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Quick TBI question

Discussion in 'The Garage' started by Russell, May 9, 2006.

  1. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Hey guys!

    Just came across something with my TBI swap, and wanted to confirm that my deduction was correct.

    The TBI system has a connector that goes to the park neutral switch on my truck. It has one orange / black wire from the ECM, and one black / white wire that goes to ground.

    On the parts truck, the park neutral switch only had one place for that connector, so I can only assume that instead of the park neutral switch keeping the starter solenoid from getting power to start the engine, the ECM simply doesn't allow the truck to start instead.

    This would mean that I need to take the yellow & purple wires that run through the park neutral switch and join them together, with a smaller wire coming off and running to the crank pin on the ECM, and run the ECM's two wires to the park neutral switch instead.

    Does that sound correct? How does the ECM keep the truck from cranking in this case? Or does the crank wire control a relay that allows / cuts power to the starter solenoid?

    Thanks!

    Russ
     
  2. 1979jimmy350

    1979jimmy350 1/2 ton status

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    when i made the harness for my TPI, i used the stock NSS, I used a TBI harness and modifyed it, i just changed the conector on the stock TBI harness so it would plug into the NSS, it works fine for me.
     
  3. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Well, I checked out the harness on the TBI truck (not sure why I didn't think about that in the first place, lol) and it appears that the truck could crank in any gear, just wouldn't start unless it was in park or neutral. So I wired it up as I described, just soldered the yellow and purple wire together, then and ran a small spliced in wire off to the ECM, then ran the ECM's wire to the park neutral switch.

    Works for me!

    Russ
     
  4. btolenti

    btolenti 1/2 ton status

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    Not sure exactly what the yellow wire is (maybe +12V hot?) but:

    1) the purple wire should be connected to the brake light switch. the brake light switch should supply +12V to the purple wire when the brakes are not applied. This setup is used to tell the transmission when to unlock the torque converter...if you have a lockup type.

    2) the black/white wire is connected to ground through the ECM. It can or cannot be used, depending on personal preference.

    3) the orange black wire should be grounded whenever the tranny is in neutral or park. Usually the neutral switch does this. You can ground this permanently, but this can cause the ECM to think the truck is in park, and may limit engine RPM.
     
  5. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Yellow wire is the crank wire coming from the ignition switch. It then went through the park neutral switch, and from there it ran directly to the starter solenoid.

    1) We are talking about a different purple wire here, lol I had previously installed a 700r4, and have a lockup kit that doesn't require the ECM. So I eliminated that wiring. There is a purple wire coming off pin C9 that is labed as the Crank wire, and is suppost to run to the starter. I can only assume that means the hot wire for the solenoid, so the ECM knows when you are cranking the engine.

    2) The black / white wire from the Park Neutral switch doesn't run directly to the ECM. It is connected to Pins A12, D1, and D6 indirectly though.

    3) The Orange / black wire hooks up to the Park neutral switch, and when in park or neutral, is grounded through the aformentioned black and white wire.

    I have no doubt in my mind that this is all connected correctly for the ECM's sake, but it seems rather odd that you'd still be capable of cranking the engine over in gear, even though it will not start. However, that seems to be the way it works, so I guess :) I don't typically turn my vehicles off without taking them out of gear anyways, lol
     
  6. btolenti

    btolenti 1/2 ton status

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    Okay, now I understand...

    Well, my truck will crank over, but it won't start unless its in park or neutral. The crank wire just tells the ECM when you are starting the truck, and the neutral setup determines if the ECM activates both the fuel and the spark.
     

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