can bus

would like some opinions on this can bus capture as this is new to me. i have no communication with the engine ecu from the cab diagnostic plug but can communicate while back probing at the ecm. I have no drivability problems just communication fault on dash. truck has left workshop for the time being and i was hoping i could get some opinions on this so i can diagnose it next time its in. please let me know if the attachment doesn't work, I haven't tried it before on here.

The CAN low signal level seems to idle at 5 V instead of 2.5V. Most of the time the signal is pulled down to ground, only a small fraction shows an adequate voltage drop of ~1V. The CAN high recessive voltage level should be close to 2.5V, yours seems to be slightly above ground voltage and again the peak to peak voltage on the signal is almost 4V instead of one volt

Then again, it seems to be a Hantek scope with a long time base selected surpassing the capabilities of the tool.

Okay I see what you mean, so getting another capture of this at another time base would be my first step I’m guessing. So what time base would you suggest I try and also are you thinking it could be a hantek issue?

The Hantek isn't too much of a help here but the main problem seems to be the voltage levels. This wouldn't be a Hantek issue though.

Alright thank you for your help, one last question have you seen this before and what did you find that caused it.

The capture does look a little strange. Was it taken at the DLC or ECM? And what were you using for the scope ground?

Anyways a message on the cluster and no comm with the ECM at the DLC ain't much of a starting point. Scan the entire vehicle at the DLC to see what modules are visible and what codes they're setting. Then maybe rescan backprobed at the ECM. Those tests coupled with, preferably O.E., network wiring diagrams should give you some direction.

Captured at the ecm and using chassis ground or block can’t remember. The code in the picture is in the vecu and becomes active while running, the other captures are at the dlc plug and cab ground was used.

The second of the last 4 pictures shows correct voltage levels on both can signals if it was a high speed can network (and it doesn't seem to be, it's another protocol). Make sure your connections are ok or/and measure using just one channel across both can + and can-.

Okay thanks for your help very appreciated thank you.

Jesse c wrote:

The code in the picture is in the vecu and becomes active while running

That explains why your first waveform looks off :silly: J1708 is a low speed, RS-485 based, protocol which your Hantek is fast enough to work with. Your first capture looks about right other than the one packet not getting pulled to ground properly. I'd check voltage drop between cab, chassis and engine grounds first. Modules sitting at different ground voltages could be the problem because although RS-485 is a "2-wire" network it also requires a common ground between modules.

Oh wow I didn’t realise that difference in protocol changed the way they looked on scope. Ok ground’s I will check, so those captures at the dlc really threw me and lacking confidence doesn’t help. Does j1708 have different voltage ranges compared to others like j1939 which is in a lot of trucks

Ok thank you I’ll try checking the grounds when I can