In the early world of automotive “black box” technology NASA gave us in the 1970s, parallel interface was the only show in town. What that means is that every sensor and actuator had to be hard-wired to the brain box and other than dashboard-mounted electronic gadgets that didn’t do much real work, the automotive computer invaded the engine compartment first, where automotive engineers had to hammer out strategies and algorithms to handle the single most complicated job computers do on a vehicle: ignition timing and fuel control.
Anti-lock brakes became mandatory on vans and pickups in 1987, about the same time computers got networked and spread their electronic tentacles into the hot and slippery innards of our automatic transmissions and our suspension systems. It was a no-brainer to realize that body-related controls such as lights, instrument clusters, entertainment and information systems would be handled by automotive networking, which called for a new generation of smart boxes that could share information among themselves, reducing the number of components and the amount of necessary wiring.
Further, this new technology brought drivers a host of choices that previously hadn’t been possible. In the service bay, smart scan tools gained the capability to see through the network into the mind of those solid state processors to help us figure out which inputs and outputs were inconsistent, and a whole new chapter in automotive service was born.
1. Frame 2. Start of Frame 3. Header Message 4. Data Byte(s) 5. CRC Byte 6. In Frame Response bytes 7. End of Frame
Each module can transmit and receive data simultaneously. A PCI Bus message typically has the following four components:
• Message Header - Is one to three bytes in length. This contains the message type and length, priority, target module(s) and sending module
• Data Byte(s) - This is the message that is being sent
• Cyclic Redundancy Check (CRC) Byte - This byte is used to detect errors during message transmission
• In-Frame Response Byte(s) - If a response is required from the target module(s), it can be sent during this frame
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So let’s talk for a bit about networks.
Networking — American Style
Different networks communicate at different speeds (measured in Kilobits per second, or Kbps). There are eight bits in a byte, so don’t say, “Kilobytes per second,” lest you inadvertently multiply your speed by a factor of eight. Different networks also utilize different protocols.
A protocol in an automotive context is the electronic language that modules can speak and understand. Network communication occurs when two or more on-board processing modules (scan tool included) share information. For example, when the PCM shares Engine Coolant Temp (ECT) sensor information over the network with the Electronic Automatic Temperature Control (EATC) module, then that represents an example of network communication, because both modules use the same ECT input to modify their operating strategies.
Another example would be the way some networked traction control-equipped vehicles use the anti-lock brake module to prevent wheel spin by pulsing the drive wheel brakes while at the same time calling on the PCM to retard ignition timing, reducing engine torque. These two modules are utilizing the same set of inputs to implement the desired strategy, like vehicle speed, wheel rotation speed and engine load calculations. The communication goes both ways in this circumstance, and the modules work together to achieve the desired result.
While all three automakers started out with simple SAE Class A (10 Kbps) networks, their scan tool hookups initially were proprietary and the early networks were slower and cheaper than the ones that followed.
Chrysler vehicles first used multiplexing on their 1981 model year Imperial Electronic Instrument Cluster. This module had two micro-computers mounted on one circuit board that communicated via a simple serial bus within the electronic cluster. It was in 1988 when Chrysler New Yorker and Dodge Dynasty vehicles were finally outfitted with full vehicle multiplexing. The proprietary multiplexing system Chrysler adopted was quite confusingly dubbed the Chrysler Collision Detection (CCD) Bus.
As outlined in the table, there are a variety of networks. The PCI bus on which we’re focused in this article replaced Chrysler’s earlier two-wire CCD network, which communicates at slightly less than 8 Kbps. The PCI is a one-wire network (usually a yellow wire with a purple stripe or vice versa) that utilizes the slower of the two SAE J1850 protocols (about 10.8 Kbps) switching between low and high voltage levels to generate signals. PCI bus low voltage is around zero volts, and high voltage is around 7.5 volts (half of charging system voltage).
PAGE 2The low and high voltage levels on a PCI bus are generated by means of variable-pulse width modulation to form signals of varying length (see illustration). Each up or down square is a “bit,” and the bus can communicate with up to 10,800 of these pulses every second. It’s almost insulting to share the obvious fact that you can’t measure these signals with a meter; use a good fast O-scope set to the 20 volt scale reading in milleseconds. If the bus is shorted to power or ground, the scope trace will tell the tale with a dead flatline, either at 12 or 0 volts.
PCI Bus messaging uses Variable Pulse Width Modulation (VPWM). What that means is that both the state of the bus and the width of the pulse are used to encode bit information. A zero"bit is defined as a short low pulse or long high pulse. A one bit is defined as a long low pulse or a short high pulse. A byte is a series of bits. Imagine bits as letters that make up words and bytes as words that make up a message.
Your Point of Contact With the Bus
Let’s have a nuts and bolts look at what’s going on behind that OBDII connector we’re so accustomed to seeing under the dash. Most of us have noted that there are dozens of slightly different pin designations, so check the model year and vehicle line for whatever you’re working on. Three pin designations that you can take to the bank are as follows: Pin 16 is always supposed to be battery power and pins 4 and 5 should always be ground.
Troubleshooting
By now, most knowledgeable spark chasers will know that P-codes are powertrain related, B-codes are body, C-codes are chassis and U-codes are network related concerns. It’s also important to understand that a no-communication issue isn’t always network related. For example, a no-start/non-communicative PCM can be the result of a shorted Reference Voltage (VREF) wire, but the PCM doesn’t depend on the PCI bus to communicate with the scan tool anyway. A blown fuse can take obviously take down a PCI bussed module as well.
As far as the PCI bus is concerned, we first need to determine whether the Chrysler product we’re servicing actually has that kind of bus. Some later model Chryslers dropped the PCI bus (for example, the 2001 Cherokee still uses the CCD bus), but there are others (like the 2006 Jeep Liberty) that still come equipped with a PCI bus operating in concert with the now-familiar CAN bus.
Secondly, we need to see which smart boxes are supposed to be connected to the bus and how many of those boxes are actually present on the vehicle in question. And we need to realize that each module has a specific termination resistor internal to its circuitry to mitigate electrical interference. With a table such as the one shown, the proper parallel resistance of all the modules can be measured. To make this simple, let’s assume there are only two
modules, each with a 120 ohm terminating resistance. With those to modules wired parallel and your ohmmeter connected to measure between pin 2 at the DLC and ground (battery disconnected), the collective resistance of both modules should be 60 ohms. Anything higher or lower would indicate a problem, and it isn’t uncommon to discover a shorted module this way.
Third, we need to check the warning lamps – if the airbag or ABS warning lamps are illuminated, we may be looking at partial bus failure. If all the PRNDL segments are illuminated it might indicate a communication problem between the Instrument Cluster and the Transmission Control Module.
Got inoperative systems or components? Think objectively and with discernment! For example, when certain gauges are inoperative and we find that they are PCI bus controlled (as is every gauge except the fuel gauge on some Dodge Neons), then we have a good launch point for our troubleshooting. Some Neons might also throw a “NO BUS” message on the odometer. Got a Vehicle Information System showing an accurate compass reading (which isn’t PCI bus controlled) but only displays dashed lines for the rest of the display? You may have PCI bus problem there as well.
We also need to recognize the fact that a bus-related DTC might not contain the word “bus,” and that a code we might interpret as a bus-related code might not mean that the bus has failed. Network bus DTCs basically mean that one module has lost communication with another module from which it usually receives data. What that means is that disconnected modules or a lack of power and/or ground to a module can send you looking for bus problems that aren’t there. Find out which modules respond (high end scan tools have a one-stop utility for this), and if you know the vehicle is equipped with that module and it’s not talking, you’ve narrowed your problem. If none of the modules on a particular PCI bus are talking, then you may indeed have a bus that has crashed.
An open PCI bus in a particular area will the modules on that leg of the bus offline, but a shorted-to-ground bus will take down the entire network and everything that depends on it. Low battery voltage can seem to be a bus problem, and arcing secondary ignition components or partially shorted electric motors can cause the PCI bus to react to the resulting electromagnetic interference, so use your senses of smell sight, feel, and hearing (and the proper instruments) to ferret out those problems if they exist.
Gather Good Data
As is my mantra, a good technician will spend a few moments whenever he or she can in gathering pertinent data from vehicles that are working right. With a mental library of dependable readings, any tech is better equipped to diagnose a concern.
Chrysler’s PCI bus has about 7.5 volts (half the charging system voltage) when the bus is transmitting data, and 0 volt when the bus is silent.
All networks have a bias voltage that originates at certain modules (Chrysler likes to call these dominant nodes and the receiving modules are referred to as recessive nodes). The network is set up not to broadcast recessive node transmissions when a dominant node transmission is under way. This bit of information isn’t all that important in the grand scheme of things – most PCI bused modules create their own bias voltage. That’s what Chrysler Collision Detection (CCD) is all about as related to networking.
In a word, when multiple modules transmit messages at the same time, the possibility exists that messages could collide with one another, as peculiar as that might sound. Therefore, messages generated simultaneously are sent in an order based on priority. Low priority messages go to the back of the line and wait in a buffer (think of it as a storage area or parking lot) until the information highway is clear, but that clearing usually doesn’t take but just a few microseconds. This collision avoidance system for network messages has been around for awhile on automotive networks, thus Chrysler named their first real network Chrysler Collision Detection (CCD or C2D).
Chrysler’s CCD network (7.8 Kbps) began to fade in the 1998 model year to be replaced by the PCI Bus (10.4-10.8 Kbps) on most models (Cherokee held on to the CCD bus until the bitter end of its existence). The PCI bus also features collision avoidance architecture like the CCD bus, but it only uses only one wire instead of two the way the old CCD system did.
The PCI bus on some Chrysler vehicles was outfitted with a central test point for bus diagnostics and Chrysler released a couple of special tools (Miller tool Nos. 8339 and 8360), which are small breakout boxes for the junction port connector (similar to the DLC breakout box), but that handy junction port connector disappeared in 2003.
Once again, higher end scan tools have a utility that checks all the buses and modules for communication at the DLC. In the case, of a dead module, it’s a good idea to attempt to access other modules on the same network to see if those modules will communicate.
If a particular module won’t communicate, check that module for power and ground connections according to the right schematic, and check the integrity of the bus between the module and the DLC and other modules. Check the bus for shorts to power or ground. It’s not rocket science. It’s bus science.
