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Odd TBI problem -- Any ideas?

Discussion in 'The Garage' started by Russell, Aug 3, 2006.

  1. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Hey guys!

    Got the TBI running in my 77 this morning! Even without the 02 sensor hooked up yet, that thing has easily got twice the torque it used to!

    Anyways, while I am getting a code for the missing 02 sensor (which will be going in this afternoon once my mother gets home with the nut for welding onto the pipe), the thing runs awesome across the board, less when it comes to starting it.

    If I grab the dizzy, and rotate it back and forth a few times to get it to pulse the injectors, the thing fires up immediately, but if I just go to crank it with the key, it just sits there and cranks, without firing. As far as I can tell, I do have power going to the injectors while cranking, and the fuel pump does run, but it won't fire up.

    Any ideas? Set minimum air so I've got about 15 counts on the IAC at an idle, and once started, the truck runs fantastic, so I'm at a bit of a loss...
     
  2. neverendingproject

    neverendingproject 1/2 ton status

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    Sounds like you may not have switched power going into the engine bay.
     
  3. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Air leak? Got all your ports and pipes connected? Timing set to 0*?
     
  4. Thunder

    Thunder 3/4 ton status

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    Injectors wont fire if ECM sees no spark reference at the dist. Is your Dist Module/ pickup coil in good shape? Do you have a small cap TBI distributor? Check the wireing and connections.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Check that you are getting an injector pulse/fuel spray while it's cranking.

    Fighting that one myself, only about 4 more months before I get to start working on it again. :)

    I'm not getting enough voltage on the injector wires during cranking, and the injectors don't pulse.

    Could be voltage to the pump as well...I've found that running voltage directly from the battery results in 100% starting.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    I'd say that it does have the spark reference, turning the distributor triggers a reference pulse. (seems to me it has to be a quick twist though, slow twist won't do it on mine)
     
  7. neverendingproject

    neverendingproject 1/2 ton status

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    Yes, you need to run a wire directly from the battery to the alternator. The alt. already has a wire, but its for the engine circuit only, and doesnt connect to the battery.
     
  8. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Winds up I had my injectors & ECM wired to an ignition source that turns off while cranking -- Switched to an ignition post with power while cranking, and she fires on the first crank every time now :D
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    No, not from the alternator to the battery...from the battery to whatever you think isn't getting enough power. Everything fed from the starter (your fuse panel) sees a large (over 2V in my case) drop from the starter load. The battery acts as the cushion, and doesn't drop nearly as much as the fuse panel feeds, IF it's in good condition.

    An alternator lead won't do you anything if the engine isn't running, alternator output is negligible when talking about cranking speeds.

    I'm assuming you are talking about making sure the battery gets enough charge, but again, in my case, it's the drain from the starter that is causing my problem, I'm certain because of a bad connection or wire SOMEWHERE in *my* truck.

    Also I gather that this rig will ONLY start if there is fuel added prior, which indicates the entire ignition system is fine.
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    And glad it was an easy fix. :)
     
  11. neverendingproject

    neverendingproject 1/2 ton status

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    No, Im saying that if I dont have the wire from the battery to the alternator, my starter is the onlything getting power. If I dont have the wire, my ecm circuit is dead and has no power to it.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Maybe that's dependant on wiring setup...everything in mine is fed through the junction block on the firewall, which feeds the battery back through the starter solenoid lug. Nothing *directly* from the battery to the alternator.
     
  13. neverendingproject

    neverendingproject 1/2 ton status

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    Huh. That could be it. Maybe mine is wired wrong, but it runs with the wire, and doesnt without it. There is only one wire running to the started right? Then the cable from the battery, but the purple wire is the only thing that hooks up down there on mine.
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    Been too long since I swapped the motor I guess, but I recall there being 3 wires down there, plus the batt. cable...purple, and two reds, the two reds being what feed the junction block on the firewall, which is where the fusible links are.

    Unless you've upgraded the alternator, I believe ALL the setups are wired like that, and I'm not even sure the newer alternator rigs were setup any differently.

    I don't see why they ran the wires off the starter solenoid, except probably to save money, and not having a longer run of thinner wiring. The battery cable is more than large enough for the typical demand from everything besides the starter, so no reason to run them any other way I guess. No clue why the alternator isn't run straight to the battery though. It may have been as simple as routing away from heat.?
     
  15. mouse

    mouse 1/2 ton status

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    The older (pre-1973 vintage I believe) had a wire directly between the alt and battery.
     
  16. Thunder

    Thunder 3/4 ton status

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    Every TBI truck(blazers,1/2 and 3/4 tons) i have worked on has a # 8 or 10 wire from + battery to alternator. At the alternator a red wire with a fuse link is connected to the #10 wire which feeds ECM/Ign. This wire gives unswitched 12V to ign/ECM circut without starter interference or voltage drop.
     

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