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How do I see J1939 signals on an oscilioscope?

marked23
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marked23

marked23

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I've done the P1689 troubleshooting steps.  I made it to the end of step #8.  According to the steps, that means my ECM is bad.  In keeping with the idea that "it's probably not the ECM," I've been trying to double-check the communication between the ECM and the VP44. 

 

I tried to put an oscilloscope on the VP44 today, to see if I could see the data.  If I see the ECM's data, then it's the VP44 computer, not the ECM.  I saw something that looked like PCM data, but I couldn't figure out how to set a stable trigger on it.

 

I put channel #1 on the black & white wires (Pins 1 & 2), and put channel #2 on the green wire (Pin 8).  Then I tried to trigger off of channel #2.  If I tuned the trigger just right, it would hold-still for a couple seconds.   I was expecting that the green wire (sync signal) would provide a perfect trigger.

 

Does anyone know the correct scope setup to see this signal? 

 

 

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