Help With Stock Tach

Discussion in 'Fox 5.0 Mustang Tech' started by 302mustanglx, Dec 11, 2013.


  1. 302mustanglx

    302mustanglx Member

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    Hello fellas I want my stock rpm in my cluster to work again. I did a carb swap and would like my bbk cluster to work like it was. I have gone and spliced into the green and yellow wire behind my cluster #11 and ran it to my negative side in the coil and on my msd box tach signal hook up and still nothing plz help I already have after market gauges but hate jumping in the car and having all my stock gauges dead except my fuel. The car is a 88 mustang 5.0 gt what am I doing wrong here guys ur help is much appreciated
  2. madspeed

    madspeed Colonel Mustard Mod Dude Founding Member

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    Moved to tech
  3. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Basically you shot yourself in both feet by doing a carb conversion.
    Unless this is a track only car, a carb conversion is a very bad idea. Track only is OK for carb, it just isn't as versatile and can't be tuned on the fly. It's take the carb apart, chase all the little tiny pieces around while your hands get soaked in gas. Then change the jets and hope you don't lose any of the pieces in the process of putting it back together.

    If you still had a working EFI system, I could tell you exactly how to connect the tach and what color wire to look for. Since you have done who knows what to the car and wiring, there is no telling where the proper connection point is now.
  4. liljoe07

    liljoe07 Active Member

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    Check your fuses first. You are using the correct wire to tap into. Fuse number should be 18.
  5. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    The Tach feed comes off the TFI circuit and expects to see ~ 10-14 volts signal input. How do I know that? I attempted to check the calibration on a stock tach to see how accurate It was. See http://www.stangnet.com/mustang-for...ach-to-be-accurate.805785/page-2#post-8333134 if you want to know more.

    That definitely will not work well when connecting to the negative side of the coil. There is a very large voltage spike when the TFI module turns off between cylinders. The stock wiring contains a 22k resistor in the dark green/yellow IDM signal line. That probably isn't a high enough resistance if the tach was connected to the negative side of the coil.
  6. liljoe07

    liljoe07 Active Member

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    If I remember correctly. The 22kohm resister is only inline with the ECM and the coil, it may be inline with the TFI as well. I'll have to check when I get back in a few days. I may be wrong. But I don't recall seeing anything between the coil and tach.
  7. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Diagram courtesy of Tmoss & Stang&2birds

    [​IMG]

    See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

    Complete computer, actuator & sensor wiring diagram for 91-93 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/91-93_5.0_EEC_Wiring_Diagram.gif

    Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

    Ignition switch wiring
    http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

    Fuel, alternator, A/C and ignition wiring
    http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif


    Vacuum diagram 89-93 Mustangs
    http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

    HVAC vacuum diagram
    http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

    TFI module differences & pin out
    http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

    Fuse box layout
    http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

    87-92 power window wiring
    http://www.veryuseful.com/mustang/tech/engine/images/mustang87-92 PowerWindowWiring.gif

    93 power window wiring
    http://www.veryuseful.com/mustang/tech/engine/images/mustang93PowerWindows.gif

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