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Converting stock 71 240Z tacho to electronic good for engine conversions


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#1 OZZ240

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Posted 08 February 2014 - 01:16 AM

I'm nearly at the end of my engine conversion and wanted to use all the standard dash gauges but my stumbling point was the factory tacho as I found out the 71 240Z tacho uses a inductive loop to pulse the moving coil in the Tacho which moves the tacho needle.

I'm using a Haltech PS1000 for my engine management and also using 6 LS1 coils as part of the ignition system, the Haltech only has a tacho output which only drives a electronic tacho so its no good to drive the stock tacho at all.

After a lot of searching I came across a website called "dinoplex" this guy had the answer for my problem it's a DYI Universal Tacho circut board he has a list of components required, circuit diagram and info on how to build your own Universal Tacho board.

I've been able to make his board without ever making something like this before so with my very limited knowledge of electronics which is none and my average soldering skills I got it to work and calibrated.

I brought most of the parts from Jaycar except for the CD14538BE CMOS dual precision monostabile multivibrator and the L78L08ACZ Low Drop fixed voltage regulator 8V. I got these parts from RS Componets, alternate supplier I found for these items was Element14 approx cost for all the components was under $30 easily. I used a experimental board from Jaycar as the circuit board.
I'm not mounting the universal tacho board inside the standard tacho as I could not make it as small as he was able to so it will be located external of the tacho and hide up under the dash somewhere.

Once you have made this cool little tacho board you need to modifiy the Tacho internals.
Remove the gauge from the dash and the black plastic lenze cover.
Next remove the tacho mechanism from the gauge housing, YOU DO NOT need to pull the tacho needle off at all. To get the tacho mechanism out, on the back of the housing are three threaded stud terminals and 2 Philips head screws, remove the wires of the three terminals and remove the 2 Phillip head screws mine was stuck so a little bit of heat from the wife's hair drier on the back of the tacho housing and it fall out very nicely.
This is where the instructions of where the universal tacho board output wires named T1 & T2 gets vague but found out that these are soldered to the moving tacho coil "+" & "-" terminals which are found just behind the gauge face and have a "black" & "red" wire connected to them this is where the T1 & T2 wires go from the unerversal tacho board. Once you have these connected up you are finished with the internals of the tacho and now can be put back together.
I connected the Haltech tacho output to the Universal circuit board and then used the power and earth that the standard tacho use to power & earth the universal circuit board once this was done started the car and found that the tacho did not work so swapped T1 & T2 around started the car again and it worked so was very happy with the end result. The tacho needle is very stable too.

This is some info I copied from the dinoplex site.
This guy does other bits and pieces for Ferrari and other Italian cars some interesting stuff he does

http://www.dinoplex....sion/index.html

The scope of this project was the design of a DIY universal replacement circuit board for electronic Veglia tachometers made in the late sixties to early eighties. The circuit is easy to assemble and based on standard 'wire through' electronic parts which should be easily available via electronic stores and mailorders. Cost of the electronic parts was around €5/$6 at the time of writing this article. Please be aware that this is a DIY project, i don't offer prebuild circuit boards.

This circuit will work with most Veglia tachometers as well as with Smiths RVI/RVC and Smiths derived tachometer designs.

This board can be used to repair your existing tacho and this universal tacho board can be used to
- repair a tacho with broken electronics by exchanging the circuit board
- adapt a tacho to a different input signal type (Coil input, Points Trigger, Electronic Ignition and ECU Signals)
- convert a 4 or 6 cylinder tacho to an 8 cylinder engine or vice versa

Features and Specs
- Compatible with most moving coil tachometers
- Enables a tacho to be calibrated for a 4, 6 or 8 cylinder engine
- Trigger input via the coil 'minus' terminal, points or from an electronic ignition tacho signal
- Supply voltage input stabilized and protected against wrong polarity, voltage spikes and line noise
- Cleans and stabilizes the input signal via a Schmitt-Trigger circuit
- Drift (output variance) less than 2% in a temperature range of 0-80º C
- Minimum supply voltage 10V (typical 12V)
- Minimum input trigger signal voltage 4-5V (typical 12V), trigger signal duty cycle 2%-96% (typical 20%-50%)
- Pulse width for output signal can be set from 1.0-2.0 ms via R1 (Veglia requires 1.4-1.5ms, Smiths 1.8ms)

Hope this helps someone now or down the line.

Cheers
Ian

#2 NZeder

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Posted 08 February 2014 - 06:49 AM

Nice find and cheaper than what I did. I took my tacho to a smiths Guage guy here in Auckland who installed a board inside my stock tacho. The board he used was one developed to do the same mod as you found but is designed for Smith Guages installed in Jags and other 60's 70's cars.

#3 NZeder

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Posted 08 February 2014 - 07:00 AM

These conversions work as the zed tacho is a license copy of the Smiths RVI Guage.  If you don't want to make your own board just google smith rvi to rvc conversion you should find completed boards on ebay.co.uk

Eg
Http://www.ebay.co.uk/itm/380831633009

#4 OZZ240

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Posted 08 February 2014 - 08:47 AM

It took a few hours to build the circuit but found it interesting to build and learnt a bit more about my Z  :D

#5 Six_Shooter

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Posted 08 February 2014 - 09:56 AM

I've been casually working on a similar conversion, using a 555 timer as the monostable section, I haven't been able to get to work as well as I would like, but have had only about half an hour to actually work on breadboarding the prototype. I plan to get back to it soon, hoping to be able to steal some time during "reading week" (week off of college classes), the week after next.

Thanks for the link, interesting design.

#6 WA240Z

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Posted 25 July 2016 - 12:14 PM

Reviving an old thread rather than starting a new one. I have just had my tacho converted to run off the negative side of the coil. While it was working with the electronic ignition upgrade it was out by about 300rpm between 2500-5000 and erratic above 5000rpm. With the conversion to the tacho completed it mirrored the dyno tacho exactly. Gauge Works in Albert Park Adelaide did the job, $160 and turn around was two days. Great service.

 

Oh, and yes, undoing the wing nuts is a pain in the neck.


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#7 PB260Z

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Posted 26 July 2016 - 10:10 PM

Hi

$160 is really good value, mine cost somewhat more - but everything in Sydney is expensive.
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