Tool Q&A: Network Diagnostics

Sept. 6, 2013
Down and dirty techniques for diagnosing CAN and fiber-optic networks.

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 break out box (BOB), though a meter, PowerProbe and scan tool come in handy too. The most important tool is the BOB. It connects to the DLC and has holes that you can put your meter or labscope in so that you can read voltage at specific pins of the DLC. You will also need wiring diagrams 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 what you are jumping.

Q: What network diagnostic strategies should I use?

A: First, we need to recognize what we're working with. 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 try your best with the following wiring diagrams, courtesy of Mitchell (See Figs. 1 and 2). On this 2006 Audi A8 we would be able to communicate to the PCM, even if the entire CAN network was 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, we would not be able to communicate to any other module that is exclusively connected to it, such as the right rear door module.

So we need to understand what module connects where in order to anticipate how we should go about diagnosing the vehicle. Sometimes you will find certain modules 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.

Now that we have all of this stuff in our head, how can we use this to 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). Don't just go about pulling random fuses and modules out. Be strategic about it. Keep the network topography in mind by pulling out the modules that you think are 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 your PowerProbe 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. Check the ground as well.

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 you do this and one of them, even when powered, does not communicate, you have your suspect right there. The preceding strategy only works when there isn't a dead gateway module, because if there is, it would block any communication to all modules "behind" it on the system. 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: How should I use my break out box (BOB)?

A: When you plug in your BOB, pin 16 is battery power, and pins 5 and 4 are, most of the time, your grounds. The LED near the jack should light up in some way to indicate that you have live power. Then, connect your scan tool to the BOB and labscope to pins 6 and 14 (which tend to be CAN high and low). CAN high and low should be mirror images of each other (when one is high, the other is low and vice versa). You might need to use different pins depending on different kinds of networks (2 and 10, 7 and 15), so know what network you are dealing with.

Then, communicate to suspect modules while your labscope is connected to the correct pins in the BOB. You should get a picture (See Fig. 3). Simply put, if your waveform looks all messy, then you have a network problem (probably created by a bad module). If it looks neat, then you do not have a network issue as it pertains to that module.

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 irregularity (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.

Q: Is there anything special I need to work on fiberoptic networks on European vehicles?

A: Fiberoptic networks usually require two main tools. First, a factory scan tool  (or an equivalent aftermarket tool, see Fig. 4) is needed to communicate to the different modules in the vehicle. The second tool is usually supplied cheaply from the dealer, a "loop connector." It allows you to visually view breaks in the fiberoptic signal.

Vehicles with fiberoptics generally always have a regular CAN network as well. Usually, only a minority of modules would use fiberoptics on a vehicle and their operation is often dependent upon the CAN network being operational. So keep in mind that a CAN problem can cause fiberoptics not to work without there being an actual issue with the fiberoptics themselves.

Essentially, the way these fiberoptic networks communicate is by using very fast patterns of light. To test them, use your scan tool to do a ring break test. Perform the ring break test by simply unplugging the fiberoptic connector from the suspect module and activating the test on your scan tool. You should see light flashing from the fiberoptic harness connector (this confirms the integrity of the fiberoptic cable), a bunch of codes should set for lack of communication to that module and the scan tool will indicate the ring break test failed (See Figs. 4 and 5). Now connect the loop connector in place of the module. You should see light going through the loop connector and the other modules will think the module you took out is online. Codes won't set and the ring break test will pass. If this happens, you just proved that the module you removed was disrupting the fiberoptics. Simply replace it.

Now, if you follow all the preceding steps and the ring break test does not pass, that means that that there is a different module in the fiberoptic network causing the issue. You will have to find the next suspect module and repeat the same testing.

Most German cars with fiberoptics use the same loop connector. For example, the Audi connector (No. 4E0-973-802) fits most brands for $28.

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