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Thread: What is the signal for the tachometer?

  1. #1
    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    What is the signal for the tachometer?

    I wanted to see what the signal for the tach looks like. I connected my VOM between a lead coming from red plug #12 and my signal ground splice. I got a steady reading of 2.06 vdc. Revving the test engine resulted in no change. Should that signal be steady voltage like that or am I measuring with the wrong type of test gear.
    I want to add a tach to the stand but wondered if an aftermarket tach would work. Also, I'm not seeing rpm on my OTC 4000E. If I understand this system the distributor should be reporting to the logic module and that should be displayed by the tach.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: What is the signal for the tachometer?

    It's a PWM signal. I think it's compatible with aftermarket tachs, but I'm not sure. I recently looked at it with my oscilloscope, and it changes quickly enough that I don't think a multimeter will do it unless is has the RPM function on it. I have one from my college days with that function and it works very well.

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: What is the signal for the tachometer?

    Cordes, thank you. Inspite of having numerous tach over the years I never really understood how they worked so its time to dig deeper. The oscilloscope info from your observations confirms my suspicions. I will try to borrow a tach to test the hookup.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Boost, it's what's for dinner... Turbo Mopar Staff Aries_Turbo's Avatar
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    Re: What is the signal for the tachometer?

    Quote Originally Posted by cordes View Post
    It's a PWM signal. I think it's compatible with aftermarket tachs, but I'm not sure. I recently looked at it with my oscilloscope, and it changes quickly enough that I don't think a multimeter will do it unless is has the RPM function on it. I have one from my college days with that function and it works very well.
    is it PWM or just a frequency signal? I would think it would be a changing frequency to indicate RPM but I havent logged it.

    Brian

    Quote Originally Posted by turbovanman
    This one is easy, I have myself to blame, I rush things, don't pay attention to gauges when I should, change to much stuff at once then expect miracles, the list is endless.

  5. #5
    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: What is the signal for the tachometer?

    I think you're right that it's just the frequency. It matches the Dist. signal, which makes sense.


  6. #6
    turbo addict
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    Re: What is the signal for the tachometer?

    It's just an on/off 0-12v digital signal, my Microsquirt output to the Daytona Digital dash with no issue.

  7. #7
    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: What is the signal for the tachometer?

    Thanks guys, I ordered a cheap tach.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: What is the signal for the tachometer?

    Update, I installed the tach with signal wire to the negative side of the coil, thre red and white wires to positive and black to negative. It worked as expected. I tried hooking the signal wire to red plug #12 wire and it didn't work however I didn't put the black wire on the signal grounds splice. I'm guessing that would make it work.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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