If at First You Don’t Succeed, Quit Trying

Posted On March 3, 2010 at 6:37 pm by / No Comments / CAN BUS

On most late-model cars, there are hundreds of messages with thousands of parameters being transferred between controllers, this leads to the idea that EVERYTHING is being able to be controlled by CAN BUS messages.  This simply is not the case.  If a controller doesn’t need to send internal data to another controller then it won’t.  Simple as that.

So why am I writing this? Because the biggest waste of time in reverse-engineering the CAN BUS is looking for a command or a parameter that simply isn’t there.  You need to have the notion that everything is not available and sometimes you won’t find it no matter how much you look.

So how do you determine how long to spend on looking for a parameter or command?  Well the answer, I think, depends on the importance of getting the parameter.  I take the example of trying to find Engine Speed on a CAN BUS.  Engine Speed is nearly universally available on today’s CAN BUS vehicles.  It is usually simple to find and scale, so not much time is wasted, but in contrast, Outside Air Temp may not be collected by a module on a car or may not be broadcast by the controller that collects the data.  In the first instance where it is not collected there are ways to find this out before hand.  You can look at publicly available wiring diagrams that show (or rather do not show) that there is no Outside Air Temp sensor on any controller.  This will save you time and frustration.

In the case of commands sent over CAN BUS (i.e. Unlock from a key fob).  These can be tested by capturing all of the messages while you pressed the unlock button, then play those messages back to the network one or two times to see if the doors unlock.  Why does this work? If the message was sent over the CAN BUS then by playing all of the messages back on the bus you should have the same result as when you pressed the button such as any parking light flashing, horn honking, etc.  If you do then you know the message is broadcast over the CAN BUS, if not then you can assume that it is most likely not available.  If it is not available, simply move on.  You can try the process again one or two more times, but you probably did it right the first time.

So good luck and as always let me know what you’re working on, I’m always interested to help.

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