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Thread: sample for in-car OBDII output on 20x4

  1. #31
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    Well you see there are some "standards" that must be adhered to.

    For example, the interface itself is a mandatory standard. Any car that is sold in California since 1996 must have this interface. However there are 3 different communications methods for the interface. GM cars use VPW (variable pulse width?) and basically you just have a data interface at 8192 baud. Unfortunately that's not very friendly for the computer to understand. There are also ISO and PWM (Pulse Width Modulation).

    There are 9 "standard" modes that you can use to access data. A mode is just a subset of commands I guess. mode 1 is mandatory across all platforms, however the more interesting modes 6 and 7 (for continuously and non-continuously monitored tests) are apparently rather manuf specific. Anyway these are all published in the J1979 and J2190 specs put out by SAE. They want to charge you big bucks to get a copy of them.

    In another thread I posted a lot more information about this and links to what I know about the modes. I'm extremely busy right now, just search on OBDII here and you'll find it.

    Yes this is also heavily dependant on the ECU that you have. The OBDII specs were created for emissions regulations, so the minimum requirements are not of much use to us. Manuf's don't have to allow any more access or write access at all. Not a lot of stock ECUs support tweaking via the OBDII port (I think), you mostly have to get aftermarket ones that allow you to do that, or get a really high performance car. The ELM322 interface that scantool uses cannot write raw data commands and can only access the OBDII spec data, unfortunately. I'm getting the sinking feeling that the information that I want to get out of this can't come directly through OBDII port via the ELM322 chip

    Via mode 1 you can get things like Fuel Trim, Intake Manifold Pressure, RPM, Speed, Coolant Temperature, absolute throttle position, calculated load value, intake air temperature and timing advance. You cannot get oil pressure, voltage, etc out of mode 1.

    There are extensive o2 sensor readings available in mode 6 (continuously monitored tests) but I can't find any information on how to access it!

    Of course I could be wrong on all of this, since you're right, finding detailed information is basically impossible because it's all copyrighted by the SAE. If you happen to find a copy of the J1979 spec or anything else like that please let me know!

    I am dying to figure out how to get all of that data. I know that everything that you mentioned plus lots of stats about the fuel system (which is what I'm interested in) can be read easily via the OBDII port because I have seen a not-very-expensive OBDII professional scantool that displayed all that information, I just have no idea how it did it.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  2. #32
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    Would it be possible to get a piece of hardware that could hook up between one of these 'high enabled' scantools, and the car's ECU, that would in effect 'listen' to the commands and the data sent back and forth? Could the commands and uses be reverse engineered from that data?

    If so, i might have access to the scan tools that the dealerships use (not that they are top of the line, but possible.)

    Also, i have found those companies online that will sell you a very functional package (i seem to remember something about a 'body' package...) with many of the features that we are discussing, could we reverse engineer those commands? Through either software or hardware....

    Just some thoughts/
    System in the works:
    Celeron 850 or PIII 933, SB Live! Value into a stock 4 channel amp, ATI vid, 10.4" sharp LCD w/ allen's controller, Elo SW touchscreen overlay and controller, GPS, TV tuner, FW DVD drive, 4 port serial card, and some other stuff....

    Loads of ideas.

  3. #33
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    reverse engineering will be of little help, but good idea.

    You see the problem isn't really how to get the data out of the ECU, it's what is the format of that data and what does it all mean. We could spy to get the commands sent to retrieve the data, but since the data will be processed internally to the scantool it'll be very difficult to do anything about that.

    I think that the ECU data format will probably vary between ECUs so it's not even very standard. I guess there is a good reason why these high end scan tools cost so much, they actually do a lot of work!

    If you have a scantool and can spy on the data stream however you should be able to cross reference the values displayed on the scantool to the data stream and figure out the mapping, but short of that I don't think that it would be of much help.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  4. #34
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    i have similar project in mind, i will post the result.

  5. #35
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    Quote Originally Posted by usernome View Post
    i have similar project in mind, i will post the result.
    You totally brought back a thread from the dead!

    Telek: how did your project turn out?

  6. #36
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    Quote Originally Posted by Brock View Post
    You totally brought back a thread from the dead!

    Telek: how did your project turn out?
    ditto! i'm very interested in doing this exact same project on my car

    anyone know what happened to the 2 pics on the first post at the beginning of this thread??

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