124 Tach calibration?

Discussion in 'Front-Engine Rear Drive Fiats' started by Pete Whitstone, Jul 20, 2019.

  1. Pete Whitstone

    Pete Whitstone True Classic

    Location:
    McKinney, TX
    I have a later tach from a 124 in my 037 project. This is the one with the shallower face than the earlier ones - not sure of the exact years but I think 76 on?

    Anyhow, as my project car has a six cylinder and the tach was intended for a four, it reads high by 1/3 as far as I can tell. Can these things be calibrated?

    If not, is there another way to go about it, like an electronic in-line signal modifier? Or am I looking at replacing the guts of the thing?

    Thanks,
    Pete
     
  2. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    I know the tach in my x1/9 has an ajustment pot inside (have to completely disassemble the dash panel to get to it) but it can be adjusted. I would guess (on the x1/9) that their is enough range to do this. So using that as a baseline I would think your's might be the same. It costs you nothing to open it up and check into it.
     
  3. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    I recall some of the old aftermarket tachs had a switch on the back to adjust for 4, 6, or 8 cyls. What circuitry did that effect? Maybe the same sort of electron magic could be made for your tach. Way beyond me to know how that could happen, but I bet some of our 'lectricity experts here know.
    Also there are conversion boxes for some tachs to run on COP ignitions, etc.. I wonder if anything like that has the capability to do this.
     
  4. That switch was pretty standard on dwell/tachs. The reason the switch is required is because the meter need to know how many firings per revolution.
     
  5. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    So what circuitry would be needed to make an existing tach change the number of firings per revolution?
     
  6. The easy way to do it, which many meters do, is have a separate scale for different number of cylinders. Some just tell you to multiply the 8 cylinder number by two for 4 cylinders. I have not looked at the circuit for these tachs in quite a while, but the old ones basically had a circuit that converted each firing to a well shaped pulse which went into an integrator capacitor that drove an analog volt meter. It is possible that the ones with switches just change the value of capacitor or a resistor associated with the integration function. They could also use a voltage divider between the integrator and meter to just change scales. It would be interesting to see the schematic.
     
  7. Pete Whitstone

    Pete Whitstone True Classic

    Location:
    McKinney, TX
    Well, not really sure I want to tear the old thing apart, without knowing exactly what I'm looking for when I get in there. I did discover these:

    https://www.summitracing.com/parts/dak-sgi-8e/overview/

    And it looks like it would do the trick. For $81, I think that might be the path of least aggrevation.

    Pete
     
    kmead likes this.
  8. Dr.Jeff

    Dr.Jeff True Classic

    Location:
    Sin City
    Pete, that looks like the perfect answer. And in the long run, I bet it is likely the most affordable way to go.
     

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