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Thread: Tacho/Igniter Voltage - Want to get stock tach to work!

  1. #1
    wisetechscottt's Avatar
    Join Date
    Sep 2015

    Default Tacho/Igniter Voltage - Want to get stock tach to work!

    Car = 87 7MGTE on AEM v1, DH61 igniter on stock coils

    Does anyone know the voltage on the wire that comes from the igniter to the Tach? I had read that the turbo tachometers received a 12 square wave signal, but I get nowhere near 12v when I volt test the tach wire on the DH61. ( or what voltage range the tachometer is expecting to see)

    Some background, I am wanting to convert to 1ZZfe coils, but when you do this you lose your Tach signal as you are removing the igniter. So I used one of the Low Side drivers to modulate a Tach signal but I am getting 12V from the low side driver but when I went to compare it to what the DH61 is sending to the tach I only got around 1V and some change.

    I really just want the stock tach to work with this setup, I know some people have gotten it to work but they have long sense left the community.

    Any help would be great.

  2. #2
    GC89's Avatar
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    Jun 2007
    Spokane, WA

    Default Re: Tacho/Igniter Voltage - Want to get stock tach to work!

    Subbed to see what you come up with as well. I have a set of 1zzfe coils sitting on the bench right now.

    The build- Centrailia or bust........Delilahs build.

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  3. #3
    wisetechscottt's Avatar
    Join Date
    Sep 2015

    Default Re: Tacho/Igniter Voltage - Want to get stock tach to work!

    So after a lot of reading and research I figured it out and now have the factory tachometer working without the use of an igniter, driving the tach signal out of the AEM v1.

    As I found this information extremely hard to come by and only with the help of some key people was a able to get it to work I’m going to lay out step by step what I did to get this to work. Keep in mind that this will all be in the mindset of a yellow plug 87 setup. (I can’t see it being much different on the 89 + AEM v1)

    This will allow you to run 1ZZFE COP in waste fire on the AEM v1 and still have a working factory tach.
    First off I was told by a couple different people that it was “impossible” to output a Tacho signal with the V1, this is simply not true. The V1 has several Low side drivers that can be used to modulate a tacho signal, I used LS3.

    (**Note – there is an error is AEMs instruction manual for the 87-88 V1 setup, they have LS3 assigned to two pins, 2C and 16C. In the first post I had tapped into 16C and this is what gave me all the crazy readings. If you are doing this use pin 2C as LS3).

    Supplies needed:
    2.2k Ohm resistor ( 1/2 watt)
    solder, (iron)
    crimpers and connectors if you want

    AEM v1 Software setup:
    1. Go to Software -> Advanced setup -> Tacho/Speedo setup
    a. On this page you only need these values:
    1. Tacho : 2
    2. Output = 128 (aem uses bit addresses to locate the LS pins, LS3 = 128)

    Wiring that needs to be done.

    2. Identify the location of LS3 (pin 2C) and tie into this, this wire will be the new tach signal wire and will go to the tach wire that previously went to the igniter.

    3. Identify a 12v power source that is powered with key on. Tap into this source and run the over to the ECU ( you could probably use the power wire for the v1, but I chose to tap into the radio fuse as I wanted the ECU to have a dedicated line)

    4. Use the 2.2K ohm resister and attach it to the 12v power source on one end and into the wire that you ran from the ECU that goes to LS3
    ** the reason we are doing this is because the Low side switch (LS3) is a ground interrupting pulse but if the line has no voltage on it you will never see the pulse. So we are creating a pull up resistor circuit to put a voltage on the signal line. Every time LS3 turns “on” it will ground the circuit and effectively take voltage to 0, then when off we go back to the “high” state. The result is a square pulse wave signal. This is the key and what we are looking for.

    5. Take the end of the wire that is going to LS3 and attach it to the wire going to the tachometer. And you’re done.

    in case the picture stops working i attempted to draw it out:

    TACHOMETER--------------------------------------|-------------------AEM v1 (LS3)
    |place 2.2k ohm resistor inline here
    -------------------- 12 volt source (key on)

    AEM v1 Tacho.jpg

    i hope this ends up helping someone else.

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