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Tacho problem

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Jessem16

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So I brought a new tacho a while back, but when I first installed it, it was flickering all over the place and would not follow the R.P.Ms properly, so I put a diode in line of the signal wire (My car having a coil) and that seemed to fix the problem only thing is that it won't follow the R.P.M speed fast enough, like when you stomp your foot to the floor the engine will build speed very quickly but the tacho will lag a bit. Would there be a fix for this ?
 
Hi.

If you want to solve this tacho problem, you have to give a way lot more info that that. And the info must be relevant too. Like what model is it, schematic if that exists, how is it connected.

But, saying that you having a coil make me a little concerned that you've may broken the tacho unit.

Is it an ignition coil to a petrol engine you're connecting the tacho onto? Well, then I guess it's broken, because even if you've connected it to the low voltage side, the voltage there may still be several houndred volts as it peaks.
And a diode over it will probably make the motor not starting as you'll kill the voltage spike.
 
Grossel said:
But, saying that you having a coil make me a little concerned that you've may broken the tacho unit.

Is it an ignition coil to a petrol engine you're connecting the tacho onto? Well, then I guess it's broken, because even if you've connected it to the low voltage side, the voltage there may still be several houndred volts as it peaks.
And a diode over it will probably make the motor not starting as you'll kill the voltage spike.
Click to expand...

I've connected a tach before, and the instructions in the box said to connect it to th ignition coil. This page confirms. That's how it's supposed to work I guess, but keep the wires away from stereo wires or you'll get a hum in your speakers. If I remember correctly, there was a selector switch inside the tach to select 8cy, 6cyl, etc. did yours have this? maybe check if it's in the right position. You should not need a diode IMO.
 
I wouldn't have trusted a site that claims (sitate) this:
For any engine, the RPM determines how much horsepower and torque is being produced at any given moment.
Click to expand...
RPM does NOT tell anyting about torque.
 
Grossel said:
I wouldn't have trusted a site that claims (sitate) this:

RPM does NOT tell anyting about torque.
Click to expand...

an error on the page does not negate all of its content. I have never seen a perfect reference. But if you must throw it out, I remain. I have hooked up a tach this way in the past and it works.
 
There's a couple of things to consider... Tachos can be built for electronic and traditional points based ignition systems, and they're different. Generally speaking, a tacho designed for one wont work in the other. There's the old ones, RVI, and newer ones, RVC. No idea what that stands for :).

I suspect you've mismatched the tacho and electronic system. While both types of tachos will generally connect the tacho the negative terminal of the coil, the reason older tachos generally won't work with these is because the pulses from the coil are much quicker than in a points based setup. And depending on what you've done with the diode, you could have fried your coil. Electronic ignition coils aren't supposed to be powered for more a moment. A diode with polarity to the tacho would have allowed it to over-charge! Assuming no damage to anything, there are ways to convert an older tacho to electronic ignition, but attempt at your own risk ( well, at the risk of the tacho being destroyed ).

On the flip side, if you have a new tacho and an older points based ignition system, I'd suggest you get an older tacho or upgrade to electronic ignition.
 
Hi, thanks for your replys.
I have it hooked up as the instructions say, Connecting the green wire (signal) to the ( - ) side of the coil
This is the second tacho of the same Brand & model I have gotten thinking the first was broken but finding out its doing the exact same thing as the fist,
I have the little switch on the back on 4 cylinder same as my engine.
and I put diode in line of the signal wire (closet to the taco its self)
as for schematic, model number and such I can not find anything of that on the internet because it was made for a short time.
 
P.S It is a 1986 Mazda B2000 with just a old style coil and distributor with points
and the coil is in good working condition :) sorry for the lack of info.
 
Well there your problem :)

I really would recommend upgrading to electronic ignition. You'll get extra performance, easier starts, and better economy. While I've no experience with Mazdas, I'm willing to bet there's some form of drop-in replacement electronic system available. At least you shouldn't have destroyed the coil... older point style coils *can* ( not for ever ) cope with constant power.

If you don't want to upgrade the ignition system then essentially you want to turn those slowish pulses from your system to quick ones for the tacho. Perhaps a simple zener diode with an appropriate breakdown voltage will do the trick... Not sure on that one... Cars are my thing, not electronics. I only know enough about electronics to be dangerous :p
 
Thanks Quad four, Yea i wouldn't have the money for an upgrade like that haha
Would you know at all how I would find out the voltage and what to break it down to ?
I'll give that a try.
 
Sorry, I got nothing to suggest here... As mentioned, I don't really have a good grasp on electronics. Might be best to ignore my suggestion of a zener too... The voltage of the peak pulses from your coil will vary greatly depending on RPM etc, and could reach into the KV. I'll be watching your thread though, would like to know how to do this myself ( will need to at some point ).
 
Thanks for the idea anyway, I'll look into it some more and see what I can find.
There must be some way or some sort of circuit to fix this problem
and maybe there might evan be a kit so you can convert.
If I do solve this problem I'll let you know on this tread :)
 
Hey, What type of tach is it? Digital or normal analog with a needle? The signal from the coil neg is very very noisy and can have many high voltage spikes. Normally a good tach will have filter circuitry however if you want to try implement your own a good thread is this one : understanding-signal-conditioningl
 
Jessem16 said:
I can not find anything of that on the internet because it was made for a short time.
Click to expand...

Probably because it's an inferior tachometer and a lot of them had this problem.
I say try another brand.

Jessem16 said:
it was flickering all over the place and would not follow the R.P.Ms properly.
Click to expand...

I just remembered that my digital tach / dwell meter would go crazy if the plug wires or distributor cap were bad. You might try changing them.
 
Last edited:
I have fixed the flickering problem with a diode in line of the signal wire But I still have bit of a lag problem,

I just remembered that my digital tach / dwell meter would go crazy if the plug wires or distributor cap were bad. You might try changing them.
Click to expand...

I did check all the connectors and there all in good condition.
 
This might be worth a try, its an ignition noise filter as fitted to a Mitsubishi 4cyl as a feed for the tacho.
 
Hey debe,
So I get 2.2K res & a .001uF cap and wire it in line.
Where would the best spot to wire in it be ? closets to the coil or tacho ?
Thanks for the diagram. :)
 
Last edited:
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