I have a computer using ODB-II interface for grabbing data (using dashcommand). I also want to use the computer for mp3, by connecting it to the ausiliary port. However, everytime i connected the ODB-II cable...the aux line-in in my headunit doesnt work, but everything is fine if the ODB-II and aux line-in are not using at the same time. Anyone knows why?
I use a auxilary cable connecting my sound card to the aux line-in of the head unit. As for the odb cable, it is just a usb to odb-II interface.
I guess its not a grounding problem, as i tried to use the standard ac-dc atx, so its not somthing relating to my m4-atx...weird.
I agree with Joe, it sounds like a grounding problem. When you plug in the OBD-II interface into your PC, it connects PC ground to vehicle chassis.
Where does the head unit get its power from? Can you connect its ground to the vehicle chassis?
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But, generally the symptom would be loud buzzing on an audio input. When you say 'doesn't work', what do you mean?
Thanks mate. Now I sorted out that it is a grounding problem. After twisted my audio input cable, I can hear the songs now. But as you said there is loud buzzing noise from speakers. Any quick way to eliminate those annoying buzzing noise?
Another issue is the dashcommand gauge reading. When connecting my PC using the OBD-II cable, the readings (e.g. RPM, airflow ratio) in Dashcommand are not stable even when my car is idle, the reading is moving up and down all the time. It happens with my 2 years old PC (with the motherboard "ECS RC410-M"), but it works just fine with my 10-years old legacy computer (at least 7 years old). Any clues?
The reason ground loops cause problems is that ground potential isn't exactly the same at all points, and these differences cause current to start flowing, and the high and low points to change. This causes interference to many things. On the audio, it is easy to notice as a buzzing noise. On a digital line (such as the OBD bus, USB bus...) it causes weird things to happen increasing the BER (bit error rate). Many protocols intended intended to be used by wire (generally if it is intended for wireless use, there are error detection and/or correction mechanisms) assume a BER of 0.