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Chrysler CAN Network Diagnostics

Friday, May 1, 2009 - 00:00
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This month we are really going to take it up a notch as far as scan tool capability. This column will highlight Chrysler's Star Mobile PC-based scan tool and show some of the CAN network diagnostic functions this tool offers. Here are just a few of the many network diagnostic function screens available.

Network Topology

This entire graphic is continuously updated with information supplied by the Front Control Module (FCM), also called the Central Gateway (CGW) Module, through the DLC. The scan tool communicates with the FCM/CGW over the separate Diagnostic CAN-C communication line at the DLC.

Each CAN network is identified by different colored lines. The CAN-B bus uses black lines, the CAN-C bus uses blue and the Diagnostic CAN-C bus is purple.

The various bus lines are shown in their respective colors if they are Responsive, as determined by the FCM/CGW.

Not Responsive ECUs will be listed in red along with the particular communication line leading to the ECU, as either part could be the cause of the communication defect.

ECUs listed in gray are determined to be Not Built on Vehicle. The Vehicle Build Configuration (VBC) for the vehicle network is stored in the FCM/CGW. If this VBC corrupts, a module could be listed as Not Built on Vehicle when it is present.

Diagnostic CAN-C Bus Test

This is a continuous test of the Diagnostic CAN-C line only, from the FCM/CGW to the DLC to the scan tool only.

Trial lists the latest test result reported by the FCM/CGW. These trials complete about every one to two seconds. Outcomes of each test trial are reported in the Result column.

Advanced LoC screen

The Advanced LoC (Loss of Communication) summary screen provides a complete vehicle communications system DTC status report.

The first row contains the ECU names. The second row shows whether the particular ECU is able to communicate with the FCM/CGW as shown by a green check mark. If the ECU cannot communicate with the FCM/CGW, a red X will be displayed.

The next column shows the number of DTCs registered against that ECU by the FCM/CGW. This counter is not the number of DTCs registered within the particular ECU. Above this column, DTCs are listed as Active (A) or Stored (S). If any DTCs are Active for a particular ECU, this column will list how many ECUs have stored DTCs against this module out of a total amount of ECUs that could set against this ECU, for instance "4 of 7" possible.

The "E" listed above the "A" & "S" row stands for Electrical fault of the bus. The "C" stands for a Communication error with a particular ECU. The "S" stands for an implausible or invalid Signal received from an ECU. The "L.A." or Logged Against column, lists the number of other ECUs that have registered communication DTCs against that particular ECU.

Now check out the "Enabling Conditions" area at the bottom of this screen. These six boxes report on the environmentals that have to be correct in order for the ECU communications to take place and the upper screen portions to be valid. If the outcomes of these six conditions are correct, the text will be in black. If particular conditions are incorrect, the text will be in red.

The Ignition Status indicates what the position of the ignition switch should be in order for the tests to be valid.

If there are any battery or ignition voltage feed DTCs set that may affect the network test, a red Yes will be listed.

The VIN mismatch box lists if any ECUs do not have the correct VIN recorded in them. Incorrect VIN can cause an ECU to not be recognized by the rest of the network, resulting in the ECU being flagged as non-communicating. This is an important parameter. Today's vehicle networked ECUs must also have the correct VIN and configuration data recorded for all functions to operate.

There are other considerations, like proper ECU Config, as determined by the VBC data, to be checked before condemning an ECU for communication faults. Obviously, if there are any CAN electrical DTCs or CAN signal DTCs, some or all the modules will not communicate.

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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