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Thread: GM Communication

  1. #1
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    GM Communication

    Ok,

    Can someone shed some light on my PID/GM CAN signal understanding dilemma.

    I understand that different modules in a car communicate to each other using one of many protocols, however lets use high speed GMLAN CAN as the example.

    Say we have the body computer talking to the engine computer. There are many signals being transmit from each of these units regularly while the vehicle is driving. E.g park brake state, current gear and so forth.

    How does PID's fit into all of this.

    Are PID's only used when a diagnostic tester is being used?
    Does the body computer use PID's when requesting/sending data to engine computer (No scan tool attached)?

    Coming back to my example above, if I wanted to spy on the current gear signal in my Holden VZ and not issue any request to do so, how do I find out what message it is contained in and what resolution is? I know GM wont tell me, but I'm out of keywords to type into google. I have been typing PID into google but I think Im barking up the wrong tree as I don't want to send any data on the bus.

    Any help would be appreciated.









    The different modules in a car communicate to each other over CAN.

  2. #2
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    most modules just broadcast things without you having to ask. It's not like obd-ii where you have to request everything you want.
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  3. #3
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    Quote Originally Posted by mechalec View Post
    Say we have the body computer talking to the engine computer. There are many signals being transmit from each of these units regularly while the vehicle is driving. E.g park brake state, current gear and so forth.

    How does PID's fit into all of this.

    Are PID's only used when a diagnostic tester is being used?
    Yes.

    Does the body computer use PID's when requesting/sending data to engine computer (No scan tool attached)?
    No. The PIDs only come into play when an external device (e.g. a scan tool) is performing diagnostic functions. The various modules in the car don't use them to talk amongst themselves.

    Coming back to my example above, if I wanted to spy on the current gear signal in my Holden VZ and not issue any request to do so, how do I find out what message it is contained in and what resolution is? I know GM wont tell me, but I'm out of keywords to type into google.
    You need a device which can interface with the GMLAN bus and go into "promiscuous mode," passively capturing all the traffic on the bus so you can examine it. Some OBD-II scan tools can do this. For example, some of the ELM-type units have an "ATMA" command to monitor all traffic on the bus.

    Of course, capturing the data is the easy part; figuring out what all the messages mean is tough, since unlike the diagnostic stuff which is standardized and well-documented, the regular traffic on the GMLAN bus is all proprietary. As you surmised, GM isn't likely to tell you how to decode them. You're going to have a long, tedious process of causing your event to happen (in your example, changing a gear) and figuring out which of the thousands of messages in your capture corresponds to that particular event. It's challenging but very rewarding.

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