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Thread: OBD2 works with the car off not on?

  1. #21
    What can I say? I like serial. Curiosity's Avatar
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    I'm wondering if you just got a defective one. Probably not easy return and get another one? Is it that little aluminum square one? That's the one I've been using that says ELM v1.5a on it which means it's a knock-off.

  2. #22
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    Yep that's the exact same one. If it was defective then it wouldn't have worked on the i30 so I can only guess its fine so to speak.

    Can I assume the ELM chip in in the metal box? So I couldn't bypass the Serial > USB interface by just plugging in the COM port straight to the laptop? Obviously if the ELM chip is in the metal box that won't work.

  3. #23
    What can I say? I like serial. Curiosity's Avatar
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    Oh, mine's straight USB. Sure, you should try the serial. Eliminate at least 1 more possible thing.

  4. #24
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    I'm afraid of it being wired differently and frying the COM port/PC by sending 12V down it... Does the OBD port send out 5V or 12V?

  5. #25
    What can I say? I like serial. Curiosity's Avatar
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    The USB to serial adapter works the same as a serial port and the data lines are 5-15V depending on how it's designed, and they aren't designed very well having only 5V to work with. So the serial port on your PC is designed to handle 15V and probably does it better.

    The OBD-II port is unregulated 12V and supplies power to the electronics in the EML interface, but that isn't connected directly to the serial port side.

  6. #26
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    Ok well I just popped the metal casing apart on the serial to USB and it contains the chipset needed for the ELM so I can't just plug the serial straight into the PC.

  7. #27
    What can I say? I like serial. Curiosity's Avatar
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    Does it look like this?
    http://www.nandtek.com/UploadFiles/ELM327_USB.jpg
    If so, don't plug that DB9 directly into the PC. Sorry I was thinking you had a serial ELM with a serial to USB adapter. That's not a serial port.

  8. #28
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    Yeah that's the one, and yes I definitely know now not to plug it straight in

    Seems like we are out of things to try now :'(

  9. #29
    What can I say? I like serial. Curiosity's Avatar
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    Yeah, I think so. If you had a DMM you could see if the volts are different between the cars. It could just be going too high for it, but the power goes directly to a 5V regulator so it shouldn't have a problem. There's something a little screwy about the electrical system.

  10. #30
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    FWIW, if OBD-II data is flowing with the vehicle off, the ECU probably doesn't need a reflash.

    If the problem occurs even when USB is connected to a laptop running on battery, it probably isn't a ground loop.

    I'd look at the OBD-II connector in the vehicle and see which pins are populated. Presumably, the OBD-II adapter has circuitry for all three common electrical interfaces, but non CARB regulated vehicles sometimes use other pins in non standard ways. If the vehicle is putting a voltage on one of the other pins, it could easily hang the little CPU in the adapter or trip the fault protection in the laptop USB chipset.

    -jjf

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