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  1. #1

    Default How to Hook up an aftermarket Tach - Square wave signal?

    I'm wanting to hook up an aftermarket tach to my 1986 LTD with a CFI 3.8 V6. The tach says not to hook it up to the negative side of the coil that it needs a square wave signal. I figured since the car is EFI it would have such a signal somewhere for the ECU and I would not have to buy an aftermarket tach adapter. Does anyone know?

  2. #2


    What brand of tach? I believe the autometers just tie into the coil

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  3. #3


    I have two, a Holley and an Arduino based tach.

    The Holley says:

    • This should NEVER be connected directly to an ignition coil, or any other high voltage source. Any connection other than a dedicated 5volt or 12volt Tachometer (square wave) type output will permanently damage the unit. If a dedicated Tach output, such as those found on the MSD ignition boxes “Tach out”, is not available, MSD part number 8918 should be purchased. This is attached directly to the coil and conditions the signal for the tach. This should work with MOST factory inductive coil ignitions

    The Arduino instructions say basically the same thing. I figure since the car has an ECU it would have a signal like that, but I can't find any wiring diagram that shows it.

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