Tacho and 123 ignition

Technical advice Q&A

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Kingkitesurf
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#1 Tacho and 123 ignition

Post by Kingkitesurf » Fri Apr 24, 2020 6:14 am

So I remember reading something about tachos going mad with 123 ignition. And it does with me. It gives way to many revs. I also remember reading something about a separate 12v wire you had to run from somewhere to somewhere to correct this... if only I could find the article again...
Does anyone here have the solution? Thanks.

BTW this is a ‘68 S1,5 FHC 4.2.
Last edited by Kingkitesurf on Fri Apr 24, 2020 7:24 am, edited 1 time in total.
Job
1968 E-type 4.2 S1.5 FHC

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JerryL770
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#2 Re: Tacho and 123 ignition

Post by JerryL770 » Fri Apr 24, 2020 6:54 am

You do not say which engine and tacho you have. It makes a difference to the advice.

If you have the later setup where the tacho takes its signal from the dizzy then do you have an RVI to RVC conversion board installed in your tacho?

This is what I have in mine:

https://www.spiyda.com/smiths-rvi-rvc-c ... board.html

and once it is calibrated properly, it works fine.

There is a lot of info on that page so keep scrolling down.
Jerome Lunt
1970 S2 FHC - Dark Blue, Red Interior, MX5 Seats

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mgcjag
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#3 Re: Tacho and 123 ignition

Post by mgcjag » Fri Apr 24, 2020 8:02 am

Hi.....have a close look at the face of your tacho....in tiny print it should say either RVI or RVC......RVI tacho do have problems working with the 123.........Re the extra wire you refer to........the 123 need a 12v connection.....some take it from the ignition side of the coil......but there have been problems known with doing this.....so instead a direct 12v from a fused ignition supply....Steve
Steve
1969 S2 2+2 & Building a C type replica

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johnetype
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#4 Re: Tacho and 123 ignition

Post by johnetype » Fri Apr 24, 2020 9:35 am

If you connect the +12 volt positive wire for the 123 to fuse 7 (this fuse is connected to the ignition switch) and not to the coil your tacho will probably work fine. You need to leave the existing wiring connected to the coil in place.

If the tacho doesn't read properly after you've tried this then you'll have to fit a conversion board as Jerry has.
John

1969 Series 2 FHC

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Kingkitesurf
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Location: Netherlands
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#5 Re: Tacho and 123 ignition

Post by Kingkitesurf » Fri Apr 24, 2020 5:45 pm

What do you mean with ‘leave the wires on the coil in place’. The one that leads to the 123 ignition as well? Run an extra wire then? Thanks.
Job
1968 E-type 4.2 S1.5 FHC

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mgcjag
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#6 Re: Tacho and 123 ignition

Post by mgcjag » Fri Apr 24, 2020 6:31 pm

How many wires does your 123 have.....typicaly for a neg earth systen they have 2 wires....the diagram that comes with the 123 shows both wires connected to the coil...the one to the neg side (Black) is ok......but instead of connecting the other wire (Red) to the pos of the coil as the diagram shows take it to a live controlled by the ignition..fuse 7 is ideal.........do not touch the wire thats already on the pos side of the coil.....Steve
Steve
1969 S2 2+2 & Building a C type replica

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david.coan
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#7 Re: Tacho and 123 ignition

Post by david.coan » Sat Apr 25, 2020 2:26 pm

My 1968 S1.5 OTS had rvi tacho, read about 50% too high after fitting a 123, sent tacho off the Speedycables for conversion, 2 weeks later very happy 😃.
David
1968 S1.5 OTS, ex USA converted to RHD

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