Interesting No Comm Waveform

Caleb Technician Mishawaka, Indiana Posted   Latest  
Case Study
2015 Chevrolet Silverado 1500 LT 5.3L (C L83 Flex) 6-spd (6L80)
No Crank No Start No Communication

Hey guys, this was so easy that it really doesnt warrant a case study. However the waveform of the failure is very intriguing to me. 2015 Silverado no comm with any High Speed Network module on pins 6 and 14 at DLC. However good comm with Chassis CAN at pins 12 and 13 and also with Low Speed at pin 1. My coworker called me over after his network resistance at 6 and 14 was 60 ohms and he wasnt sure were to go. Because he doesnt have a scope I decided to use a meter on 6 an 14 to measure the voltage before hooking up my scope. I had 2 volts on CAN low pin 14 and 3 volts at pin 6 Key On. Well at that point I decided to hook up my scope. I first scoped Chassis CAN at pins 12 and 13, that network waveform was great. I then scoped the High Speed at pins 6 and 14 and got this:


Wow!!!! Absolutely a perfect waveform but no work/no messages being transmitted whatsoever. After looking at a High Speed Network diagram there are some modules that are on Chassis CAN and High Speed CAN. I an effort to find our issue quickly we decided to focus only on the modules solely on the High Speed network since the Chassis CAN appeared to be okay. The ECM is the quickest to unplug, after he unplugged it our waveform instantly changed.


Wow lol I never get it on the first shot but thats cool! Anyway a new ECM later an this truck lives to die another day. Im just curious if anyone know what this signal is. It was strong enough to overcome the entire network. It also is 100% repetitive, never tries to transmit any messages. The resistor was still intact. Is this a CAN transceiver that is no longer connected internally to the actual computer inside the ECM?? I have no idea what Im saying but I hope the question makes sense. I wish I would have gotten a waveform of just the ECM but I didnt. Anyway curious if anyone knows what this signal is?

Todd Mobile Technician
Morinville, Alberta
Todd Default

Why did you think the first waveform you posted was good?

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Caleb Technician
Mishawaka, Indiana
Caleb Default

I didn't mean good as in a good CAN waveform. As far as CAN goes it's worthless. I just meant good because of its crisp lines an perfect mirroring. Sorry for the confusion.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default

My thoughts exactly Todd. Didn't look right to me.

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Caleb Technician
Mishawaka, Indiana
Caleb Default

I responded to Todd's question

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Jaxon Technical Support Specialist
Stafford Heights, Australia
Jaxon Default


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Sean Technician
Forest Lake, Minnesota
Sean Default

It might be “ringing” or echoing of messages. It can happen when terminating resistors are compromised. Was 1 of the resistors in the ECM by chance? I had a Cadillac recently with a very similar signal caused by a near open network separating the ECM’s resistor from the rest of the bus.

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Chris Diagnostician
Lansdale, Pennsylvania
Chris Default

Caleb, It doesn't appear to be the result of a single (or dual) failed terminating resistor, as that generally leads to a relatively valid waveform (varying durations of signal high/low) with some comm errors stacking up over days and weeks. You mentioned the CAN transceiver not being connected to the computer inside the ECU and that seems to be partially valid. The transceiver does in fact…

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Billy Mechanic
Deming, New Mexico
Billy Default

Well amungst the old guys that is whats called PFM (pure ......... Magic) hey as far as spending time on it unless its just for fun if it works ship it cause in the end the main thing is does it go down the road with no yellow lights on the dash.

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Gary Owner/Technician
Owings, Maryland
Gary Default

Caleb, I agree with Chris Groff, my understanding of the inner workings of the CAN Node are this: The node typically has three functions, contained within 2 physical sections. (1) The CAN Controller Section, which is responsible for MCU communication (back and forth, on the Node's data sheet it shows as a RCV/TNX function) between the PCMs IC and the CAN Node. You can literally think of this…

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