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Getting put through the diagnostic wringer with CAN Interior Bus issues

Friday, December 1, 2017 - 09:00
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Pushing through a diagnosis with a plan of attack is the best course of action to keep you focused and making diagnostic decisions accurately and efficiently. These decisions should be based on your test results, whenever possible. Although, sometimes instinct and gut-based decisions can’t be helped. Duplicating a customer’s complaint, pinpointing the root cause of the symptom and verifying the fix is absolutely essential to a solid repair. So, what to do if a vehicle comes back with the same issue? You may recognize this subject vehicle — it had me eating crow when I saw it roll back into our parking lot last week! That is, of course, until I fixed it — again!

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The saga begins
The 2008 Chrysler Town and Country came to shop with the complaint of erratic gauge operation and cycling of the windshield wipers all while hearing an audible warning chime. As strange as this series of complaints sounds, I witnessed it myself. I began my diagnosis at the DLC. A scan of the entire vehicle and all of its nodes was carried out and without surprise, multiple Diagnostic Trouble Codes were stored in many of them. All of the DTCs were based about the CAN Interior Bus functionality and loss of communication between modules. There was quite a handful of DTCs and attempting to follow them, hoping for a conclusion, would just lead a tech to confusion and drive him/her in circles. Lots of times modules like to tell on one another. Sometimes we get lucky and a bunch of modules will point a finger at a single module. I wasn’t so lucky in this case. When faced with challenges with erratic symptoms like this one, related to bus integrity, I find it best to print out a road map to help me navigate myself to the problem. The road map in this case, is the wiring diagram for the CAN Interior Bus (Figure 1).  With this topological diagram in hand, I can devise a plan to narrow down the shortest route to the problem. 

Figure 1

The first order of business after printing out the roadmap is to sit back and regroup. Jumping into a diagnosis like this one is very tempting but let’s think about this for a second. The CAN Interior Bus runs the entire length of the van. It's in every door, inside the dashboard, behind all the trim panels and even into the headliner — connecting 25 modules in a network. Do you really want to roll those diagnostic dice and hope for a “7”?

I start with the facts I do have. The wipers cycle, the gauges drop out, a chime sounds. This is what I know. I want to print wiring diagrams for these components (wipers/cluster, for starters) and look for common points between them. It seems all of the malfunctioning systems do share something in common — the Totally Integrated Power Module (TIPM). This device serves as the gateway in this vehicle’s communication network. It also drives the wiper relay. I’m not one for silver bullets, but I will state these devices have a high failure rate for all sorts of various malfunctions. A road test was conducted with a scan tool interfaced at the DLC. Two objectives were in order; first, to determine the scenario required to reproduce the problem and second, to see if the scan tool lost communication when those symptoms were exhibited. The problem was very repeatable. Just driving the vehicle around the corner I was able to reproduce the concern multiple times but I couldn’t pinpoint a why or how as of that point. The fault wouldn’t present itself sitting still so the first thought was water intrusion or a short somewhere in the bus. I attempted to shake the vehicle and rock it back and forth but to no avail.

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The leap of faith
I couldn’t think of a way to temporarily eliminate or bypass the TIPM without disabling the vehicle. This is why I thought it best to replace the TIPM initially. The premature decision to replace the TIPM was made on my order and as you guessed it, the problem remains. Now, I’m married to this vehicle and I have to get this resolved. It’s still as broken as it was and now so too, is my pride and conscience. This was very embarrassing for me. If I could take it back, I would but I can’t. Humbled with a belly-full of that yummy crow, I proceeded to find the cause and fix this van, now more determined than ever! 

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