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  1. #1
    FEP Supporter vdubn's Avatar
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    Default MSD Digital 6, Do I need a Tach Adapter, Which is the Tach Lead

    So, I got my 1985 GT (converted over to EFI with full 86 harnesses) fired up for the first time last night, and it runs great, but has a few wrinkles to work out.

    First is the fact that my factory tach is not working. After some research, it looks as though I may need an MSD Tach Adapter, part number 8920. I am running an MSD Digital 6 (not to be confused with the 6A or 6AL series MSD systems), and I am having a tough time finding accurate info as to what I need, and how to connect it to my car. I called MSD today, and they said that they couldn't give me any specific information about how to wire it to my car, but they said just don't connect the Tach Adapter to the coil... AT ALL. MSD shows wiring diagrams to connect their Tach Adapter to a 6A or 6AL, but nothing for the Digital 6.

    This was especially confusing, as I thought that the tach lead in our cars connected to the negative side of the coil.

    The Digital 6 has a tach lead connector, so I can wire the Tach Adapter to that, I just need to know what signal wire I need to connect the Tach Adapter to, so that it will send my factory tach the appropriate signal.

    Hopefully someone has been through this, and can help out.

    Thanks in advance
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  2. #2
    FEP Supporter vdubn's Avatar
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    I found this diagram, that shows the tach signal wire (Dark Green, With Yellow stripe) goes to pin 4 of the EEC, and I assume that it also goes to the Tachometer, right? I also need to find out if I can connect this wire straight to the tach output on my MSD Digital 6 Plus, or will I need a Tach Adapter to bridge between the MSD and my factory tach? Also, if I clip this wire from the negative side of the coil, will it cause any issues? I can do some testing tonight to see, just curious to hear from anyone else who has run into a similar situation.

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    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  3. #3
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    Really surprised that MSD's Tech support can't help you. Have you tried giving them a second call to see if you get a different tech support person? Might get a better answer form a different person. Wish I could help. I have the 6AL programmable box so hopefully I don't run into the same issue.

  4. #4
    FEP Supporter vdubn's Avatar
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    Yea, I was surprised as well. You would think they could get any information for any vehicle, especially the cars that they market their products to... I think that I may have it figured out though. I will post back how I had to wire it all to get it to work.
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  5. #5

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    1986 GT T Top- stock except for magnaflow cat back
    1990 LX vert- 500hp V3 and all that stuff
    2013 GT- usual bolt ons

  6. #6

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    This probably won't help much but I found in some notes I made about a white wire going to the tach adapter. I'll see if I can dig any more up.

  7. #7

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    Your results may vary, but my Digital 6 ran the factory tach in my 82 Mustang just fine. Put a female spade on a jumper, plugged it into the Digital 6, and another connector for the green wire in the harness for the tach.

    Then the second tach died, so I am adding a pedestal mount....

  8. #8
    FEP Supporter vdubn's Avatar
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    Quote Originally Posted by Oldrd View Post
    Your results may vary, but my Digital 6 ran the factory tach in my 82 Mustang just fine. Put a female spade on a jumper, plugged it into the Digital 6, and another connector for the green wire in the harness for the tach.

    Then the second tach died, so I am adding a pedestal mount....
    There are two wires that connect to the negative side of the coil... and my MSD has a white wire that connects to those. I could just jump from the white wire over to the spade on the side of the Digital 6 and see if I get Tach, right?
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  9. #9
    FEP Supporter vdubn's Avatar
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    So I tried to connect from the spade on the Digital 6 to the negative side of the coil.... No luck. I think I need to cut one of the 2 green/yellow wires in the harness that normally connects to the negative side of the coil, and jump it to the spade on the side of the MSD.

    Does anyone know how to tell which of the green/yellow wires goes back to the tach?
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  10. #10
    FEP Senior Member qikgts's Avatar
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    You should be able to use a multimeter to figure things out. I'd ohm them out trying to locate the one going over to where the Duraspark box was then very simply, the other would be for the tach itself.
    '85 GT

    The other one... 2016 F-150 XLT Supercab Ecoboosted with 3.73's...

  11. #11
    FEP Supporter vdubn's Avatar
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    Ok, so I had a 50-50 chance and I had to cut one either way. Of course, as luck would have it, it was the second wire I tried.

    So, I start the car, let it idle, I have a nice tach signal...Idling at 900 rpm. Shut the car off figuring I'm good to go.

    Start the car later in the day, drive off, and as soon as the tach hits 1500 rpm, it drops to zero. If I put the clutch in, I get tach signal back.

    Any ideas?
    Last edited by vdubn; 02-11-2018 at 08:08 PM.
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  12. #12
    FEP Member 83GLCoupe's Avatar
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    I'm not sure what the answer is, but if you are using the EFI harness with the above mentioned wiring diagram I wouldn't have cut either one of the wires from the coil. Looks like 1 goes to the TFI module and the other goes to the ECU through a resistor wire. The ECU probably sends the tach a signal so you need to take that wire from the back of the tach and run that to the tach output of the MSD.
    -1983 Mustang GL Coupe
    -2015 Taurus SEL (DD)
    -1982 Oldsmobile Cutlass Brougham (Wife's Toy)
    -2012 Nissan Xterra

  13. #13
    FEP Supporter vdubn's Avatar
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    Yea it looks like maybe I cut the one to the ECU, but not sure. There is a connector under the dash that has the same green/yellow wire, that goes back to the tach. Interesting that the car still runs though. The first wire I cut, car wouldn't start. Strange. I'll get that wire reconnected and send a wire from behind the dash and see if that changes anything. Something tells me the tach is shot, as I should get good tach signal either way from that wire, not just the first 1500 rpms.
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

  14. #14
    FEP Supporter vdubn's Avatar
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    I installed the 8910 MSD Tach Adapter today, in place of the MSD adapter harness that goes between the MSD Coil and the MSD Digital 6, and reconnected both green/yellow wires to their original wires in the harness. Tach performs perfectly, and car runs great.
    85 GT Hatchback, T-Tops. 2012 Coyote 5.0L Swap, T56 Magnum, GT550 Dept of Boost kit, with Eaton M122 SC, RAM 11" Powergrip clutch, BBK Long Tubes/OR X-Pipe, Flowmaster Axle Back, Stifflers DS Loop, and Rear X-member, AJE Tube Crossmember with AJE-Mod Mounts, 3.73 Posi 8.8, 5 lug Cobra 13" fr and 12.6" rear disc swap, 2004 Hydroboost Swap, MM SubFrame Connectors, MM Panhard Bar, MM front A-Arms, MM rear control arms, MM CC Plates, MM Coilovers w/Koni dampeners, MM Torque Arm (soon to be installed).

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