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Thread: Calibrate Smiths Tachometer

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    Obi Wan RAC68's Avatar
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    Calibrate Smiths Tachometer

    Hi All,

    A good while back, I replaced a capacitor in my Smiths tachometer as it had obviously failed (cracked apart) and I was reading high. Recently, I compared my readings with a friends hand held unit and the comparison seemed to indicate I was again reading a few hundred RPM high again. Since I had also converted the gauge to neg. ground when replacing the capacitor and also drilled a hole in the case to access the potentiometer, want to calibrate the tach, I understand there is a laptop program that, with the correct cables, will generate the frequencies to accurately transmit frequencies representing specific desired RPM settings for the inductive pickup of the gauge to read and be adjusted to.

    1. Does anyone know of this laptop program and where it can be downloaded?
    2. Does anyone know how the cables are made up and attached from the Laptop to the gauge?

    In my search, I read an article that suggested using a 60-cycle AC florescent lamp and a piece of white tape attached to the crank pulle, you would see 4 marks standing still at 1800 RPMs and 3 marks at 2400 RPMs. Although I don't doubt this would provide good comparisons to adjust the gauge to, I would rather have greater accuracy at 1000 RPMs and 5000 RPMss well as 3000 RPMs.

    Any suggestions or input?
    Ray(64BJ8P1)

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    Yoda
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    Re: Calibrate Smiths Tachometer

    Why not put an external tach on the ignition and compare it to the tach? Not high tech, but easy. If your speedo is accurate, you can 'close the loop' at speed with with one of the charts that solve for either speed or RPMs in a given gear.

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    Re: Calibrate Smiths Tachometer

    Do you have a digital timing light? That should tell you the RPMs. Or, if you have it available, try a stroboscope.

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    Yoda dklawson's Avatar
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    Re: Calibrate Smiths Tachometer

    +1 to the advice given by both Bob and Blueskies.

    Most digital, dial-back timing lights I have seen have a built-in tachometer that you could use as a reference during calibration. If you don't have one of those an aftermarket tach from the parts store could be your reference. However, for most of us that would mean you need to buy another gauge. Some old tuneup dwell/tachs may have high-RPM ranges that would allow you to use them as a reference.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

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    Obi Wan RAC68's Avatar
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    Re: Calibrate Smiths Tachometer

    Hi Guys and thanks for your responses.

    I had intended to use a tachometer to sync the Smiths to but my old unit is definitely out of calibration and so far the same with those of 2 friends. So, yes, it would be easier but if I am going to calibrate the unit I want to make sure it will be as correct as reasonably possible.

    The documents I found indicated that a laptop program was available that could transmit accurate appropriate frequencies that would be selectable. That would mean I could sustain a chosen frequency for 1,000 RPMs without fluctuation and as long as it takes for me to adjust the potentiometer. I could also do this at 3,000 and 5,000 to set the accuracy of the tachometer readings where most important for my driving style. However, since the document was not recent, I could not find anything further (like where to get the application or how to hook up the tachometer to the computer.

    Well, if I can't find anything about the program or how to use it, I guess I will be left with buying a new tach and synchronizing my Smiths to it.

    Again, thanks for your comments and suggestions,
    Ray(64BJ8P1)
    Last edited by RAC68; 03-11-2020 at 04:45 AM.

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    Re: Calibrate Smiths Tachometer

    Last time I tried it, I just wedged something under a throttle stop to produce a relatively stable RPM. Better than a foot or finger trying to hold rpm steady. It worked O.K. but probably off a few RPM. I only did 1000 rpm and 2500

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    Yoda dklawson's Avatar
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    Re: Calibrate Smiths Tachometer

    You asked a couple of times about PC software to use for calibration.

    The link below is for the Spiyda Design RVI tach adapter board. It converts a Smiths RVI tach to work like a later (Smiths RVC) voltage pulse sensing tach. Down the page you will find some files for using the PC to generate calibration signals. NOTE: It is my understanding that the calibration signal was/is to be connected to a calibration input point on the adapter board. Obviously with the original Smiths board you are not going to have that. You will have to figure out where the input signal must be connected.

    https://www.spiyda.com/tachometer-electronics.html
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

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    Re: Calibrate Smiths Tachometer

    Hi All,

    In my search, I noted an obvious program being used by an individual calibrating a tachometer on YouTube but without reference. However, I also viewed another strip that described the use of a multimeter to read the engine's Hertz (Hz) reading to determine an accurate RPM. Since my multimeter has the ability to read Hz, I will be trying to identify how to set up the gauge, in conjunction bdcvg's suggestion to set the accelerator, for comparison and setting the Smiths tachometer.

    In short, Hz represents cycles per second. Since the Healey's engine is a 4-cycle engine, with 2 revolutions per each spark, and 60-seconds/minute, arriving at RPMs requires the Hz reading to multiplied by 2 revs x 60-sec =120. As an example, this means that 10-Hz would translate to a 1200-RPM reading on the Smiths tachometer and 30-Hz would be equivalent to 3600-RPMs . As this approach is based upon a conversion formula, the Hz reading produced by setting the engine to a consistent speed could be translated to a tachometer correction within that range.

    Since the multimeter provides very accurate readings and, if I can get this approach to work, could serve as a very good calibration method. I will hopefully have the answer by the beginning of next week.

    Ray(64BJ8P1)

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    Re: Calibrate Smiths Tachometer

    Hi Doug,

    Thank you for the direction to the wave files. Although I did enter that site earlier, I obviously was not observant enough and passed over it thinking I wasn't interested in the change. However, as you pointed out, maybe there is a way to use the approach (with wave files) to drive and calibrate Smiths RVI tachometers as well.

    Again, thank you for your direction,
    Ray(64BJ8P1)

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    Re: Calibrate Smiths Tachometer

    Ray,

    You may be able to borrow a tach/timing light from an auto parts store. I see my local O'Reilly's has signs that they loan tools.
    John, BN4

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    Re: Calibrate Smiths Tachometer

    Hi All,

    Today I successfully calibrated my Smiths Tachometer with my Hz setting on my multi-meter. To get the reading you need to attach an inductive sensor on the positive line of the meter. Although you could attach a length of wire and wrap it around the #1 spark plug wire and increase the number of wraps until you get a reading, I chose to use the plug-able inductive clamp from my adjustable Sears timing light and attach the meter line to a clamp pin in the attachment socket. to identify the correct pin of the 4 available, I used a pin to penetrate the insulation of the wire entering the clamp and proved for continuity.

    The Hz readings were translated to RPMs through the calculation previously posted and I found that, at idle my tachometer was reading 400 RPMs high. As per the suggestion by "bdcvg" to use an accelerator wedge, I use his suggestion to get higher on the RPM scale and compared the tach reading to the calculated meter result and found a slight difference after the initial adjustment idle calibration.

    Well, that's it.
    Ray(64BJ8P1)

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    Re: Calibrate Smiths Tachometer

    Congratulations and job well done!

    Thanks for posting the conclusion and your results.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

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    Re: Calibrate Smiths Tachometer

    I have drilled a hole in the rear of the tach to access the potentiometer on both of my Healys. I connect an accurate test tach to the coil and adjust the Smith Tach to match up. Have always been happy with results, it is quick and easy.

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