Diagnosing Chrysler PCI Communications
identify which modules are communi- cating on the PCI bus where ours can only report that it isn’t communicat- ing with the TCM. Not quite the same thing. Now what? Why not unplug the ABS? Because
we couldn’t reach the module! We removed the fuses for the module and solenoid and still had communication with the ABS. We checked the wiring again and it shouldn’t be. Now we were faced with replacing
a module. Which one first? To choose, we needed to understand the logic of which module is doing what. Because the TCM and ABS are both tied direct- ly to the Front Control Module, we decided to start there. We were lucky: A local salvage yard had one with the exact label code, so we purchased it, installed it, and were faced with the same results. Disappointed, I reexamined the
situation. Does the BCM filter the TCM info and feed it to the PCM and Instrument Cluster Panel (ICP) module, or does the Instrument Cluster Panel module do the work? The BCM turns the backup lights on. The Instrument Cluster Panel module lights up the cho- sen gear display, but who’s the traffic cop? I opted for the Instrument Cluster Panel module. Again, the local yard had the mod-
ule. We installed it, and again, no change. Frustrated is an understate- ment. We’re not comfortable that the BCM would do this, but couldn’t find any documentation that specified which module is responsible for the decision making. Perhaps the engineers didn’t think it worthwhile to document this logic. The BCM isn’t a module to sim- ply swap out. It has learned functions and shouldn’t be swapped from another vehicle. This is when I called Randall on
the ATRA HotLine. Randall suggested I read some material Cliff McCormick wrote for ATRA’s 1995 Technical Seminar Book (pages 61–88) regarding networking the CCD bus and how it works, along with suggestions for deal- ing with no communication issues. I decided to go back and reconfirm
my PCI communication from the TCM. The TCM is located in front of the inner fender cover of the driver’s front wheel (a great location in our snow belt).
62
Figure 2
Knowing the TCM, ABS and Front Control Module are tied together, I cut the PCI wire near the TCM (from pin 43) and checked for PCI activity with the scope and found none! What a ter- rific discovery after numerous hours of anxiety. I went back to previous years’ wir-
ing diagrams to learn that pin 43 is part of the SCI bus and the PCI bus didn’t exist yet. In simple terms, we messed up by using the wrong TCM for this model year. We replaced the TCM with correct one and all issues resolved. Why write about a stupid mistake?
SCI, PCI and CAN bus communica- tions are now part of our lives. Sooner or later you’ll be dealing with them. Unfortunately, most of us don’t really know or understand them. We’re faced with fixing transmission issues without ever touching the transmission. Too many shops pass this type of
problem off to the dealer. In today’s economy, we need to be capable of doing all we can to resolve our custom- ers’ problems. We need good informa- tion, system knowledge and under- standing, and the right tools. ATRA has a wealth of knowledge,
information, and experience. Don’t wait until you’re desperate to ask for help. Become familiar with your tools. Take a minute and look at PCI, SCI, and CAN bus communication signals
with your scope and learn what you should see. That will help when you have a problem like this. I remember a VSS problem I ran
into a few years ago. At first glance the signal looked fine. Later, after great frustration, I discovered that the voltage amplitude was too low (2 volts vs 6+ volts); moving the sensor deeper into case corrected the problem. It took a lot of reading to learn
that we were looking for 7.5-volt PCI signal on this Caravan. Now we know that we needed to verify the PCI signal from the TCM, knowing that the TCM, ABS and Front Control Module are tied together. The Instrument Cluster Panel mod-
ule contains the Front Control Module, fuses, relays and wiring connectors for a good deal of the system. It can easily become corroded and should be a good place to begin your diagnosis. We could have cut the PCI wire here as well. Typical wiring schematics don’t pro- vide the connector wiring information, so you may need ATRA to help here. The key to any of these diagnoses
is to learn the system, and use the tools that are available to you. And maybe the most important tool of all is the ATRA HotLine.
GEARS May/June 2010
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