connecting OE tach to multi coils
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  1. #1
    1000+ Posts jo proffi's Avatar
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    Default connecting OE tach to multi coils

    As the title suggests, my challenge of the week is trying to work out a simple and hopefully cheap solution to the issue of connecting the factory dash gauge tacho to the multi coils set up.
    My set up has 4 coiled ganged together in pairs to make a twin wasted spark version.
    The coils are plain old bosh ones driven by a bosh module, so nothing fancy like COP.

    I believe the tacho is a neg driven one, and when i connected the signal wire to one of the coils it works fine, but at half speed of course.
    I didn't try and connect the neg of both banks figuring I'd create crosstalk by doing so.

    I know one solution would be be to go down to the second hand boat shop and buy an outboard motor tacho designed for 2 cyl motors and simply wire that up to half the cols (one bank), but that means gutting the dash and the dash is quite sexy as it is, so this will be the last option I explore when all others have failed.

    A net search details a few different circuits with diodes and resisters but there is enough variation in their designs to confuse the heck out of me.


    Does anyone here have any clear advice (for dummies) for getting around the problem???



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    Jo

  2. #2
    COL
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    Hi Jo

    Does OE tacho have a switch on the back to select different number of cylinders, or there may be a pot there that is used for calibration of the tacho. You may need to change a the resistors near the pot if there is one to get the range needed.

    I have converted a 7000 RPM tacho into a 10000 RPM tacho by just recalibration and obviously rescaling the face.
    Regards Col

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  3. #3
    1000+ Posts jo proffi's Avatar
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    Yes this is the other option, halve or double the resistance of a resister in the gauge and simply run half the tach signal.
    This task does fall well outside my skill set but thats not to say i dont have a mate who is an instrument repairer who would understand how to do this, and have the test equipment to make sure it is calibrated.

    He has already looked at the internals of the gauge and whilst he didn't suggest your simple method (as i wasn't even as far as knowing it worked at half speed), he did suggest that as the electronics were pushing 30 years old, the caps and some of the other components he recognised would probably be starting to get out of range and should be replaced anyway.
    Jo

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  4. #4
    1000+ Posts PeterT's Avatar
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    This is how to do it, provided your original tacho is looking for a +12V square wave input. Some tachos rely on the large voltage (~400V) produced by the coil collapsing, in which case the circuit won't work.
    Attached Files Attached Files

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  5. #5
    COL
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    Hi Peter

    Thats a nice easy solution.
    Regards Col

    1973 Renault R12 Station Wagon
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    2002 Renault Laguna V6
    1973 Alpine A110

    http://alpine-a110.weebly.com/

  6. #6
    1000+ Posts jo proffi's Avatar
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    Cheers, Peter.

    Jo

  7. #7
    1000+ Posts jo proffi's Avatar
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    Brilliant !!!!!!


    How easy was that with the right info.
    It worked first go. ( I meant first go at me making the lead up)


    You've made my day Peter.


    Jo

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