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  1. #1

    Default Aftermarket big tach wiring, stock 2.3, stock ignition

    I have searched all over to find some answers, but cant seem to find much. I have an aftermarket tach that I want to hook up to my motor. its a 1985 thunderbird motor, with a brand new RFW wiring harness. I have the tach green wire hooked up to the tach output from the wiring harness, obviously, but the tach still doesnt register correctly over about 2500 RPM. I have also tried putting a pull up resistor (i think 10k) between +12v and the green wire, still same thing. even tried switching the tach cylinder setting between 4, 6, and 8 cylinder, same thing. Is there something im missing? is my tach just garbage?

  2. #2


    So it's a used aftermarket tachometer? Maybe garbage, maybe not. Is the other end of that wire that is tach output that you connected to from the wiring harness, connected to the coil's negative?

    In general, for tachometers, green goes to the coil's negative terminal (or the aftermarket ignition box's tach output), red is fused and to battery positive, black to battery negative, and white to power for the tach's illumination with the dash lights at night.

    Which aftermarket tachometer do you have (we could find instructions online if you haven't any), and what's all included with your ignition?
    1986 Mustang convertible 3.8L ---> BUILD THREAD
    1983 Mercury Cougar 3.8L LS
    1986 Ford Thunderbird 5.0L ELAN
    1966 Ford Fairlane sedan 200-6
    1966 Ford Fairlane GT (390ci 4spd)
    1981 Mercury Marquis Brougham
    1980 Capri RS Turbo
    1971 Mustang Fastback
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  3. #3


    Is your stock ignition a TFI or Duraspark?

    If TFI, the variable dwell and constant current features may confuse an aftermarket tach if it's thresholds are too low. Try hooking the tach to the PIP wire instead of the coil. The PIP signal is a nice 12 volt square wave.

  4. #4
    FEP Supporter 4-barrel Mike's Avatar
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    Apr 2010
    Portland, Oregon


    Autometer? Or?

    1985 ascMcLaren 5.0 SC Roadster
    My '78 Fairmont build -

  5. #5


    it is a brand new tach that I got. honestly i kinda cheaped out and just got an ebay tach, nothing too fancy, because I couldnt find a used autometer or something nice like that.

    im not sure of the circuit diagram for the RFW harness, but i think the tach output wire is coming from the ECU. since the harness is meant to "replace" a stock harness, and the stock tach would get its signal from the ECU, yeah?

    I have TFI ignition (1985 thunderbird engine), with the gray "box" on the side of the distributor. what/where is the PIP wire? is that the "spout" wire?

    this is the tach I got (go easy on me, I have an autometer, and all autometer gauges in the

  6. #6


    yesterday I ran a wire from the negative on the coil to the tach. I tried it with and without the pickup resistor. it didnt seem to change anything. when I revved the engine up it felt like it was nearing redline, but the tach was only reading up to 3500 rpm

  7. #7


    The PIP is the top pin in the 6-pin connector (#1 in the pic below). This is the wire that sends the signal from the Hall sensor to the computer. Try back-probing [slide a needle in next to the wire] on the PIP pin and hook your tach signal wire to that. If the tach works then you can do a real splice into the PIP wire for a more permanent installation.

  8. #8


    Just tried tapping into that pip wire and the reading was still off. Gonna try to borrow a known good tach and see if maybe mine is just crap

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