What symptoms show up when the J1850 PCI bus goes open or gets shorted?



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Thread: What symptoms show up when the J1850 PCI bus goes open or gets shorted?

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    Exclamation What symptoms show up when the J1850 PCI bus goes open or gets shorted?

    Electrical Black Magic or explainable phantom faults? If the bus shuts down what happens to the vehicle control modules?

    1996 Plymouth Grand Voyager 3.8L LE FWD
    Everything's Working - No Drippin' or Burnin'
    428,500 Kilometers - 267,800 Miles

    1989 Voyager 2.5 liter Turbo-Automatic (deceased in a freeway spill 2006)
    1987 Voyager 2.2 liter 5-Speed Manual (Burgundy Red - very clean)

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    PCI BUS Description

    PCI bus can NOT "shut down".. depending on type of fault (sort circuit to ground vs. one/several module(s) loosing connectivity) you may experience anything from no visible fault, loss of cluster, to no function at all...

    to get a better idea, read the following:
    Quote Originally Posted by FSM
    PROGRAMMABLE COMMUNICATIONS
    INTERFACE (PCI) BUS
    DESCRIPTION
    The Programmable Communication Interface Mul-
    tiplex system (PCI Bus) consist of a single wire. The
    Body Control Module (BCM) acts as a splice to con-
    nect each module and the Data Link Connector
    (DLC) together. Each module is wired in parallel to
    the data bus through its PCI chip set and uses its
    ground as the bus reference. The wiring is a mini-
    mum 20 gage wire.
    OPERATION
    Various modules exchange information through a
    communications port called the PCI Bus. The Power-
    train Control Module (PCM) transmits the Malfunc-
    tion Indicator Lamp (Check Engine) On/Off signal
    and engine RPM on the PCI Bus. The PCM receives
    the Air Conditioning select input, transaxle gear
    position inputs over the PCI Bus. The PCM also
    receives the air conditioning evaporator temperature
    signal from the PCI Bus.
    The following components access or send informa-
    tion on the PCI Bus.
    • Instrument Panel
    • Body Control Module
    • Air Bag System Diagnostic Module
    • Full ATC Display Head (if equipped)
    • ABS Module
    • Transmission Control Module
    • Powertrain Control Module
    • Travel Module
    • SKIM
    PS: Theoretically, PCM NGC (next gen controller comprising of PCM + TCM) does not need PCI bus to control the engine / trans - all sensors feed directly to it EXCEPT SKIM needs to send an "allow start" command...(or something like that..)
    BUT everything depends on software and who knows how it's programmed...
    relevant vehicle: '05 -=C=- T&C Ltd @~110K

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    All those things can constantly talk to each other over 1 wire? That's crazy lol.
    Candy the van. '98 Sport 3.8L 132,200 miles. Used trans at ~96k. Great piece of my life and a fine van.

    '69 GTO drop top
    www.facebook.com/people/Andy-Greif/1410438927

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    SKIM? (Not milk) SKIS (No snow involved)

    Dam those acronyms!

    SKIM = (Sentry Key Immobilizer Module) has programmable security key code.
    SKIS = (Sentry Key Immobilizer System)
    ATC = Automatic Temperature Control (display)
    EVIC = Electronic Vehicle Information Center

    So the PCI bus does not shut down because it's only a wire. But the modules can shut down so your car may shut down. I gues wires only corrode or break. Wires don't shut down. Only active electronic circuitry shuts down?
    It appears the BCM is connection central. If the BCM loses power are the data
    links running through it affected or are they just a hardwire feed through the module?

    I think it's a really good idea on the new system if the engine management can not be affected by a bus controlling periperal components.
    Last edited by TheDevilIKnow; 05-01-2010 at 04:37 AM. Reason: Additional comment

    1996 Plymouth Grand Voyager 3.8L LE FWD
    Everything's Working - No Drippin' or Burnin'
    428,500 Kilometers - 267,800 Miles

    1989 Voyager 2.5 liter Turbo-Automatic (deceased in a freeway spill 2006)
    1987 Voyager 2.2 liter 5-Speed Manual (Burgundy Red - very clean)

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    Quote Originally Posted by TheDevilIKnow View Post
    Dam those acronyms!
    ...
    So the PCI bus does not shut down because it's only a wire. But the modules can shut down so your car may shut down. I gues wires only corrode or break. Wires don't shut down. Only active electronic circuitry shuts down?
    It appears the BCM is connection central. If the BCM loses power are the data
    links running through it affected or are they just a hardwire feed through the module?
    ...
    PCI Bus can't shut down due to the fact that it's not a "centrally" controlled system - (in that) pieces of the bus are controlled by independent components connected to the PCI INTERFACE, which is provided by the single signal wire (and ground reference)...
    The PCI bus is (in all likelihood) a CDMA/CD type architecture (but unlike Ethernet, does not have a hub OR repeater..)

    Note: <<The
    Body Control Module (BCM) acts as a splice to con-
    nect each module and the Data Link Connector
    (DLC) together.>>

    If taken literally, that means that BCM contains the solder/PCB/physical connection of the wires running to all the modules on the PCI bus.. Thus if BCM looses power, it should not effect PCI bus operation (should only drop BCM from the bus)..
    HOWEVER, it the "splice" fails (due to poor solder contact / component failure, etc) some or all of the modules may loose communication capability..
    AND if the signal wire is shorted to ground, no communication will be possible...
    relevant vehicle: '05 -=C=- T&C Ltd @~110K

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    Quote Originally Posted by atoman View Post
    ...The PCI bus is (in all likelihood) a CDMA/CD type architecture (but unlike Ethernet, does not have a hub OR repeater..)...
    I don't think SAE J1850 is CDMA/CD type architecture, it is defined by it's name, SAE J1850. Here's an interesting link describing some of it's characteristicsJ1850 I don't know anything about those people at that site, it just looked like an interesting description.

    so what happens when it "shuts down?" i.e fails? I'm living that right now. I have a random failure between the steering column and the rest of the system. When it "fails":
    * Airbag fault light lit;
    * SKIS fault light lit;
    * Car will not start, no starter whirl, no clacking, nothing;
    * three cycle key trick will not display OBDII codes in odometer
    * I can prove it's not the starter or battery using a jumper wire from the battery+ to pin 87 on the starter relay; doing that during the failed mode causes the starter to spin but the engine will not fire.

    In my case jiggling the steering column will sometimes restore the circuit and poof! everything works, the car starts, all fault lights are out.

    It apparently is documented that if the PCM does not receive the correct message from the SKIM it will not allow the engine to begin the start up sequence. See for example the text at the top of this page(note: may or may not be specific to chrysler)

    I've looked under everthing and unfortunately do not find the anticipated worn out wiring harness shorting out on metal or other wires. I am reluctantly planning to take the car to the dealer now. anybody want to bet on how many ECU's they'll try to sell me?
    2008 Mazda Tribute i sport 5-speed
    2007 Infiniti G35
    2003 Chrysler Town & Country eX
    1996 Ford Probe GT
    1993 Ford Probe GT - on life support
    1992 F-150 XLT
    2000 John Deere 4410

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    err, that should be CSMA - Carrier Sense Multiple Access (not CDMA - no such thing, lol)
    CR vs CD - slightly different ways of handling "collisions"...

    If you/anyone want(s) to geek out on the bus have a look at THIS.(if this link works..)

    As for the described symptoms:
    you could be looking at anything from clock spring / airbag / steering wheel - if you have remote radio controls / wiring in column OR BCM...

    Since moving the steering wheel effects the symptoms, it could be a wiring issue... (most likely in the clock spring..)

    Luckily, BCM can be diagnosed by a knowledgeable Tech with the right tools...
    Quote Originally Posted by FSM
    The BCM has internal diagnostic capability that
    assists in diagnosing the system error. When an
    OPEN or a SHORT circuit exists, the diagnostic tool
    can be used to read the BCM faults. The faults are
    very descriptive in identifying the appropriate fea-
    ture that has faulted.
    The only two faults that the BCM logs that con-
    clude the replacement of a BCM are faults;
    # 01 - Internal BCM failure (replace BCM)
    # 1F - J1850 Internal Hardware Failure (replace
    BCM)
    Otherwise the appropriate diagnostic procedures
    for each of the features should be taken when the
    BCM logs a fault.
    But this is getting off topic...
    relevant vehicle: '05 -=C=- T&C Ltd @~110K

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    Quote Originally Posted by atoman View Post

    As for the described symptoms:
    you could be looking at anything from clock spring / airbag / steering wheel - if you have remote radio controls / wiring in column OR BCM...

    Since moving the steering wheel effects the symptoms, it could be a wiring issue... (most likely in the clock spring..)
    Thanks for the links and the additional education. I forgot J1850 was one of the three OBDII protocols, thanks for the reminder.

    I appreciate the comments on the symptoms, but I'm sticking with my personal diagnosis of the dropped / failed / shut down J1850. Why?

    It wasn't moving the wheel, it was moving the column. Airbag fault clears same time as others. IF it were clockspring, that fault would not go away.

    sentry key immobilizer module in steering column MUST communicate with the PCM in order to initiate engine start-up. nothing happens. Note what the factory manual tells us:
    * wrong key: starts then shuts down
    * faulted: will not start.

    Also the three cycle key sequence will not drive PCM fault codes to odometer display. No link, no display.

    we'll see. I suppose if I were out tracing wires instead of..........I might have fixed it by now lol.

    oh, CDMA is a cell phone protocol.....wireless! that would solve my problems.
    2008 Mazda Tribute i sport 5-speed
    2007 Infiniti G35
    2003 Chrysler Town & Country eX
    1996 Ford Probe GT
    1993 Ford Probe GT - on life support
    1992 F-150 XLT
    2000 John Deere 4410

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    How do you confirm for certain it's a J1850 PCI failure? Found this in the manual, unfortunatly cannot confirm it (of all times for the van to start)

    (1) With the ignition switch in the Off position, on Electronic Vehicle Information Center (EVIC) and Compass Mini-Trip Computer (CMTC) equipped vehicles simultaneously depress and hold the STEP and the RESET buttons. On Compass Temperature Module (CT) equipped vehicles depress the C/T and the US/M push buttons.
    (2) Turn the ignition switch to the On position.
    (3) Following completion of these tests, the electronics module will display one of the following messages:
    a. Pass Self Test (EVIC only), PASS (CT, CMTC) - The electronics module is working properly.
    b. Failed Self Test (EVIC only), FAIL (CT, CMTC) - The electronics module has an internal failure.
    The electronics module is faulty and must be replaced.
    c. Failed J1850 Communication (EVIC only), BUS (CT, CMTC) - The electronics module is not receiving proper message input through the J1850 PCI data bus circuit. This can result from one or more faulty electronic modules in the vehicle, or from a faulty PCI data bus. The use of a DRB IIIt scan tool and the proper Diagnostic Procedures manual are required for further diagnosis.
    There, maybe that is useful information to help someone later
    2008 Mazda Tribute i sport 5-speed
    2007 Infiniti G35
    2003 Chrysler Town & Country eX
    1996 Ford Probe GT
    1993 Ford Probe GT - on life support
    1992 F-150 XLT
    2000 John Deere 4410

  11. #10
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    the easiest way to verify OBDII - PCI - J1850 operation is to connect a scan tool to the OBD port and see if it can communicate with any modules...
    relevant vehicle: '05 -=C=- T&C Ltd @~110K

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