It’s not rocket science… Although they do use network topology
The Issue
Occasionally, when you’re working with vehicular diagnostics, you may come across a certain scan tool issue. Either you have no communication with the scan tool, or you may retrieve communication codes indicating that one or more specific modules are not communicating. Always start by familiarizing yourself with the type of network topology (structure) used for the communication network on the vehicle you are working on. This will help you to be aware of possible faults and help you strategize how to locate a problem. For most of the common issues, there are only a limited number of ways the system can fail.
The Solution
Check for TSBs. Sometimes software updates can solve a problem.
Check for any aftermarket equipment, modifications or prior work that could have been done by someone else. If other work has been done, check to see what part of the CAN bus wiring is near where that work was performed. You should also check any connectors in that area that could have been disturbed or are loose. Always disconnect them and check for bent pins.
Ensure you have the correct vehicle information entered in your scan tool.
After retrieving fault codes, take some time to understand what the DTCs mean and what they are telling you. Keep in mind the module reporting the communication code is communicating and in many cases knowing which modules are reporting each DTC can be all you need to narrow your pinpoint testing to a few tests. Plot the problem on a wiring diagram. The codes may identify which module or CAN bus is in the network or even which portion of the circuit is having a problem.
Examples of system failures:
- Wiring/terminal/interconnect failures
- Hardware failures (individual module pulling the bus low or high)
- Electro Magnetic and Radio Frequency Interference
- Software issues
- Power and ground issues to various modules.
- Car stereo/alarm hack installers and Scotch-lock connectors.
- DLC Insurance monitors and Loan company modules
- Aftermarket remote start systems
Try communicating with other modules. It is possible the entire network is not compromised but only one or more modules are down. This can be done by jumping the scan tool to a network splice to the suspect modules or module(s). Depending on the system, this could be done easily at a splice pack or gateway module. In the case of one or multiple modules being down check the wiring diagram to see if they share power or ground supplies.
Check for a data signal at the DLC using a scope. DLC breakout boxes are handy but you can test directly at the terminals. This will confirm no data transmission and can eliminate the possibility of a faulty scanner. This could also narrow down which network you need to be concerned with. While you are there at the DLC, check for proper power to pin 16 and grounds pins 4 & 5. Some scan tools require those to work while others may not. Sometimes there can be a bad ground that prevents the engine from running unless a scan tool is plugged into the DLC. This is because the scan tool is supplying that missing ground between pin 4 & 5.
Check at the DLC for CAN LO and CAN HI voltages. On CAN LO, anything below 2.48V is a short to ground and anything over 2.5V is short to power. CAN HI should be above 2.5V. If it’s not, there is a short to ground. The system can be shorted by a circuit or module (shorted internally). In this case, dropping modules off the bus can be productive. Just be sure the CAN bus circuit does not require the module to complete it, in this case you will need to jump CAN+ to CAN+ and CAN- to CAN- when disconnecting the module connectors so the circuit remains complete but with the module dropped out. If one wire is compromised or a terminating resistor is damaged, the noise on the lines will be greater than usual and show up on a scope waveform.
Check the terminating resistors. CAN twisted pair systems have two terminating resistors in parallel to each other on each end of the network. Each resistor is 120 ohms so the system will measure at a total of 60 ohms between CAN HI and CAN LO. If one wire of the twisted pair shows no data transmission, this is most commonly an issue with wiring and not a module.
99% of possible communication problems are covered above and if you work logically, you can find it.
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