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Thread: Engine cranks over while in the Run position

  1. #1
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    Default Engine cranks over while in the Run position

    Doing a 2005 6.0 DBW swap into a 96 k1500. Went to fire up and the engine cranks over in the run and crank position. When I made the harness (grafted the 6.0 into the s10 stock harness) I utilized the PCM starter circuit (12v gets sent to the PCM on crank and PCM then grounds starter relay). For some reason the crank wire from ignition switch is reading 10.5V with key in run position. It reads 12.5V in crank position. 10.5V is apparently enough to tell the PCM to start cranking engine.

    Any ideas? Is my ignition switch bad?

  2. #2
    Joe (Moderator) joecar's Avatar
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    The ignition switch, the starter relay and the BCM are in series... if the BCM recognizes the ignition key it completes the ground, and the starter relay activates.

    Battery voltage is sent to the PCM during start, but the PCM does not does cause the starter relay to activate.

    Sounds like something is incorrect in the wiring or the ignition switch.

    Do you have a wiring diagram for your vehicle...? If so, post it here.

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    This is a similar diagram to how i wired up the starter circuit.

    Click image for larger version. 

Name:	2005_tahoe_starting_diagram.jpg 
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  4. #4
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by bubba68ss View Post
    This is a similar diagram to how i wired up the starter circuit.

    Click image for larger version. 

Name:	2005_tahoe_starting_diagram.jpg 
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Size:	119.7 KB 
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    Ok, I was incorrect... your PCM activates the starter relay... (interesting, BCM is not directly involved);


    For some reason the crank wire from ignition switch is reading 10.5V with key in run position.
    There should be voltage at the ignition switch START pin only when the key is in the START position.

    Is the PCM seeing residual voltage...? Test this by connecting a 2K resistor between the START wire and ground...
    if this fixes it then the PCM was seeing residual voltage, otherwise the problem is in the ignition swith and/or wiring.
    Last edited by joecar; December 6th, 2014 at 10:15 AM.

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    If the resistor fixes it would this be a permanent fix?

  6. #6
    Joe (Moderator) joecar's Avatar
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    Quote Originally Posted by bubba68ss View Post
    If the resistor fixes it would this be a permanent fix?
    I'm not sure...

    it might indicate that the PCM may be missing a ground connection.

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    UPDATE

    Ignition switch was ok, so I traced the crank voltage wire from the switch and noticed it split into two smaller wires. One went to the PCM and the OTHER went to snaking off somewhere up into the dash and who knew where else... so I spent some time at the library and that wire went to something called 'Diagnostic Energy Reserve Module' which was back feeding the 10V and causing my problem. I isolated that and everything works great now. Not sure what that module does... but it seems to only be active during 'cranking'?

    OK, this is what I found out on the DERM

    Circuit Description
    When the ignition switch is first turned ON, Ignition 1 voltage is applied from the GAUGES fuse to Redundant Indicator Ignition 1, terminal B2, and to the AIR BAG warning lamp which is connected to SIR Indicator, terminal B1. The AIR BAG fuse applies system voltage to the Ignition 1 inputs, terminals A9 and A10. The inflatable restraint Diagnostic Energy Reserve Module (DERM) Responds by flashing the AIR BAG warning lamp seven times. If Ignition 1 is outside of the normal operating voltage range, the AIR BAG warning lamp come on solid with no DTC set.
    When the engine is being cranked, Ignition 1 voltage is applied from the CRANK fuse to the DERM at the Crank input. The DERM responds by grounding the SIR Indicator output until Ignition 1 voltage is removed from the Crank input. This results in the AIR BAG warning lamp being on during cranking.
    After cranking, the DERM will flash the AIR BAG warning lamp six times.

  8. #8
    Joe (Moderator) joecar's Avatar
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    Thanks for the update... hmmm, interesting (I'm wondering if your airbag/SIR system has a fault).

  9. #9
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    Will disconnecting my airbag wire under core support and steering wheel bag cause this?

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