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Diagnosing the PCI Bus

Wednesday, April 1, 2009 - 00:00
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The PCI protocol is a Chrysler specific adaptation of the SAE J1850 specification. The system uses a single wire variable pulse-width modulated signal to communicate between the modules on the vehicle.

The PCI Bus is a "star configuration" with all modules connecting to one or more central contact points. Some vehicles use a splice pack called the Diagnostic Junction Port (DJP), which serves as the central contact splice of the bus. The DJP is located behind the knee bolster under the steering column. The DJP can provide an access point to isolate most of the modules on the bus in order to assist in diagnostics and probing. Be aware, though, that at times there might be other "splices" farther upstream, so consult your DLC schematic for that vehicle's specific layout.

Some vehicles use the BCM as the central connection point for the PCI Bus. All modules are wired through separate pins at the BCM connector and connected internally in the BCM. The single wire PCI data line can be found at DLC terminal 2. The bus voltage is at 0 volt when no modules are transmitting and around 7.5 volts, depending on how many and which modules are transmitting at the time.

Each module has its own PCI Bias and Termination functions integral to the module. This gives each module the ability to both transmit and receive data. Each module has a specific termination resistance, which is typically stated in the service literature. Dominant nodes terminate the bus through a 1,200- to 3,300-ohm resistor and a 3,300-pF capacitor. Typically, the Mechanical Instrument Cluster (MIC) and/or the PCM are the dominant nodes.

Standard nodes terminate the bus through an 11K-ohm resistor and a 330-pF capacitor. When checking a module termination resistance to ground during diagnostics, be sure to wait a few minutes for the termination capacitor to discharge before using an ohmmeter to test the circuit.

Now let's look at the DRBIII PCI diagnostic functions. The first is the J1850 Module Scan. This function is located under the System Monitors section of the main menu. When this selection is made, the DRBIII will query the PCI bus to see which modules are able to communicate on the bus. Figure 1 shows page one of the two-page list of modules that responded. Use the DLC schematic from your service information to find a list of all possible modules the particular vehicle may have.

The second function is a unique feature called Monitor Function. This function is found by moving from the main menu to a particular module of concern submenu.

This function lists both the individual data parameters and the values of those parameters that the selected module receives from another module on the bus. In our example in Figure 2, we selected the BCM section from the main menu. Here we see a list of three data parameters that the BCM collects from the PCI bus that it needs in order to make decisions for the outputs the BCM is responsible for. In this example, the BCM needs to know the Engine Speed, Vehicle Speed and Engine Lamp (MIL requested status) in order to tell the Instrument Cluster what information to display. The information flows for a particular vehicle function is typically stated in the service information System Description section for that function. This scan tool function is necessary in the following and similar scenarios.

Say we have no engine RPM reading displayed on our cluster. Is the cluster assembly defective because it has lost its ability to display a known data value? Is the BCM defective in that it did not send the required data to the cluster assembly? Is the ECM defective because it did not send the requested data to the BCM? Without the above listed Monitor Function, it is sometimes impossible to answer such questions during the diagnostic process. Without this function, we usually will be left with making sure the engine RPM signal got into the ECM and perhaps scanning the ECM to see if it can at least display the RPM signal.

How will we know if the ECM sent the RPM data to the bus or if the BCM recognized the data ? If the BCM sent the data to the cluster, without again using the Monitor Function under the cluster module sub menu, we can't know the answer.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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