How to feed an original tachometer?

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Peter C in Canberra
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How to feed an original tachometer?

Post by Peter C in Canberra » Fri, 07 Nov 2008, 03:07

Hello All,
I would like to send an appropriate signal to my Daihatsu Charade's original tachometer if it can easily be done. One diagram I have shows a single connection called 'tachometer pulse' while another diagram shows the connection as 'from ign. coil'. I assume I could generate an appropriate pulse four time per rev. (was 4 cylinder, EFI) with something attached to the back end shaft of my motor. While I can find plenty of detailed info on most aspects of conversion this is one that doesn't seem well documented as far as I can find. Is there a good primer on this somewhere or would someone like to tell me all about how to do it and where to get the bits?
Thanks,
Peter.
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How to feed an original tachometer?

Post by woody » Fri, 07 Nov 2008, 04:08

As far as I know, this is how tachometers have been fed since Moses rode forth in a Triumph. It comes from the low tension lead on the coil in a petrol motor (the small one that goes to the distributor, not the 5th spark plug style lead). I'm only guessing that the signal varies between 0-12 volts, mostly ~12V when the coil is charging and 0V when the spark plug is firing.

The tricky bit I just remembered is that ICEs are 4 stroke, each spark plug only goes off every 2nd revolution so 4 pulses per 2 revolutions, I.E. 2 per revolution.

As for a rotary encoder to do that, you could paint your end shaft in 90 degree sectors black + white and use an optical pickup. a4x4kiwi had teeth put on his tailshaft mount so he could use a magnetic? encoder.

Danfoss AC controllers have a pulse output which can be used for tachometer or electric speedo use.

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How to feed an original tachometer?

Post by Electrocycle » Fri, 07 Nov 2008, 04:33

most EFI engines use a 5v or 12v logic pulse to the tacho, which is nice and easy to replicate, but if it senses from the low side of the coil it'll usually get about a 300v spike as the coil fires.
You can either modify the tacho itself to work with a low voltage pulse, or make a small transformer pulse generator using a small 240v to 9v transformer.
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How to feed an original tachometer?

Post by Richo » Fri, 07 Nov 2008, 04:41

I suspect that there will be input filtering on the tacho.
So it should still work 0-12V even though a 300V spike may come in.

As for the sensor...
A reed switch with a dual pole maget on the shaft. (N-S-N-S)
Or you would need an encoder then a divide circuit.
So the short answer is NO but the long answer is YES.
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How to feed an original tachometer?

Post by Peter C in Canberra » Fri, 07 Nov 2008, 04:51

Would a reed switch be fast enough to switch, say 2x 5000/min? Wouldn't it fatigue and break pretty quickly if it could keep up?

Do I understand correctly then that a suitable feed would be from 12V via a resistor with some sort of switch to ground that closes twice per revolution?

I'm not sure what the dual pole magnet is for. Could you not have a reed switch normally open and then two magnets on opposite sides of a disk on the shaft? The magnet passing twice per rev would close the switch twice to give two ground pulses, wouldn't it? A pair like that would be balanced also.

thanks,
Peter.

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How to feed an original tachometer?

Post by Electrocycle » Fri, 07 Nov 2008, 06:09

a 12v pulse may work, but some of them definitely need the higher voltage spike to work in standard form.

Some speedo sensors use reed switches (BMW in diff ones) so can probably do a tacho pickup, but for the same effort you could use a hall effect sensor or make an optical pickup for not too much more.
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How to feed an original tachometer?

Post by tdean » Fri, 07 Nov 2008, 19:57

You can buy a complete inductive tachometer system from VDO for around $190 or contact a local auto instrument specialist and get him to modify and calibrate your existing tacho to work with an inductive pickup which you could run off the shaft.

I just did a quick search and rang this guy from Victoria who is sending me info on tachos.

http://www.automarineinstruments.com.au/who.html

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How to feed an original tachometer?

Post by juk » Fri, 07 Nov 2008, 21:10

Just use a speedo with a different scale. Or even just put a smaller scale inside the speedo perimeter scale. Since the two are a fixed ratio in most EVs you'll be cooking with gas.

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How to feed an original tachometer?

Post by Richo » Fri, 07 Nov 2008, 21:13

Yeah poor little reed switch.
Hall switch sounds better.
Your magnet arrangement sounds fine.
I was more talking a disk/ring that has 2N/2S so has the same effect.
But is a nice shape to glue to the end of a shaft.

So many ways so many choices...
So the short answer is NO but the long answer is YES.
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How to feed an original tachometer?

Post by Peter C in Canberra » Fri, 07 Nov 2008, 22:22

Thanks for the help. With some further reading I found mention of a hall effect device at Jaycar (ZD-1900) for $19.95. It has a slot so that a vane can pass between two halves. If soft iron passes through the slot it changes the magnetic path in the device which turns a transistor on or off. It has three leads. One for +12V, one for ground and one output. With a "pull up" resistor (1K-10Kohm) between the +12V and the output it will swing between up towards 12V or down towards ground. I think I could have two bits of iron (some transformer lamination?) sticking up from opposite sides of the pulley I will have on the tail shaft of my motor. I think that should do the trick to feed my original tacho. I will do some experiments and report back!

I am not concerned to have high precision from the tacho. Partly I want a visible warning to other drivers of the car that they might be getting close to a new red line I'll mark on the tacho. Partly I just think the car will look nicer if both the original speedo and tacho are doing something. IE, dead dials look a bit ugly. If I were the only driver I would probably not worry.

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How to feed an original tachometer?

Post by Richo » Sat, 08 Nov 2008, 00:46

I think the speedo is a legal requirement anyway.
BMW is nice since the sensor is in the diff so is not affected.

So you need a steel disk with 2 holes punched in it.
Slip that onto the shaft.
Then make a bracket up to hole the sensor so the disk is in the slot of the sensor.
You need a 470R to 1k on the output pull-up to 12V(KEY ON).
This way it will give 12V out normally then when there is a hole it will drop to 0V.
Punch more holes to inrease RPM to suit the Tacho.
So the short answer is NO but the long answer is YES.
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How to feed an original tachometer?

Post by Peter C in Canberra » Mon, 10 Nov 2008, 15:32

Here is a picture of the device from Jaycar (actually a picture of one like it stolen from elsewhere).Image
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How to feed an original tachometer?

Post by Electrocycle » Mon, 10 Nov 2008, 16:08

yep, that'll do it.

You could also go for an optical pickup and save a few dollars on the sensor.
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How to feed an original tachometer?

Post by Richo » Tue, 11 Nov 2008, 00:47

A hall should be more reliable over it's life as would be less prone to brush dust clogging optical sensor/detector.

10k pullup might be fine for digital but in automotive into tacho might be prone to problems and noise.
1k would be more realistic.
470R is the output current limit with about 14V so don't go lower than this.

Edit: typo
Last edited by Richo on Mon, 10 Nov 2008, 13:49, edited 1 time in total.
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How to feed an original tachometer?

Post by a4x4kiwi » Thu, 08 Jan 2009, 00:30

For the Hilux, I needed to feed it a 24v signal as per http://www.kolumbus.fi/juha.niinikoski/Tacho/Tacho.htm (Thanks for the link Johny)
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How to feed an original tachometer?

Post by Peter C in Canberra » Mon, 30 Mar 2009, 21:27

Hi,
This is just to report back that I now have the wheels off the ground so I can test the tacho set up as above. I am disappointed to report that I got no response from the tacho dial. I suppose I either wired something wrong, totally misunderstood the requirements of my tacho, or it needs more voltage than 12V dipping to 0V twice every rev. From the link above I see the idea of rectifying the output of a mains-fed variac to see if it needs a bigger voltage pulse. I may try that later after the car is registered, a higher priority for now than having a functioning tacho. Hopefully I will report back again in a few months with a solution.
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How to feed an original tachometer?

Post by Johny » Mon, 30 Mar 2009, 21:32

Hi Peter. I know you don't want to pursue this at the moment but did you check that you got pulses from the hall efect device? (You didn't mention it.)

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How to feed an original tachometer?

Post by Peter C in Canberra » Mon, 30 Mar 2009, 22:35

I have not tested the output in the car but I did test it manually with a 12V battery to power the device and just turning the disc by hand so that I had either a gap or metal in the slot of the device. In those circumstances I got either 12V or 0V at the output.
You are right though. On one hand I should test that I am getting a signal out of the device in the car with the motor turning. On the other hand I should test what signal is sufficient to get the tacho to respond.
Peter C.
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How to feed an original tachometer?

Post by Johny » Mon, 30 Mar 2009, 22:57

Sorry Peter, I've checked your posts but I can't remember the make and model of the car you are converting - and roughly year. I'll try to find something out...

Edit: Forget it, I just looked 2 lines up at your signature....
Last edited by Johny on Mon, 30 Mar 2009, 11:57, edited 1 time in total.

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How to feed an original tachometer?

Post by Peter C in Canberra » Tue, 31 Mar 2009, 00:24

Thanks. It may be relevant that it was the 1.3L four cylinder EFI version originally rather than the 1L, 3 cylinder carburetor version. I have a circuit diagram for the car. The ignition coil is shown with 4 connections. One was to the middle of the distributor, one was to the chassis. One was to 12V-On supply and also to the distributor. The final one was to the ECU, tacho and A/C Amp and also the distributor. I put a fuse in the 12V-on line and used it to supply the vacuum pump, the tacho sensor and main contactor via various safety interlocks. The output of the sensor I returned to that 4th wire that I deduced must be headed to the tacho and the final connection from the sensor was to the chassis. I'd be happy to hear of anything you can deduce from all that!
Thanks,
Peter.

Edit: PS The diagram for the gauge shows the tacho part as having three connections; one to the common 12V supply to the gauge, one to the ignition coil and a connection I don't understand. That last one is shared with turn and hazard light indicators and goes to a reed switch whose other end goes to the ECU. The turn and hazard lights and their indicators all work normally.
Last edited by Peter C in Canberra on Mon, 30 Mar 2009, 13:35, edited 1 time in total.
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How to feed an original tachometer?

Post by evric » Tue, 31 Mar 2009, 01:16

Peter,

This is the circuit I am using with my Barina. It may work in yours too.

Image

Ric.

[Edit] Updated circuit diagram 16/4/09
Last edited by evric on Thu, 16 Apr 2009, 09:48, edited 1 time in total.
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How to feed an original tachometer?

Post by coulomb » Tue, 31 Mar 2009, 02:56

evric wrote: This is the circuit I am using with my Barina.

Wow, that should get the tacho's attention!

What size pulse do you get out of that? 100v peak? Any ringing from the effectively tuned circuit of the inductor and capacitor?

I guess the tacho must be used to a fair old spike from the ignition primary.
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How to feed an original tachometer?

Post by a4x4kiwi » Tue, 31 Mar 2009, 03:04

Here is a similar approach in the link from my blog http://a4x4kiwi.blogspot.com/2009/01/ta ... s-now.html but somewhat more complex
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How to feed an original tachometer?

Post by evric » Tue, 31 Mar 2009, 05:41

coulomb wrote:
evric wrote: This is the circuit I am using with my Barina.

Wow, that should get the tacho's attention!

What size pulse do you get out of that? 100v peak? Any ringing from the effectively tuned circuit of the inductor and capacitor?

I guess the tacho must be used to a fair old spike from the ignition primary.


I haven't measured the pulse yet and yes there is ringing - similar to the original car circuit.
I am working on an associated RPM cutout at about 6,000 RPM which hooks into the above cct at the junction of the 470ohm and 100ohm resistors.
I feel that this is a necessity to prevent motor self destruction when someone floors it in neutral.
Once I've finished the whole cct I'll publish it on my website.
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How to feed an original tachometer?

Post by Peter C in Canberra » Thu, 16 Apr 2009, 18:07

Ric,
Do you have any updates on your circuit?   I might try it while the paint dries on my battery box. Do you have it actually making your Barina's tacho move or is it still theoretical?
Thanks,
Peter C.
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