An Interesting Jeep CAN Diag

Mike Mobile Technician South Carolina Posted   Latest  
Case Study
2008 Jeep Wrangler Rubicon 3.8L (1) 4-spd (42RLE)
No Communication With Used Ccn

I got this call to "program" an 08 Wrangler instrument cluster "CCN".… No comm when I arrived and the customer decided to have me diagnose it. The story was that the truck needed an engine and in the months the truck sat was deciding whether to pull the trigger on a new engine someone had taken the known good CCN out of it to put into another Wrangler. When the customer finally did decide to replace the engine it was installed and ran along with a used cluster which they could not get to work. The TIPM and the radio would immediately set codes U0155 for no comm with the CCN.… The wipers would also stick on high, another symptom of a no comm CCN. I noticed that a connector had already been spliced in assuming that they had tried that in an effort to repair the vehicle and there was another cluster in the backseat with junkyard numbers on it in the back seat.… I checked for proper powers and grounds at the connector as well as made sure all of the wire colors matched. Everything looked good. Next, I scoped the can lines at the CCN and found a good clean square waveform.… I also noticed that the CCN would power up correctly and seem to function correctly and show a mileage for a second or two before switching to a "no bus" message. The other used cluster had the same symptom. One last thing that I noticed happens to be a nice feature exclusive to the Micropod II diagnostic tool. Codes are listed as stored as well as active and that is a live reading. I noticed that the U0155 was switching back and fourth from stored to active. I was trying to prove my theory that both of the used units were for the wrong vehicle when I noticed a very brief glitch in the CAN waveform. I watched it closely for a while before finally catching it in a recording to go back and review and here is what I saw.… I also noticed that every time I was able to observe this abnormality on the scope the U0155 code could be seen switching from stored to active. I found it interesting that this symptom was so intermittent as I was assuming that this abnormality was only from communications sent from the CCN and it could only be seen happening every 30 seconds or so but sometimes it would take a minute or so to catch it. I could assume this to be because of the refresh rate of the scope but when set up with a trigger it seemed to correlate with the active/stored message on the scanner which showed stored most of the time and would only briefly switch to active seemingly when the cluster was trying to send a message. I wonder if anyone has any thoughts on that as I would assume if we could identify the messages sent from a cluster they would be much more frequent than every 30-90 seconds. I assume this is likely due to the communication failure. Anyway, I printed a screenshot of the waveform and began to pack up my tools. As I'm loading up one of the technicians comes over to see what I found and I show him the waveform explaining how even though they were certain that both used clusters came from the same truck I was able to prove that they were not because the CCN is trying to communicate in a different way than the other modules on the bus. As I was going over the waveform with the tech it dawned on me what some of you have probably already figured out. Especially with the hint that another connector was spliced in. Looking at the voltages the CAN high and low signals are switching. I went back and made some adjustments to the scope to capture this screenshot which makes it obvious when the trace voltages are aligned.… I swapped the pins in the connector for the CAN high and low and all of the codes cleared and the cluster worked properly.

Remember how I had checked the wire colors? Well I had but I had missed something. Turns out the connector they had wired in was from a different year. The CAN High circuit on an '08 is pin 16 with a white with orange tracer wire - the CAN low on a '10 is a white wire with an orange tracer on pin 15. The CAN low on an '08 is a white wire on pin 15. The CAN high on a '10 is a white with grey tracer wire on pin 16.…

I also learned that the connector had been taken with the cluster as the vehicle that received the original cluster had a melted connector. I had overlooked that grey tracer and almost cost myself a brand new cluster which I'm sure isn't cheap. I thought this case was interesting because I guess I might have assumed that crossed can lines might have shut down the entire bus or at least caused issues in other modules but in this case all of the other modules seemed to not only still communicate with each other but also identify that the only module causing the issue was the CCN which made the diagnostic much easier.

And yes, we all know that is not a proper CAN wiring repair. I do not perform repair work but only sublet diagnostics for repair shops so it was up to them to repair that harness correctly if they so chose to do so.

Jasper Technician

Wow, great find. The switching of those wire colors is absolutely diabolical.

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Mike Mobile Technician
South Carolina

Thanks! I'm just glad the colors were close enough for me to have an excuse for missing it this first time!

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Albin Diagnostician

Excellent write up and thanks for sharing.

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Jeff Diagnostician

Great write up, Thank you for taking the time and sharing.

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Adrean Diagnostician

Great find ! Those dam wires had that happen to me before and cost me a ecm . great write up as well

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