Thread: Stock Tach Circuit

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  1. #1 Stock Tach Circuit 
    Young Brit
    Join Date
    Feb 2012
    Posts
    52
    My TR8 is running a 4.6 distributerless, using a MegaJolt and a Ford EDIS coils to drive the ignition.
    That works perfectly but now I am trying to drive the stock tach. The MegaJolt people have an adapter board to drive old tachs.
    I have installed the board but am not getting any tach movement. Measuring the output from board using a digital volt meter
    I get about 12v at idle, rising to about 19v as I blip the throttle. Does that sound about right, I have no idea what the output should be?

    Thanks

    Graham
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  2. #2  
    Hi Graham,

    I don't think the stock tach wants to see voltage, it should count the pulses from the ignition circuit.

    -Darrell
    ---------------------------------
    Darrell Walker
    66 TR4A IRS-SC CTC67956L
    81 TR8 SATPZ458XBA406206
    Vancouver, WA, USA
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  3. #3  
    Young Brit
    Join Date
    Feb 2012
    Posts
    52
    You may be right, but using a digital volt meter I think the pulses are getting damped out, that is just a guess.

    Graham
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  4. #4  
    I don't think a DVM will respond quickly enough, you would probably need a scope. Though some voltmeters do have an RPM function that might work (they are designed to connect to the negative side of the coil, just like the stock tach).
    ---------------------------------
    Darrell Walker
    66 TR4A IRS-SC CTC67956L
    81 TR8 SATPZ458XBA406206
    Vancouver, WA, USA
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  5. #5  
    Young Brit
    Join Date
    Feb 2012
    Posts
    52
    I think I have answered the question. Some automotive digital volt meters have an RPM scale, Harbor Freight sell one that also has a temp scale for $30.
    Using a friends meter I was able to get a good reading out of the adapter board. So the problem must be in the instrument cluster. I have a spare cluster
    so I will strip out the tach and see if I can get it to work stand aloan.

    Graham
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