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Thread: Tacho Calibration

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    Default Tacho Calibration

    Is it possible to have an 8000 rpm tacho recalibrated to 7000 rpm and changing the tacho decal to suit



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    Ummm...depends on the tacho...ie; they started out cable driven, then moved to ignition points for trigger pulse, then moved to coil back EMF for pulse in the first electronic ignition days (some used pulse from ECU to recreate points switching)....and I suppose now they're out to CAN bus or PWM drive from ECU.....pick your poison =)

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    Yes of course its a late model electronic one as below and actually I need to recalibrate to 6000rmp with the application of the Decal in link 2



    Last edited by RedXT; 30-05-22 at 02:17 PM.

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    M'kay.....now it depends on what driver circuit they've used....or more specifically, if they've used a dedicated IC to drive the meter (and if so, what IC and what operational parameters you can muck with)....

    ....consider the basic tacho circuit like this below....



    In the above, R4 sets the full scale deflection of the meter itself....this is the first area one looks at to do a mod like this...ie; change the meter full scale defection so at max_rpm 6000 it's at full deflection...

    ....I've done mods similar to this, mainly with speedos (ppl changing diff ratio when their speedo is electronically driven, not cable driven) that need to be re-calibrated so they're accurate with the revised gearing...and sometimes you get lucky when the OEM has included a trimpot on the driver board you can twiddle to obtain calibration ~ other times not, and you have to mad a trimpot in place of fixed resistor -:- tachos tend to be the same, you don't know what they've done until you exorcised the circuit board and had a squizzy at the circuit implementation...

    ...as noted in the ad, it'll accept either points switching input, or that from electronic ignition....(that's a bit grey, but my guess is input trigger is switched to ground)....which is different from electronic ignition with tacho driven by ignition coil back EMF (around 50vdc peaks)...so I'm thinking simple driver circuit setup for 8cyl/4stroke impulse pattern input...(wouldn't surprise me if it was the generic driver board for other aftermarket tachos that can switch between 4/6/8 cylinder engines but setup for one application)...

    ....often, after achieving the mod itself, you may get into a boofle wrt gauge needle travel linearity versus the marked graduations of the replacement decal....again, depend on the circuit employed ; custom made decal is the quick way out of that...

    ...are you prepared to open it up, to have a look inside at what they've done?...

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    IIRC that model ford was points driven. Haven't touched one since about 1980 so could be wrong.
    I'm out of my mind, but feel free to leave a message...

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    im quite sure they were points driven. The trouble is removing the dial face to apply the decal without breaking it. and while its apart connect it to the car and see if you can adjust a trimpot to suit the scale. If neccessary drill a hole in the casing so you can trim the adjustment from outside the case.

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    Quote Originally Posted by lsemmens View Post
    IIRC that model ford was points driven. Haven't touched one since about 1980 so could be wrong.
    It was points driven but I changed to electronic ignition and these tacho have been modified to suit

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    ...are you prepared to open it up, to have a look inside at what they've done?

    I am still waiting for it to be delivered, but when it arrives I will open it up and take some pics

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    as an aside, what is it for? i.e What project are you undertaking?
    I'm out of my mind, but feel free to leave a message...

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    ...are you prepared to open it up, to have a look inside at what they've done?
    Quote Originally Posted by RedXT View Post
    I am still waiting for it to be delivered, but when it arrives I will open it up and take some pics

    Good'o.... I'll keep an eye out =)

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    Im making up an XT GT Dash which had a 6000 rpm tacho, hence the decal which is ford xt but instead of the XW XY 8000rpm ($350) taco I am now thinking to get this one



    No need for calibration and at $50 if I stuff it up removing and replacing the needle shit happens

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    Quote Originally Posted by RedXT View Post
    Im making up an XT GT Dash which had a 6000 rpm tacho, hence the decal which is ford xt but instead of the XW XY 8000rpm ($350) taco I am now thinking to get this one



    No need for calibration and at $50 if I stuff it up removing and replacing the needle shit happens

    Provided you're not after OEM look an don't mind the hour meter....should work.


    edit: but not sure if that'll do single ignition coil back EMF pulse.....what ignition module did you use?
    Last edited by wotnot; 31-05-22 at 11:31 AM.

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    Quote Originally Posted by wotnot View Post
    Provided you're not after OEM look an don't mind the hour meter....should work.


    edit: but not sure if that'll do single ignition coil back EMF pulse.....what ignition module did you use?
    The bazel and glass will be removed and the hour meter will be covered ( and perhaps disconnected) by the new Decal

    I assumed the pulse from the engine coil would work the tacho as it does now

    I am using a Bosch MEC 717 foe electronic ignition like below


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    How are you switching the coil?.... reluctor type pickup in the dizzy, and remote coil driver/ignitor?

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    Quote Originally Posted by wotnot View Post
    How are you switching the coil?.... reluctor type pickup in the dizzy, and remote coil driver/ignitor?
    God mate Im not sure there are no points just disy with bosch module see link


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    Ah, ok....magnetic/reluctor pickup, with HEI module built in....should be ok. (I was just wondering if it ran a MSD box or not =) The HEI module shouldn't be a problem =)

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    Thanks mate I will let u know how I go

    thanks again

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    Is this a genuine GT or just a look alike? I'm thinking that the aftermarket tacho would be good on a lookalike but, if genuine, it would probably be more of a hindrance than help should you either show it, or try to sell it.
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by lsemmens View Post
    Is this a genuine GT or just a look alike? I'm thinking that the aftermarket tacho would be good on a lookalike but, if genuine, it would probably be more of a hindrance than help should you either show it, or try to sell it.

    The OEM stuff (usually made by Smiths or VDO) fail in various ways, be it the driver board, or gauge coil/jeweled bearings/return (clock) spring ~ when it's a mechanical failure, they can be horrowshow expensive to have rebuilt.

    When it's just the driver board --> ...which is a good treatise of this area as well.

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    Its actually an 69 XT Fairmont, the only difference between fairmont and GT is badges, stripes, Driving Lights and Dash, there are places that do repo dashes complete and they cost $2500, but the 6000 rpm tacho and gauges are not available by themselvs.

    I have the original motor and dash etc if I sell it and have allready done one dash with XA gauges 10 years ago , so I just want to try building a repo one myself

    Real Dash,

    My Dash with XA Gauges

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