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Changing to electronic ignition - tacho mod

PostPosted: Sun Jun 09, 2019 5:45 pm
by Bits
img_2807.jpg and
Tacho is negative earth.
Hi all,

I am changing my points to electronic ignition (installed in the existing dizzy) and I believe the tacho needs a mod to work properly?
Where do I go for the part to fit myself or do I need to send it off to get it installed please? 8)


Regards,

Mark.

Re: Changing to electronic igniytion - tacho mod

PostPosted: Mon Jun 10, 2019 1:44 am
by Chrispy
I understand that there is a few options:

Put a dedicated power wire to your tacho rather than the one which also feeds the coil. Apparently it sometimes works and saves extra work

Change over to a RVC style Smiths tacho.

Install the Spyider (spelling?) conversion board into the original tacho which makes it similar to a RVC style tacho.


I like points :)

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 6:55 am
by Bits
Thanks Chrispy. And for noticing the spelling - now rectified! :D

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 7:18 am
by Chrispy
Not your spelling - I meant my spelling of Spiyda :lol:

https://www.spiyda.com/tachometer-electronics.html

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 2:03 pm
by pharriso
I highly recommend the Spiyda conversion.

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 2:55 pm
by Bits
Thanks Phil,

I will let the guy who is doing the electrics know.

Regards,

Mark.

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 5:30 pm
by m750rider
If you use a dedicated power supply instead of the power supply that runs through the back of the tachometer as you do with the points distributor, you will not need to replace or convert the tach. It will work fine as is.

Bob

1969 S4 SE

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 6:14 pm
by geni
bonjour Bob
pourriez vous ĂȘtre +explicatif pour mieux comprendre
cela m'intéresserait beaucoup
geni

hello Bob
could you be + explanatory to better understand
that would interest me a lot
geni

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 6:21 pm
by Bits
Thanks Bob,

I'm not electrically minded. Could you sketch it out please?

Many thanks,

Mark.

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 8:45 pm
by billwill
m750rider wrote:If you use a dedicated power supply instead of the power supply that runs through the back of the tachometer as you do with the points distributor, you will not need to replace or convert the tach. It will work fine as is.

Bob

1969 S4 SE


I'm not at all sure about that, I think the minimum that you need to do is swap over the positive and negative wires inside the tacho.

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 9:00 pm
by mbell
The standard RVC setup works measuring current pulses to the coil trigger by it operation, therefore the power line to the coil runs through the tacho. If you add Electronic igition and power it from the coil, the current pulses generated by its working also pass through the tacho and can cause problem as the the tacho sees more pulse and therefore can miss read.

Moving the power for the electronic module from a separate source means the current pulses for the electronic ignition aren't passing through the tacho and therefore shoudln't be measured by the tacho. So this is often enough to get a RVC tacho working with electronic ignition.

This said some cases the Tacho still miss reads, possibly due to aged tacho or change in current pulses from the electronic ignition. Swapping to RVI internals is a solution for this but requires changing the wiring in the car by adding an additional sense wire adding between coil and tacho.Then bridge the the coil 12v supply outside of the tacho.

Re: Changing to electronic ignition - tacho mod

PostPosted: Mon Jun 10, 2019 9:23 pm
by billwill
billwill wrote:
m750rider wrote:If you use a dedicated power supply instead of the power supply that runs through the back of the tachometer as you do with the points distributor, you will not need to replace or convert the tach. It will work fine as is.

Bob

1969 S4 SE


I'm not at all sure about that, I think the minimum that you need to do is swap over the positive and negative wires inside the tacho.


Oh, this was related to a different topic, one on converting the whole car to negative Earth, in which case the positive and negative need to be swapped inside the tacho case. However if the car is already negative earth as this topic title implies, then the swap will have been done already.

=======

THis topic has been covered in great length before, see: viewtopic.php?f=38&t=29435&hilit=tacho+circuit
Unfortunately the external diagram of the tacho circuit linked from my messages in that topic all seem to be dead now.

I have found another such diagram.

Image
which comes from a useful pages about these Smiths tachometers
https://www.sw-em.com/Smith%27s%20Tachometer.htm

Re: Changing to electronic ignition - tacho mod

PostPosted: Tue Jun 11, 2019 2:03 am
by prezoom
Last time I checked, Moss Motors had the conversion of the tack from positive to negative in their technical section. Just a matter of moving a couple of wires.

Re: Changing to electronic ignition - tacho mod

PostPosted: Tue Jun 11, 2019 9:17 am
by Bits
The polarity of the tacho is marked on the face of it, therefore this is the polarity of the car. Also check at the battery!