Wave on Can Bus

Paul Owner/Technician Wisconsin Posted   Latest   Edited  
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2015 GMC Savana 3500 6.6L (L LGH) 6-spd (6L90)

We are getting a “wave” in the Can Bus signal between NOx 2 and the level & temperature emission reduction fluid tank module. We replaced the module a couple of weeks ago and that corrected the problem for a short period of time. Code set was a P11CB NOx 1 performance signal high. I attached a copy of the wave form. Verified powers and grounds as well as terminating resistors.

2015 GMC Sierra 3500 with 6.6 Duramax

Anyone run into this or have an idea?

Thanks!

UPDATE

It appears we have an issue with the power supply from the Glow Plug Control Module. Clean voltage going into the GPCM but the 12 Volt feed coming out of it has a voltage drop/surge that corresponds with the CAN BUS. I attache a couple of pictures.

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Caleb Diagnostician
Indiana
Caleb
 

Year, make, model, engine type etc?

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Paul Owner/Technician
Wisconsin
Paul
 

Oops! 2015 GMC 3500 6.6 Duramax

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Glenn Owner/Technician
Texas
Glenn
 

Hello Paul, Vehicle info? A signal high code usually means a break in the circuit wiring, connector pins or a rub through. Load test the wiring after inspecting carefully, if it passes, then the NOx sensor is suspect.

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Dmitriy Analyst
Ontario
Dmitriy
 

Paul, what do you use for grounds of the scope? You are looking at the same signal, just in different locations, but the waveforms are off by about a volt — this means there is that much voltage drop on one of the grounds!

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Jordan Apprentice
Ontario
Jordan
 

Hi Dmitriy, can you please explain this? Just following along and am not totally understanding this. Can a voltage drop on the power side not cause this as well?

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Dmitriy Analyst
Ontario
Dmitriy
 

This is about how Pico4425A works: for each channel it measures between the input lead and the ground lead. Unlike many other scopes, the ground leads do not have to connect to the same place. So, in case we connected the inputs to the same wire (assume it’s good), but ground points are different, the difference between measurements will be due to the voltage difference between the ground…

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Andrew Technician
Utah
Andrew
 

I have seen this before. One of the modules on the CAN bus is shifting the bias voltage. What you need to do is unplug each module one at a time and jumper the CAN bus connections so that the rest of the bus will be functional. When the bus signal straightens out you've found your offending module. You'll then need to voltage drop check the power and ground circuits while the circuit is loaded…

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Glenn Owner/Technician
Texas
Glenn
 

Hello Paul, Make sure the scope grounds are at the battery negative and nowhere else, a scope is very sensitive about this.

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Justin Technical Support Specialist
California
Justin
 

diag​.​net/msg/m1kudxq6dy…

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Matthew Diagnostician
Kentucky
Matthew
 

GM busses are inherently noisy, or at least a lot of the ones I’ve seen. might want to use a math channel to confirm if this is an actual problem or not. Any time I scope a gm can it looka very ugly in comparison to other Manufacturers. this also being with no communication faults. the buss is just noisy. Justins shared article above is very helpful information to isolate wether ur chasing a…

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