Interactive Case Study - Intermittent HS-CAN fault

Doesn't look like a mirror image on the can hi on the zoomed in capture. In the wiring diagram near the IC there are a few splices that could be a problem area for the green crusties. I would most likely hook up my other two channels to can hi and low at the IC and see if it looks the same. Maybe do a wiggle test on harness under dash and around splices. Lastly unplug IC and see if pattern changes. 

Still new to this but hoping to learn something here! 

Just checked back on the diagram looking for something in common that could cause issues for those two modules. The splices that I already mentioned and connector C212 could be an area with possible corrosion

Looks like CAN + is being pulled low. It starts looking a lot like CAN -. My knee jerk reaction is to suspect the cluster just because of the codes and symptoms. I think it's either in the cluster or the connection at the cluster.

This reply is to Tyler and Nicolas - they are having similar thoughts. 

The PCM and IC have the termination resistors. 

I measured the resistance of DLC pins 6 & 14 (battery disconnected):

Initial resistance reading

Resistance with IC unplugged

Resistance with PCM unplugged

Is that useful information? 

Did you do this test at the start? Or later,I like to test this first on u codes. 120 ohm , ecm side is were the problem is .Looking forward to this discussion I can use the help.

I did this very early in the diagnosis process. I really don't like disconnecting the battery or connections until I have some direction.

You are correct, my resistance readings does indicate the problem is on the PCM side. Would you rule out the IC for now? 

Yes, I would look at my can on the scope with the pcm out of loop. Is it normal looking now.

No it scares the heck out of me! Puts a damper on replacing the IC??? Is there any reserve (KALM) voltage on line?

KALM - I'm not familiar with that acronym. Is that short Keep Alive Memory? 

Yes, I think the old acronym was KAM. I'm questioning if there may have been other voltage present when testing. Lets say a ghost source????

This would lead me towards the pcm side for sure. But not sure what my next step would be at this point need to think on that

Have you looked at the high/low CAN with the PCM or the IC disconnected? 

No, I did not. I wish I would have. 

Me Too! LOL

BTW - Not everybody can see your CAN circuit resistance checks unless they click on your reply to Tyler and Nicholas.

Hi Rick, anyone who hasn’t seen a reply will see it in its un-collapsed view...

I would be on the same page of condemning the PCM once and if a voltage drop is performed on powers and grounds to module. However looking at the network diagram the IC is the gateway for HS and MS networks. Yes the IC has coulds for the HS network BUT from fault test results we have ALL the HS modules talking and faults in the MS networks.

Definitely a good case study. I struggle with communicatiln fault codes. Will be taking an ATG class later in September in Plymouth. But besides the point. Looking to hear others chime in, with their thoughts. 

I believe the issue lies within the wiring to the PCM or the PCM itself. I say so because of the resistance values you posted. 120 is consdered normal with one terminating resister disconnected. In my opinion a reading of 430 ohms is a problem. I’d check CAN wiring integrity to PCM (ohm test wires from DLC to PCM) & check PCM powers and grounds. Load testing the wires. 

Thanks for sharing. 

Given those images I would keep the scope live and disconnect/reconnect modules one at a time until it looked normal. I'd start with ABS because it is outside the cabin.

I appreciate all of the replies and the feedback.

Most guys pointed out that there was too much resistance through the PCM termination resistor. This is where experience can interfere with logic. I've encountered a couple of Ford Five Hundreds with PCMs causing weird/random communication faults. With 430 ohms of resistance through the PCM at the DLC, I called a bad PCM on this one. The PCM was replaced and I came back out to program it. The vehicle was still acting normal, but the CAN waveform looked the same as before, and I still had around 92 ohms at 6 & 14. 

I got tunnel vision. On the first diagnosis, I didn't even consider the high resistance being a circuit fault. Reviewing the wiring diagram, I began to wiggle the connectors in the harness. As soon as I touched C212 (in the passenger kick panel), the resistance dropped back to 60 ohms and the HSCAN waveform returned to normal. It took about 5 minutes to diagnose it correctly. I cut the HSCAN circuits out of C212 and spliced them together bypassing the connector. 

Tyler Flight nailed this problem right off. In his second reply to me, he mentions C212 being a possible suspect. Awesome job Tyler!

CAN diagnostics can be challenging. If you've ever misdiagnosed one, don't feel bad - we've all done it, just learn from it. Hopefully, you can learn through some of my mistakes.

Even a blind pig finds a truffle every now and again! lol. The resistance measurements had me second guessing myself and lead me toward the pcm also. I enjoy this type of format really gets the brain working as someone who does not get many CAN faults coming through the door. Would you have seen 120 ohms if you put the ohm meter on the pcm pins with it removed? Did you check resistance with both terminating resistors unplugged? Thanks Robby 

Great question! Yes, I did measure the network resistance at the PCM. 

This was the resistance at the DLC with the PCM unplugged:

This was the resistance at the PCM - PCM unplugged:

I did not measure network resistance with both terminating modules unplugged. 

Awesome I sure learned something from this! Hope others did aswell. Great post Robby would love to see more of these interactive studys!

Glad you learned something. I know did. It's amazing how much you can learn by writing a case study on one you've diagnosed. You'll see steps that you either missed or steps you did that were unnecessary. You should try it sometime. I really liked the interaction seeing how other guys would approach that diagnosis.

Its also easier to stand back and look at the big picture when the vehicle is not in front of you, without distractions. Really shows that taking a coffee break stepping back, reviewing wiring diagram and code set criteria and making a sensible attack plan can most of the time lead to a efficient repair and less wasted time.

20/20 hindsight... next time, check the leads for less than 5 ohms per lead. Anytime I get one of these cases that looks like I'll have to spend $$$, I always check things again using two entirely different methods of testing to insure the first test results were accurate. IE... from one direction and then from the opposite direction or two different tools, scope and meter, scanner and meter... etc... . Great find though... LIKE 

Thanks Gonzo! I'll keep those things in mind.

I appreciate you chiming in!

love this stuff... :) 

I absolutely love this! Great post Robby! Sharing the story a piece at a time while encouraging others to engage by asking, "What would you do"? Everyone involved has a great opportunity to walk away with something. I feel like I heard that recently.

I would love it if we could describe in more detail what happened and, why it happened. How the terminating resisters affect these parallel circuits and why. What do the parallel circuits do? Why do we need them? What does terminating resisters mean? What happens when our resistance values change. Let's talk about network communications like we never have. Let's talk about why we have a 2.5 volt bias on CANHS. Why we have a bias on any circuit. Why we have two signals mirroring each other. Let's talk about which modules supply the bias circuit and whether or not all modules are capable of supplying the bias voltage. 

I'm not asking too much am I?

Maybe, for this thread, we focus on this case study but maybe hopefully someone will start a new thread with deeper look into network communications than we are used to. 

Robby, I really appreciate that you put yourself out there. Showing that you're capable of making a mistake shows great character. This lesson was multi layered for sure.

Thanks Brin. You are asking a lot of questions 😜- I thought you were one of my kids for a second there 😁. But it's not too much though. 

I agree, most of the things you are asking about would be better in a new thread. CAN is a subject you dive off deep into very fast.

Regarding the questions on termination resistors, I hope we can discuss their purpose in another thread (the theory behind their purpose is pretty complicated). For diagnostics though, it really doesn't matter what purpose they serve. However, it is important to know that they are there and where they are located. The CAN standard specifies each termination resistor is 120Ω (+/- 10%). With 2 of these resistors in parallel, this would give a total resistance spec of 54Ω - 66Ω. You can see that demonstrated in this step of an CAN pinpoint test from a Ford:

In the case of this Ford Five Hundred, I had 91.5Ω of total resistance. The termination resistors are located in the PCM and IC. Looking at the CAN waveform, there was problem with the CAN (+) circuit. There was 430Ω through the PCM and the fault was found in C212. That means there was 310Ω in the HSCAN (+) circuit in C212. I've made a note in the wiring diagram showing the resistance:

Because of the unwanted/additional resistance, we now had a series/parallel circuit:

Doing the math, we end up with very close numbers. 

1/120 = 0.0082

1/(120+310) = 0.0023

… = 94.34Ω