Network Diagnostics

Down and dirty techniques for diagnosing CAN and fiber-optic networks.

Q: How should I use my breakout box (BOB)?

A: When you plug in your BOB, pin 16 is battery power, and pins 5 and 4 are, most of the time, grounds. The LED near the jack should light up in some way to indicate that there is live power. Then, connect both a 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). Different pins might need to be used depending on different kinds of networks (2 and 10, 7 and 15), so it is good to know what network the vehicle has.

Then, communicate to suspect modules while the labscope is connected to the correct pins in the BOB. A reading like Fig. 3 is typical. Simply put, if the waveform looks messy, then a network problem may exist (probably created by a bad module). If it looks neat, then there probably is not a network issue as it pertains to that module.


Q: Is there anything special I need to work on fiber-optic networks on European vehicles?

A: Fiber-optic 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, a "loop connector," is usually supplied cheaply from the dealer. It allows users to visually view breaks in the fiber-optic signal.

Vehicles with fiber-optics generally have a regular CAN network as well. Usually, only a minority of modules would use fiber-optics 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 fiber-optics not to work without there being an actual issue with the fiber-optics themselves.

Essentially, the way these fiber-optic networks communicate is by using very fast patterns of light. To test them, use a scan tool capable of doing a ring break test. Perform the ring break test by simply unplugging the fiber-optic connector from the suspect module and activating the test on the scan tool. Light will flash from the fiber-optic harness connector (this confirms the integrity of the fiber-optic 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. Light will be seen going through the loop connector and the other modules will think the module is still online. Codes won’t set and the ring break test will pass. If this happens, the diagnostic proved that the module that was removed was disrupting the fiber-optics. Simply replace it.

Now, if the ring break test does not pass, that means that that there is a different module in the fiber-optic network causing the issue. Move onto the next suspect module and repeat the same testing.

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

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