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Thread: Tacho tech

  1. #1
    Chookhouse Chooning Automotive Encyclopaedia Hen's Avatar
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    Default Tacho tech

    There seem to be a fair few threads about this already, so I hope this can bring a bunch of information together so anyone can get an understanding of how their tacho works, and more importantly how to fix it if it doesn't.

    What do they do?
    A tacho (or correctly a tachometer) give you an indication of the speed an engine is running (usually in RPM).

    How do they work?
    They are fed an electrical signal from your ignition system or ECU, the frequency of which is directly proportional to the engine speed. They then process this signal, and move your tacho needle (or digital display) to a certain point corresponding to the frequency of the signal.

    Not too bad so far really. Now it starts to get a bit more interesting. There are two types of signal a tacho can receive: voltage spike or 5V square wave. And a given tacho will only work correctly when fed the correct signal type.

    Voltage spike.
    A voltage spike is used in older systems, all those without EFI and some EFI systems using a coil and dizzy. In such a system the signal from the negative terminal of the low tension side of the coil is fed directly to the tacho. This signal is usually 0V (as the negative side of the coil is earthed through the points/ignitor). However when the coil fires, not only is the huge voltage for a spark generated in the high tension side of the coil, a voltage spike (well over 12V) is also seen on the negative terminal of the low tension side, and hence on the wire leading to the tacho aswell. The signal looks like this:



    In this case the tacho just monitors the frequency of these spikes and converts that into a needle movement.

    5V square wave.
    This signal is just as the name suggests, a 5V square wave whose frequency is given by the engine speed. It is now used in almost all cars and aftermarket gauges. It would look like this:



    This signal is generated by the ECU and fed to the dash, which then moves your needle.

    Tachos are tuned differently depending on their use
    Not only does a tacho need the correct signal type, it also needs the correct signal frequency. Consider an old school V8 doing 6000rpm (50 combustion cycles a second). It will spark 400 times a second, so the dash needs to know that 400 voltage spikes a second means 6000rpm. However a 4cylinder engine at the same rpm will only spark 200 times a second, so its tacho needs to know 200 spikes a second = 6000rpm. Clearly connecting the V8 to the dash made for the 4cylinder car will mean the tacho is going to display an incorrect reading.

    Now that's the theory, but how do you get your tacho to work?
    That depends on your problem. These I think can be split into a few different groups. The problems and solutions are listed here:

    - New engine/ECU outputting 5V square wave mating to voltage spike tacho
    Tacho wont work at all, or jump around randomly. You need to use a coil negative emulator or a tacho modification. The emulator converts the 5V square wave from your ECU into a series of voltage spikes. The circuit is HERE. This tends to give a somewhat sluggish needle though, maybe a bigger inductor would help. A .pdf guide for modifying the hardware in an AE86 dash to overcome this problem (and accept twin coils too) is provided HERE

    - Multicoil equipped engine mating to a single coil dash
    The tacho will be out by a factor equal to the number of coils if you only connect the signal from one coil. The signals from all the coils need to be combined and fed to the dash, as shown HERE. That is for 4 coils, for any other number, just feed all the coil signals in from the left.

    - 6cyl engine mating to 4cyl dash (or other combinations)
    The tacho will read 50% too high for a 6cyl engine -> 4cyl dash. I have no direct experience with this problem, however it has been rumoured that most late Toyota tachos have a trim pot behind them that allows these problems to be adjusted out as shown HERE. Or add your own as shown HERE

    Now some engines have funny setups (eg 1UZ has twin 4cyl dizzys), but most problems should be able to be overcome using a combination of the above techniques.

    I should also point out this diatribe is based on my personal experience, and there may well be mistakes or areas in which I am uneducated. If you have anything to correct or more to add, please do so.

    Thanks
    Hen

  2. #2
    Official Off Topic KING! Conversion King stradlater's Avatar
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    Default Re: Tacho tech

    Thoughts on the 1uz tacho setup, wouldn't you just be able to join the two negative coil terminals to get a signal that has one pulse per fire of EITHER of the coils and then put that into your voltage spike tacho (of course if it's accepting a V8 signal).

    Ofcourse, if your 1uz ecu has a "tacho out" and your plugging it into a square wave tacho, you'll have no worries.
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  3. #3
    Current UZA80 owner Chief Engine Builder JustCallMeOrlando's Avatar
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    Default Re: Tacho tech

    Either that or you use a 4 cylinder tacho
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  4. #4
    Toymods V8 Member Too Much Toyota CrUZida's Avatar
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    Default Re: Tacho tech

    Bingo, 1UZ's just use a 4cyl tacho.
    Peewee
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  5. #5
    Junior Member Conversion King timbosaurus's Avatar
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    Default Re: Tacho tech

    Quote Originally Posted by Hen
    There seem to be a fair few threads about this already, so I hope this can bring a bunch of information together so anyone can get an understanding of how their tacho works, and more importantly how to fix it if it doesn't.
    Great idea Hen! Should clear up a few questions for some!

    Something I cant understand about the "add ur own potentiometer" article (although i didn't have a good read) is how putting one across the incoming square wave signal can adjust the reading? A pot does sweet FA to the frequency of a signal... only affecting the voltage and maybe the speed of the edges.

    have i missed something?

    EDIT: no need to requote the whole article

  6. #6
    Chookhouse Chooning Automotive Encyclopaedia Hen's Avatar
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    Default Re: Tacho tech

    Timbo - quoting from the linked page:
    The Toyota tachometer accepts a pulsed ignition signal from the coil. This signal is converted into a proportional DC signal that drives an analog current meter in the dash to display engine rpm. Its reading can be scaled to correctly display the correct engine rpm by adding a calibrated resistive shunt to the meter input
    So the tach doesn't actually count the frequency, instead it converts it to a DC equivalent. So the added pot just scales this DC equivalent.

    That means my inital explanation isn't actually correct, might have to read some more and update it.

    Hen

  7. #7
    Junior Member Conversion King timbosaurus's Avatar
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    Default Re: Tacho tech

    Sounds weird... i thought he said it connects between the two inputs on the tach. would seem odd to have an analog signal into the tach in these digital days

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    Fuel Economy Warrior Carport Converter Vios-GT_07's Avatar
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    Default Re: Tacho tech

    hm..

    okay i'm a noob about this.. but i have a 5 inch APC tacho, and was wondering how do i connect it to a 1NZ-FE (yeh the toyota echo thing), that runs DLI ignition i.e. individual coil packs..? doesn't have a distributor i can see.. bloody modern engines.. grrr.. i can't find a simple enough write-up that explains how to wire up the tacho that previously picks up signal from the one coil.

    that, or are there any aftermarket tachos, preferrably 3.75" to 5" with a shift light that can hook up to a modern engine that uses multiple coils like the 1NZ ?
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  9. #9
    Junior Member Domestic Engineer urantia's Avatar
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    Default Re: Tacho tech

    very good, this info should sort out my cousins aw11 with /beams, the tacho jumps around eratically, so it looks like the beams ecu is putting out a square wave form and the old tacho is expecting voltge spikes from the old 4age dizzy.

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  10. #10
    Excited Member Grease Monkey thomasbl's Avatar
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    Default Re: Tacho tech

    This is a great write up. As more people keep posting up useful information like this, the toymods forums will continually become a better resource. Cheers, im sure lots of people will find this useful.
    Tom

  11. #11
    Toymods V8 Member Too Much Toyota CrUZida's Avatar
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    Default Re: Tacho tech

    Quote Originally Posted by Vios-GT_07
    okay i'm a noob about this.. but i have a 5 inch APC tacho, and was wondering how do i connect it to a 1NZ-FE (yeh the toyota echo thing), that runs DLI ignition i.e. individual coil packs..?
    Does the motor have a DIAGNOSTICS box under the bonnet anywhere?

    If it does, and you can find an IG- wire, thats the tacho driver.

    Failing that, what do the ignitors look like?
    One of those wires (black) will be a tacho driver.
    Peewee
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    2013 86 GTS

  12. #12
    Toymods Pimp Chief Engine Builder Norbie's Avatar
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    Default Re: Tacho tech

    If you can find the pinouts for the 1NZ-FE ECU, there's a very good chance one of them will be a tacho output. Note this will be a 5v square wave signal so this method may require the use of a signal conversion circuit (as described above) if your tacho is designed to be connected to an ignition coil.

  13. #13
    Is a Chief Engine Builder wilbo666's Avatar
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    Default Re: Tacho tech

    I would like to add that when I was playing with the tacho on the 1JZ, it had a +12V square wave output, not +5V... (there is the option that I measured it incorrectly on the cro....but I don't think so...).

    Regardless it doesn't effect the situation that much

    Oh and another thing, the -ve of the coil is ussually at +12V (not gnd), it gets pulled to gnd after the coil....the tacho ussually sees +12V, then Gnd and then the back emf spike that is used to trigger the tacho...

    Cheers
    Wilbo

  14. #14
    Fuel Economy Warrior Carport Converter Vios-GT_07's Avatar
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    Default Re: Tacho tech

    Quote Originally Posted by CrUZida
    Does the motor have a DIAGNOSTICS box under the bonnet anywhere?

    If it does, and you can find an IG- wire, thats the tacho driver.

    Failing that, what do the ignitors look like?
    One of those wires (black) will be a tacho driver.
    hm... didn't get a good look at that... there's more plastic under there than i care to remember.. but yea there is a diagnostic port.. and i vaguely understand the need for a signal converter.. now i have to figure out where to find one of them and how it works i suppose..

    edit.. if it gets too messy i'll just settle for an Apexi RSM item... or one of those Greedy display units which plugs straight into the diagnostic ports..

    also, i've noted a few places where the tacho signal can be taken from, somehow from the individual coils which i have yet to understand but i think i'm close, from the ECU's rpm signal off the loom, and also just basic ignition trigger... another way i've seen it done is to modify the tacho itself to read the 5v signal as opposed to the 12v...
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  15. #15
    Toymods V8 Member Too Much Toyota CrUZida's Avatar
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    Default Re: Tacho tech

    Most new aftermarket tacho's will work without a signal booster.
    ie, they have one built in.
    Peewee
    1985 MZ12 Soarer - 1UZ Powered
    2013 86 GTS

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