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TBI Wiring schematics

Discussion in 'The Injection Section' started by justinf, Sep 7, 2005.

  1. justinf

    justinf 1/2 ton status

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    Does anyone have detailed wiring schematics or diagrams for a 93 TBI 350? I believe the motor came out of a 2WD pickup with the 4L80E. I have everything hooked up and the motor will run if I dump gas in the throttle body. It will not fire the fuel injectors, so I believe I am missing a wire or two that needs to be hooked up, possibly for the transmission, but the diagrams I have (out of the Haynes manual) are not very helpful at this point.

    BTW, this is going in the jeep in my sig.
     
  2. HarryH3

    HarryH3 1 ton status Author

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    The best schematics can be found in a factory electrical manual. The Driveablity and Emissions section is also very helpful in doing the swap. When I did my swap I bought the manual for the '87 Corvette that the engine came from. It was VERY helpful! Hopefully, someone has the right books to help you out. If not, www.helminc.com sells the factory manuals.

    One of the most common errors in wiring these up is wiring the switched power feed to a circuit that turns OFF when the key goes to the start position. With the exception of the headlights, most other circuits get switched off when cranking. If your ECM is wired to a circuit that switches off then the engine will never start... :wink1:
     
  3. justinf

    justinf 1/2 ton status

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    No, the injectors and computer are wired to a source that remains on when cranking. I had an 87 TBI in this jeep, I have replaced it with a 93, and now everything appears to be working except the computer will not fire the injectors.

    I did some reading on pirate and I am wondering if maybe the vehicle the engine came out of had a VATS system that is screwing things up for me.
     
  4. k5-newbie

    k5-newbie 1/2 ton status

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  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Did they end up putting VATS in trucks? I know they did in cars, wasn't sure if that made it to trucks or not. Should be a VATS module in the harness if so.

    Not firing injectors could also be a distributor issue, although I'm assuming you left that in place.

    I'd be surprised if the tranny has anything to do with the engine running, there should be a limp mode that at least allows the truck to move.
     
  6. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    I was thinking possibly a faulty oil pressure switch / fuel pump relay? Usually no fuel from the injectors is a bad ignition module, but if it runs when you pour gas down the throttle body, then that means that the ECM should be pulsing the injectors because the dizzy should be sending signals to the the ECM. Try renting a set of noid lights, unhooking both injectors, hooking up a noid light, and see if they are pulsing when you are cranking the engine. If they arn't, check to make sure that the injectors have 12 volts when the engine is on, and cranking. The ECM provides the ground for the injectors, so if you've got 12 volts, and no pulsing, something else is wrong, and you'll need to find a no-fire TBI flowchart.
     
  7. k5-newbie

    k5-newbie 1/2 ton status

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    I wheeled with a guy this weekend who had a Samurai on 44's with a 4.6 FI setup and he was having some wiring issues that made the injectors not fire, so it could be wiring but dont think its anything to do with the tranny. I have to look at this coming weekend and see where his problem is, hopefully you will figure it out before then but if not I may have some info for ya
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Good point. Difference between no fuel and no injector pulse. You can make a noid light with a 194 (dash) bulb.

    You can have spark and not be giving the ECM a reference pulse unfortunately. That's one of the service manual tests, to check if the ECM is "seeing" the firing events as the engine cranks, with a scantool.
     
  9. justinf

    justinf 1/2 ton status

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    I am not sure about the VATS. I am going to check with the guy I bought the engine from to double check what it came out of.

    As far as the distributor is concerned, it was left in place. I was thinking the distributor should not be an issue since it will run if gas is dumped in the throttle body.

    I have a noid light, the injectors are not pulsing while cranking. The injectors have 12V in "run" and "start" I have already checked that. this is what makes me think there is another signal that I need to hook up back to the computer. I am guessing it is looking for another signal before the computer will tell the injectors to fire.

    I could change the ignition module, I have another complete distributor (out of the 87), but would a bad ignition module allow the engine to run when fuel is dumped in?

    As far as the fuel pump circuit, I have not used the harness fuel pump circuit. I ran my own circuit. I did however remove the relay, and hooked up 12V (hot at all times) to the orange power wire that goes back to the computer, as well as hooked 12V (hot in "run" and "start") up to the grey wire that would have gone back to the fuel pump, this wire also ties back to the computer (according to the Haynes diagrams) for an "elec fuel pump in" signal. However when i checked continuity and for power back to the computer connector, the grey wire from the fuel pump relay split somewhere in the harness and split into two wires (a blue and a blue/green/teal color).

    I am going to check the other harness I have (it is another 93 harness I believe) this one is partially taken apart to help me trace wires down. I want to see where and why the grey from the fuel pump splits since it is not shown on the wiring diagrams I have.
     
  10. justinf

    justinf 1/2 ton status

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    So does the ignition module provide the reference pulse to the ECM? I don't have any diagrams in front of me, so I can't look right now.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Yeah, that's from the distributor.

    Since the engine WILL run with fuel, that means you've got spark, but if the injectors aren't pulsing, the ECM isn't commanding them for some reason.

    I don't believe there is any other input that will stop the injectors from pulsing.

    Part of the ECM's job is to "time" the fuel pump run time vs. how long the engine has been off (think I've got that right) so I believe that is what the wire is from the ECM regarding fuel pump. There will more than likely be an ECM pinout on diy-efi.org in the GMECM FTP section for yours, which is bound to be precise compared to Haynes.

    The stock setup uses the oil pressure switch as a failsafe for the relay failing, nothing else. If you've bypassed all that, then you should eliminate any of that circuitry.
     
  12. justinf

    justinf 1/2 ton status

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    Well, I can try changing out the ignition module tonight.

    I have looked at diy-efi.org and there is no computer listed that ends in 625 like I have. As far as I can tell by looking quick, the 94/95 truck computers appear to be close, but not exact. There are some differences.

    So the oil pressure switch will not kill the fuel pump when the oil pressure drops? That is what I had always heard, but wasn't sure since the diagrams I have seen did not show the circuitry to accomplish it.

    Either way, I don't use it, the relay is gone, the wires are still in the harness only because I didn't want to undo the whole harness to get them out.

    Any other tips and/or suggestions? Or wiring diagram sources?
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    Whats the exact ECM #? 12277625? Try over at thirdgen.org?

    No, the oil pressure switch is wired in "across" the relay. Power can go either path, but the relay would have to be de-energized or die before the oil pressure switch would have the capability of killing the engine.

    (interesting reading on fuel pump circuit) TBI stuff is wired similar, although I think earlier TBI uses an additional component.

    If you've got an Autozone handy, I know they have scanners available. An OBD1 scanner should allow you to test the RPM "output" of the module as the ECM should be seeing. No idea if they'll let you take it home though.

    I made a post about troubleshooting that has a GM crank/no-run chart.

    Of course I find a post that says the pickup coil is what triggers the ECM, but since that signal goes into the module...
     
  14. justinf

    justinf 1/2 ton status

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    I don't know the exact number off the top of my head. I will have to check the computer tonight. I may look around at thirdgen.org later today.

    I will look for your trouble shooting post.

    thanks for the help.
     
  15. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    To test if the ECM is receving a pulse from the dizzy, at least with my TPI, I was told to unplug all the injectors, install a noid light into one of the injectors, unplug the dizzy's harness, and very quickly tap 12 volts onto the white wire. It turned the noid lights on when you did that. However, TBI may not work the same way, so make sure you do some reading up on that before you start putting 12 volts onto different wires, lol
     
  16. dyeager535

    dyeager535 1 ton status Premium Member

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    The problem with that I see is that the white wire might not be the right one.

    http://92b4crs.tripod.com/86wiring/dia-pics/22-0.jpg That's TPI, later TBI has got to be very similar.

    I'm not quite understanding what each one of those wires coming OUT of the ignition module is "telling" the ECM. Maybe one of our resident electrical experts can deceipher that in plain terms.

    Maybe I've got it...tan is "hot" based on the ignition switch. When it's hot, the white wire sees that voltage on the tan wire, which the ECM then sees. When the engine is running/cranking, I assume the ppl/wht wire then indicates via the pickup coil that the engine is rotating to the ECM as well. If it weren't for the noid light test on the white wire, I'd assume the white is the fuel pump trigger, and the ppl/wht is the injector trigger. I could be totally wrong though, I'd like to understand that diagram. Same design TBI/TPI except how ESC is handled later on. (internal to the ECM)

    I'm at least partially wrong, ECM pinout shows that white AND ppl (based on pin #'s above posted wiring diagram page) white don't receive voltage unless the engine is running. Looks to me like PPL is what should trigger the injector pulse. Putting 12V to either one is too much.
     
    Last edited: Sep 7, 2005
  17. justinf

    justinf 1/2 ton status

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    Just taking a quick look, I would have guessed the tan wire and the white wire formed a feed back loop to control spark timing, while the ppl/white was the reference pulse off the pickup coil to the ecm. But I would need to look it over closer and possibly have more info to figure out exactly what is happening.
     
  18. utchev

    utchev Registered Member

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    http://www.autozone.com/servlet/UiB..._us/0900823d/80/08/79/51/0900823d80087951.jsp

    Autozone's (of all places) website and a very knowledgeable GM parts counterman at my local dealer (Murdock in Bountiful, Utah) helped me track down some issues on my '89 TBI system (I have one in my Jeep and in my tow rig). I don't know if that'll help you on your '93 or not.


    {I had issues with the EVRV "widget" on my Jeep when swapping the harness/motor in if that's any help.....I wasn't getting a signal to start up the fuel pump} See figure 4....


    Oh, and the 'thirgen.org' F-car brotha's got this TBI/TPI wiring down pat. Worth searching their forums.....
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    ECM pinout agrees with your diagnosis. :)

    So with that known, it would seem the pickup coil or the module could cause a no pulse condition.
     
  20. justinf

    justinf 1/2 ton status

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    Seems right, I am going to do some parts swapping, and change out the pickup coil and module with the one from the 87 TBI I had in there.

    I think I am also going to browse through thirdgen.org tonight.
     

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