Interactive Case Study - Intermittent HS-CAN fault
I would like to share this case study, but in an engaging fashion. I know from experience, if I get involved with discussion, I always carry something away from it. There are some very sharp guys on this network, so I know how intimidating it can be to post something. To try and take away some intimidation, let me say, first off, that I misdiagnosed this vehicle, had to go back, and diagnose it correctly.
I’ll call this an “Interactive Case Study”. I’ll present my initial findings and some data, then I’d like to see how you would approach this diagnosis (similar to Brin’s Mechanical Testing Techniques Exercise).
2006 Ford Five Hundred
Intermittently, a bunch of lights will come on the dash, and the Odometer will read dashes.
Of course, being intermittent, when I get to the car, the customer’s concerns are not present. No warning lights are on and everything appears to be working normal. These are the DTCs I got from the vehicle scan:
- Pass - (ABS)
- Pass - (ACM)
- Pass - (GEM)
- Pass - (PCM)
- Pass - (TCM)
- U0002-60- (IC) - High Speed (CAN) Communication Bus Performance
- U1900-20- (IC) - (CAN) Communication Bus Fault - Receive Error
- U1900-A0- (RCM) - (CAN) communication bus fault Fail - (4X4)
- Fail - (EATC)
- Fail - (DSM)
- Fail - (PAM)
- Fail - (RETM)
Based on the customer’s concerns, I expected a network issue – and the DTCs confirmed that. I cleared the DTCs, but no DTCs returned.
My plan was to scope the HS-CAN signals using a DLC BOB, road test the vehicle, and watch for any unusual. Without even driving the car, this is what I immediately found:
Initial HSCAN waveform - zoomed in
Again, I have not yet verified the customer's concern.
How would you precede with diagnosis based on this data?
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
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
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
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
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…
Thanks Gonzo! I'll keep those things in mind. I appreciate you chiming in!
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
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