Diagnosing Chrysler PCI Communications
by Wayne Russell
members.atra.com www.atra.com
connected to each other through the Serial Data Communication Interface (SCI) HI and LO wires, a 2-wire com- munication protocol still used on our 2002 Caravan. Next we began to look at the PCI
Figure 1
C
hrysler can use two com- munication formats, often in the same vehicle. The format
we’ve become used to seeing since 1989, Chrysler Collision Detection (CCD), is a 2-wire system; the sys- tem introduced almost a decade ago is PCI, which is a single-wire system. Of course, we now have the OBD-II required CAN BUS. As Randall Schroeder, ATRA
Senior Technician, mentioned in a past ATRA Tech seminar, “It took one com- puter to put a man on the moon; it takes three to get a Caravan to shift down the road.” We need the PCM, TCM and BCM to make this system work. Here’s a real situation that I recent-
ly faced in my shop: A 2002 Dodge Caravan had the check engine light on (code P1698; no TCM communication), no backup lights, and the gear selector display was lit up. The vehicle worked fine otherwise. Simple enough. What I quickly realized is just how
much I didn’t know and how little good information there was to guide me. To survive in the wilderness we needed a compass and a map. To survive in the
60
wilderness of computer system diagno- sis, instead of a compass we need a scan tool; instead of a map we need wiring diagrams and system knowledge. Like most shops, we use aftermar-
ket equipment and information, which is good. But these pose challenges when communication is an issue. Some of the best diagnostic tools for a 41TE transmission are a TCM/PCM, a sole- noid block, and a battery charger. These three “tools” can help you diagnose the unit much faster than your scan tool when you’re faced with no communica- tion. Although the Chrysler DBR scan tool would be the best choice, it isn’t always an option. We began by substituting a
known-good TCM (from a prior model Caravan), which wasn’t particularly helpful: Same code, symptoms and the Caravan worked fine otherwise. The transmission and engine both worked great. How can that be if the control modules aren’t communicating? Our scan tool looks at Programm-
able Communication Interface (PCI) communication through the OBD-II connector. The TCM and PCM are
path. With help from the ATRA 2007 seminar book, we had a good overview of the system. We found the Diagnostic Junction Port (DJP), which is where all PCI wires go and are connected to each other through the terminals in the cover (figure 1 ). From there we checked the continuity of the PCI circuits between the TCM, PCM and other modules. We didn’t find any opens or shorts to ground. Any module on the PCI bus can
bring the system down, so which one? How would we know what we were looking for? ATRA has access to Chrysler OEM information and we pored over numerous pages to find out. What we needed to look for at the
Diagnostic Junction Port was “activity of about 7.5 volts” on the module PCI wire (figure 2). For example, when we looked at the HVAC module, the scope would be flatlined (0 volts) until we pushed a button on the heat control to cause the module to do something; then we observed activity. The body module was active all the time, so you have to keep activity in mind while observ- ing the PCI wire for each respective module. One significant obstacle: the Front
Control Module (FCM), ABS and TCM are tied together in the Intelligent Control Module (ICM). When we were checking PCI activity at the Diagnostic Junction Port, what were we looking at? Chrysler’s DRB-III scan tool can
GEARS May/June 2010
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