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BCM shorting out? http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=3&t=38469	Page 1 of 1

Author:	GAWilliams [ Sun Dec 21, 2008 6:17 pm ]
Post subject:	BCM shorting out?
I've had my 05' Liberty Limited since new... and have had a problem about once every few months (but twice this past week -- hope it's not getting worse) like this: Car starts, runs and drives fine, but several things don't work: The miles per gallon display shows "--- MPG" The other caluclations requiring speed or distance don't work (trip miles, etc.) The odometer below the speedo still works The steering wheel radio controls do NOT work At night, with headlights on, the displays do NOT dim to the nighttime brightness level, but stay bright -- although the backlight for the guages does come on. If I turn off and restart the KJ, everything works fine. It's never been something that the dealer has been able to find becuase of that... has anyone else tackled this problem?

Author:	Guest [ Wed Dec 31, 2008 4:40 am ]
Post subject:
If your speedo does not work that is an indication of a bad wheel speed sensor which is located on the top of your rear differential. I've had to replace mine 3 times due to various off-roading 'incidents'. If your wheel speed sensor is not working then your mileage will not change and your speedometer will show zero. I'm not sure what is causing the other issues though. It could also be that you have a loose connection or bad relay that ties these circuits together somehow.

Author:	Guest [ Wed Dec 31, 2008 5:21 am ]
Post subject:
After looking in the service manual for curiosity sake i noticed that the BCM controls those items you describe. However, that does not necessarily mean the BCM is at fault but it is a common denominator. The BCM has a number of circuits that connect to it that involve relays that could also be faulty. You can get the BCM checked with a scan tool from the dealer. From Service Manual - There are two different versions of the BCM: base and premium. The base BCM is a subset of the components in the premium version. Basically, the base version BCM does not support the following features: Compass Mini-Trip Computer (CMTC), fog lamps (front and/or rear), Remote Keyless Entry (RKE), remote radio switches, or Vehicle Theft Security System (VTSS). Both versions of the BCM utilize integrated circuitry and information carried on the Programmable Communications Interface (PCI) data bus network along with many hard wired inputs to monitor many sensor and switch inputs throughout the vehicle. In response to those inputs, the internal circuitry and programming of the BCM allow it to control and integrate many electronic functions and features of the vehicle through both hard wired outputs and the transmission of electronic message outputs to other electronic modules in the vehicle over the PCI data bus. BCM INFO The microprocessor-based Body Control Module (BCM) monitors many hard wired switch and sensor inputs as well as those resources it shares with other electronic modules in the vehicle through its communication over the Programmable Communications Interface (PCI) data bus network. The internal programming and all of these inputs allow the BCM microprocessor to determine the tasks it needs to perform and their priorities, as well as both the standard and optional features that it should provide. The BCM programming then performs those tasks and provides those features through both PCI data bus communication with other electronic modules and through hard wired outputs through a number of driver circuits, relays, and actuators. These outputs allow the BCM the ability to control numerous accessory systems in the vehicle. The BCM operates on battery current received through a fuse in the Junction Block (JB) on a nonswitched fused B(+) circuit, through another fuse in the JB on a fused ignition switch output (run-start) circuit, and through a third fuse in the JB on a fused ignition switch output (run-acc) circuit. This arrangement allows the BCM to provide some features regardless of the ignition switch position, while other features will operate only with the ignition switch in the On, Start, and/or Accessory positions. All of the battery current circuits are connected to the BCM through the JB/BCM connector. The BCM receives ground through five separate circuits. Three of these circuits are connected to the BCM through a connector and take out of the instrument panel wire harness on three separate ground circuits, while the other two circuits are connected to the BCM through the JB/BCM connector. All of these circuits are grounded through a splice block located in the instrument panel wire harness with an eyelet terminal connector that is secured by a nut to a ground stud on the driver side instrument panel end bracket near the JB. The BCM monitors its own internal circuitry as well as many of its input and output circuits, and will store a Diagnostic Trouble Code (DTC) in electronic memory for any failure it detects. These DTCs can be retrieved and diagnosed using a DRBIIIt scan tool. Refer to the appropriate diagnostic information. BCM Diagnostics The hard wired inputs to and outputs from the Body Control Module (BCM), as well as other hard wired circuits for this module may be diagnosed and tested using conventional diagnostic tools and procedures. However, conventional diagnostic methods may not prove conclusive in the diagnosis of the BCM, the Programmable Communications Interface (PCI) data bus network, or the electronic messages received and transmitted by the BCM over the PCI data bus. The most reliable, efficient, and accurate means to diagnose the BCM and the PCI data bus network inputs to and outputs from this module requires the use of a DRBIIIt scan tool.

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