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Automotive network diagnostic strategies

Wednesday, March 1, 2017 - 09:00
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“May you live in interesting times” is a quote that always caught my ear growing up. After Googling it, I found that originated as an old Chinese curse. In the automotive repair industry we certainly are living in very interesting times. Technology is advancing at such an alarming rate that technicians and shops are in a constant scramble to keep up. Some may regard this a curse indeed!

For others, those you are willing to invest in themselves and are eager to adapt, they hopefully see this as an opportunity to learn and to thrive. Communication networks and their diagnosis are a prime example of both the challenges and the inevitability of this technology moving forward.

The Trick to it all

Communication networks come in many different configurations and protocols. The trick as I see it is not to become overwhelmed or intimidated by the task or diagnosis at hand. Having a good plan of attack is one of the keys to reaching a successful conclusion. Having an understanding of how the network is laid out and the protocol or protocols it communicates with are essential. A wiring diagram and a quick read of service information will usually provide a quick insight of the network’s layout and communication characteristics.

Ford Network Tool

I believe similar to any automotive electronic system diagnosis, the first step is to garner as much information as you can with the least amount of effort. For me, this is almost always by plugging a scan tool into the DLC to communicate with modules, pull codes and look at the data stream. But what do we do or how do we start if we cannot communicate with the network, certain modules are unresponsive, or we have those ever pesky “U codes?” We will examine some of these issues and some commonsense approaches to develop a systematic plan of attach (POA) to aid us in no communication and U code issues.

In general if we can talk to other modules on the network bus there should be some form of U codes set. The way I like to simplify it is; the module with the U code is stating that “the problem is will u (you) and not me.” In other words if a 2008 Cobalt PCM has a U0140 for communication fault with the Body Control Module, generally the fault is not with the module that reported the U code, in this case, the PCM. So in this example, our POA will concentrate our efforts and testing at the BCM.  My POA would start simple with the scan tool and perform a network test if offered, if not I am going to attempt communication with the suspect module or in this case, the BCM. Suppose I have the ability to do a complete vehicle scan and multiple modules report code U0140 and the network test is performed and all the modules are on the bus except for the BCM. These results in itself tells me quite a bit if I employ the diagnostic tool on my shoulders. I know that the bus cannot be dead or shorted to power or ground due to the fact I can communicate with other modules on the bus and that they have pointed the finger at the unresponsive BCM. This greatly helps to streamline my process doesn’t it? Now I going direct to the BCM and do my testing there.

Figure 1

One simplification or universal rule of thumb I use is that all modules require at the very least three things and some a couple more. In order to communicate, all modules require power, ground and the physical layer for data communication or the serial data line (network bus integrity) Some modules may also require a “wake up” signal and some modules will lose communication if a sensor voltage supply circuit to shorted. This being said, I will reference my SI for a diagram of the 2008 Cobalt’s BCM (Figure 1). I want to verify that I have power and ground. I like to use a headlight or a headlight bulb to load the circuit and perform a loaded voltage drop test. If I have solid power and ground, my next step is verify the network connection or the integrity of the serial data bus between the BCM and the rest of the vehicle. I prefer to use a lab scope for this, but a wiring diagram and multimeter could also be used. If the power, ground and serial data circuits are good and I have no communication with the scan tool and have U codes for the BCM in other modules, the BCM is most likely the issue. This is a fairly fundamental failure of a module that is pretty straightforward in its diagnosis. Things can get a lot more challenging, so let’s look at some others and some tools and techniques that will aid us in our diagnosis.

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Multiple modules

Perhaps my failure is several modules that do not communicate or that I have multiple U codes for communication issues on a vehicle with a standard CAN High and Low Bus. Something to consider, a lot of standard Controller Area Networks or CAN networks use some sort of termination resistance on the Bus. Most use 120 ohm resistors at either end of the Bus. The resistors can be built into a module or a physical resistor that is plugged into the bus or even the fuse block. Ohms Law tells us that the result of these two 120 ohms resistors wired in parallel is 60 ohms so if we were to disconnect the battery and place an ohmmeter across pins 6 and 14 of the DLC connector, that’s the resistance I should read. Remember to use a DLC Breakout Box (or BOB) or the correct test terminal in the DLC connector to avoid creating terminal tension issues (Note: There are exceptions — like Dodge vehicles —always consult a wiring diagram).  If I see 60 ohms of termination resistance, I have a fairly good idea of the integrity of the CAN bus.  If I see 120 ohms I know I have some testing to do. I could be looking for an open in the Bus, a blown termination resistor or a defective mode that houses the termination resistor that is open internally. I can also use a decade box or a 120 resistor and my wiring diagram to strategically substitute while observing the termination resistance at the DLC. 

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