture, so I decided to step back and think about variables with these modules, as in correlation to the current that each module produces. Out of the four modules that coded, the inlet and outlet NOx sensors are the largest current carrying devices (due to the heater circuits within them).
I looked at the schematic again to find a common power source for all the modules. The aftertreatment power relay supplies voltage to all these modules. I decided to test at the relay and monitor the total current along with CAN-Hi and CAN-Low (Figure 3).
Drawing the diagnosis
I captured a key-on event and zoomed in to see the details. The CAN bus exhibited a disturbance (the YELLOW and RED channels) in correlation with the increase in aftertreatment system current (GREEN trace) (Figure 4).The current clamp gathering the data was positioned at the aftertreatment power relay. It was capturing current from both NOx sensor heaters. We now have to determine which of the two sensors was causing the disturbance. Keep in mind that the bus is not setting DTCs anymore. The information that helped me here was derived from the scope only.
For my next test, I used two low-amp current probes to monitor the current of both the intake and outlet NOx sensors (simultaneously/separately). As can be seen from the capture (the GREEN trace) the intake NOx current is repeatedly going high and low. The BLUE trace (for the outlet NOx current) is staying relatively steady (Figure 5). I then zoomed in and added the CAN circuits (YELLOW and RED traces). The disturbance correlated with the current of the outlet NOx sensor (Figure 6).
I thought this was odd, but I still thought I was on the right track. I decided to seek clarification of what was learned on this capture by checking a known-good circuit, (from a different vehicle) under the same conditions (Figure 7). They are mimicking each other!
Ok, now I was getting somewhere! After seeing this, I sampled only from the intake NOx sensor, looked at CAN-Hi, CAN-Low (YELLOW and RED channels), voltage (GREEN channel), and current (BLUE). I placed my scope ground reference at the sensor ground. When I referenced my ground at the sensor I was checking the voltage drop across the load. I checked voltage and ground at the same time, freeing up a scope channel.
Looking at the capture, I again noticed that the CAN network was being affected by the intake NOx sensor’s heater operation. The voltage drops coincided with the current increase from the intake NOx sensor (Figure 8). After gathering this information, I deduced that the excessive current from the intake NOx sensor was bringing down the network. hink of voltage as pressure and the flow of air as current. A bigger air hose allows for more flow/current, resulting in more of a drop in pressure/voltage. This told me the fault was not caused by a wiring problem (this would be added resistance). If it was, there would be voltage drop present, but also less amplitude in the current ramp.
What happened here is the intake NOx sensor module was repeatedly resetting because the heater was internally shorted. Every time the current went up (on the BLUE channel) the module shut down, and the current then went away. The effect of the excessive current is the voltage drop (visible in the GREEN trace). This repeating excessive current was interfering with the bit timing of the CAN bus (as indicated on the YELLOW and RED traces).
After I replaced the intake NOx sensor all was well; and according to the scope capture, the new intake NOx sensor’s functionality looked like that of the known-good capture I took (Figure 9). Notice the current of the intake and outlet NOx sensors are similar. You will see no major disturbances on the CAN bus circuits, as you saw in the faulted captures.
This bus now runs great with no issues. If I didn’t have a scope to help with this diagnosis I would just be throwing parts at it, hoping for a fix. This problem was elusive as all DTCs were inactive and the operation of the vehicle seemed OK. It is now more important than ever to have access to a scope and know how to use one.
