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MG Midget and Sprite Technical - Rev counter/electronic ignition - electronic

Bit of advice from electrical gurus here would be most welcome as I am a complete electrical ignoramus! I have a Nodiz pro ignition moudle installed – sort of ECU I suppose - which works fine with the coil pack and the Ford Duratec engine. All installed in a 1971 MG Midget!

However, I was told the RVI tachometer requires a tacho driver to work. Hence I purchased a 2 input tacho driver - from trigger wheels - wiring the orange wires of the tacho driver in to the blue and green wires (the negatives of the coil pack) essentially stripping a small portion of the insulation of each and soldering each of the orange wires in. Other end went to the tacho - the original terminal from the coil. All as per instructions. Engine does not start and nothing on the tacho! Removing the tacho driver – engine starts and runs fine as before.

Checked connections, tacho etc. and I cannot see anything obvious. Any clues about how to fix would be much appreciated! I believe the RVI tachos don't like elctrinic ignition. Is the tacho driver incompatible perhaps?

Any suggestions are most welcome

you're way beyond my level of technical incompetence.
Had to google your request and came up with this

Any help?
Jeremy MkIII

Sorry I can't help but for other readers just to clear up
>>I believe the RVI tachos don't like elctrinic ignition.<<
some might not but others work fine.

There's a Smith's 'The Care of Instruments' booklet on the internet somewhere that might give you some background info.
Nigel Atkins

The RVI is current driven, usually derived from the ignition to coil lead. I presume the Trigger Wheels unit converts a voltage pulse from the ignition/ coil pack to the equivalent current as drawn by a conventional coil. There's not a lot of information on the Trigger Wheels website. I can only think there's a problem with where the orange wires are connected. What are the instructions for connecting?
Bill Bretherton

Yeah the RVI Rach and electronic ignition don't always like each other

I seem to recall that there was a later tach that played well with electronic but it wasn't called RVI

1 Paper

Prop, the later Lucas tach is a RVC which works from voltage pulses so is more compatible with electronic ignition.
Bill Bretherton


There are apparantly two solutions. Buy a later RVC tacho and install the tacho driver - or gut the existing RVI tacho, replace with new electronics, and wire into the Nodiz electronic box.

Both ain't cheap! 150 notes plus....I'll see how it goes.

Oggers, Richard Woolley might be worth contacting for an RVC tacho. I've dealt with him and he was very good value.
Bill Bretherton


I have the same Nodiz Pro unit, and had similar problems with the Smiths tacho.

Mine is an early tacho that has been retro-fitted with later innards by Speedy Cables many years ago, but do not know exactly what they are!

Try contacting Nodiz, they have some circuits that get the Smiths tachos to work, but not always 100% - ours sometimes sticks around 4,000 rpm on the way up, and then jumps to the correct rpm.

The solution for ours was to use the innards of a standard 12v automotive relay, to create a 'kick' to the rev counter, using 'back emf' - you don't need to understand the theory, just go by the Nodiz instructions!

The Nodiz forum may also be helpful.

Richard Wale


Thank you, but I did contact Nodiz and they put me on to a company called Spiyda who offered the gutting and retro-fitting. Very helpful chap actually. I'll think about it!

Incidentally started the beast up today after 3 months or so restoring the interior. First time, very smooth...engine bay to tidy up, carbs to changeout, a few other jobs and then unleash the power!

I have a spiyda converted tacho in my Elan and can confirm that it works ok with a powerspark electronic ignition module. Not the same scenario that you have though.
Simon Wood

I think there are 2 types of RVI as well. One with a coupling loop outside and one with it inside.

It works by looping the ignition wire (the one carrying the current pulses) through another loop which feeds the electronics. The loop within a loop forms a simple transformer. Fitting electronic ignition cuts down the current in the ignition wire so the tacho doesn't get enough signal to work. One way around this is to increase the number of turns the ignition wire makes in the tacho's loop.

I did this many, many years ago when I fitted electronic ignition to an MG 1300 saloon.

MG Moneypit

This is an image of an RVI with external loop. The white wire loops around as shown. It it's this type try looping it around a few times more. It might work.

The image is from the MGB Stuff website.


MG Moneypit

This is the inside loop RVI. You need to dismantle this type to try the same trick.

I might add that when increasing the number of loops be careful not to cut into the insulation on the ignition wire.

Image from scimitarweb site.

MG Moneypit


Oggers has a coil pack so I'm guessing the Trigger Wheels tacho driver takes a voltage pulse from the ignition module and converts it to a larger current to drive the RVI tach, this current being derived directly from a switched 12v source you'd think. Probably simpler to use a RVC tach.
Bill Bretherton


Oggers has a coil pack so I'm guessing the Trigger Wheels tacho driver takes a voltage pulse from the ignition module and converts it to a larger current to drive the RVI tach,this current being derived directly from a switched 12v source you'd think. Probably simpler to use a RVC tach.
Bill Bretherton


Your posts seem to be appearing twice.

It did the same on the 'what's your daily' post last night.
Dave O'Neill 2

It might just be the angle of the photo but the RVI with the external loop looks a bit tight.

From the Smiths Care of the instruments booklet -

“The pulse lead should form a symmetrical loop and should not be tight enough to pull the plastic former out of line, as this may well cause a poor electrical connection at the iron core. Poor connection may well result in intermittent operation of the tachometer.”

Nigel Atkins

Dave, it's my fault I think. I edited, in both cases, but still posted the original as well.
Bill Bretherton

Yes, the mod I did was for a transistorised ignition with normal points. In my case the current dropped because points interrupted the current through the base-emitter connection on the switched power transistor. I expect nowadays there would be some optical or magnetic interrupter with a tacho drive signal from the ignition module or something like that.
The RVC solution sounds best as it works on voltage rather than current.

MG Moneypit

This thread was discussed between 01/02/2018 and 04/02/2018

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