Q: What network diagnostic strategies should I use?
A: First, it is important to recognize what the problem is. Let’s say that a module does not communicate or a group of modules does not. If several modules are “bad,” chances are that a “gateway” module is down, taking down all the modules that are connected to it.
This might be hard to visualize, so let's look at the following wiring diagrams, courtesy of Mitchell 1 (See Figs. 1 and 2). On this 2006 Audi A8, a technician can communicate to the PCM, even if the entire CAN network is down. Why? Because it has a signal wire that connects directly to the DLC. Now, if the gateway module (the databus on-board diagnostic interface) were to be down, no communication would be possible to any other module that is exclusively connected to it, such as the right rear door module.
So it is necessary to understand what module connects where in order to anticipate how one should go about diagnosing a vehicle. Sometimes there will be certain modules that have communication and they all happen not to be connected to another module, where a bunch of other modules that lack communication not so coincidentally are connected to! Understanding what connects to what can be a major clue.
How does this information help diagnose network issues? The easy way to go about this is to start unplugging modules (which most of the time can be done easily by pulling their fuses). Be strategic about it. Keep the network topography in mind by pulling out the modules that appear suspect and are thereby taking down the other modules. The network might simply come back to life after pulling the plug on one of these modules.
Don’t replace that module so fast! With a Power Probe or with hot wires, try bringing power direct to the “bad” module(s). If it now communicates, then it was just a power issue to that module.
Use this same method to test modules that are down in situations where a bad BCM might have cut power to the ignition switch as an anti-theft strategy, thereby taking power away from all your other modules, killing communication. Just directly power them up. If they start communicating, they are good. If one of them, even when powered, does not communicate, the suspect module has been found. The preceding strategy only works when there isn’t a dead gateway module. Always check grounds to the modules when doing this.
Lastly, check resistance in the communication lines with a meter. If your meter says “OL,” you have a short to power or open circuit somewhere.
Q: What equipment do I need to get started?
A: Network problems are rare, but they happen and we need to get tooled up. Thankfully, quite a few of these tools are probably already in your box. They include a labscope and a breakout box (BOB), though a meter, Power Probe and scan tool come in handy too. The most important tool is the BOB. It connects to the DLC and has holes where a meter or labscope can read voltage at specific pins of the DLC. Wiring diagrams are also important in order to look at where to connect on these modules in order to figure out which module might be preventing communication from the other modules. Lastly, it never hurts to have some alligator clips and pieces of wires to jump circuits (for example, to see if you can get communication to a module). Just be careful to connect to the right wire.
Q: How do I interpret a labscope waveform when diagnosing a network issue?
A: Network waveforms, specifically CAN, tend to be high- and low-voltage square wave signals, usually between 0V to 5V. Certain systems might go between 0V to 7V or even 0V to system voltage. Sometimes, the system might have an “intermittent” voltage spike to a higher voltage, but watch out. This may be normal and it can be a strategy used to wake a module up.
Typically, voltage drop-outs, spikes and irregularities (seen as a fuzzy waveform) can indicate a problem. On CAN vehicles, for example, you should see a perfectly inverse (“mirror image”) relationship between CAN high and low. Of course, comparing against known good waveforms always helps too.