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MG Midget and Sprite Technical - Midget 1973 tacho

Hey

After a bit of help from you guys here if poss

Got a 73 1275 midget

I've fitted a accuspark /powerspark electronic ignition dizzy to my car
But the tacho doesn't work

I've not used the car with points system fitted so not sure if it's a tacho fault or the tacho isn't compatsble with this electronic ignition system

Anyone know how I can check this - is there a specific tacho that will work with electronic ignition from accuspark ?

Thanks
richard weaver

Richard,

You may need to investigate getting one of these (or similar):-

https://www.spiyda.com/magento/index.php/vehicle-electronics/tachometer-electronics.html

I have recently fitted one of these to my Elan +2 after getting the same issues as you

Simon
SA Wood

I'm guessing (maybe) that your tacho is an RVI tacho (should state the model number next to the 'Made in UK' on the bottom of the dial).

These are not compatible with electronic ignition.

You need an RVC tacho instead. There is an extra wire then to connect from the coil to your tacho.
TonyJH

Your right I have the rvi 2340/01a tacho -

Is there a full number for the correct one ?
richard weaver

Or anyone got a spare rvc one ?
richard weaver

The RVI tacho works by sensing the current through the wire that goes to the contact breaker. It senses this current by means of Inductance (the I in RVI). On the back is a male and a femail connector. This is designed to ensure it can only be connected up one way so the current goes the right way through the sensing circuit.
I'm remembering this from many years ago when electronic ignitions were a new idea. I had a Riley 1300 at the time and fitted the new fangled electronic ignition to the car and the Tacho promptly stopped working.

The reason it stopped working is the electronic ignition reduced the current flow through the sensing wire so there was not enough to trigger the tacho.

I took the tacho out, took the back off to have a look see, to try to figure out how to make it work. Inside I found the white wire did one turn around a flat piece of metal that was soldered at both ends to the circuit board. This arrangement forms a transformer where the white wire is the primary and the flat strip was the secondary. The current through the wire induced a current in the flat strip which provided the signal for the tacho to register. I was being trained as a radio engineer at the time so I reasoned that if the current was reduced in the primary, in order to restore the current in the flat strip you needed more turns. So I undid one end of the white wire and wound a couple more turns around the flat strip and re-assembled everything. The tacho burst into life, problem solved!!!

Now I don't know (or more correctly can't remember) what type of electronic ignition it was, and I have no idea what type they use now, but it has to be worth a try.

Rob
Rob aka MG Moneypit

Found this on t'internet.

http://www.mgexp.com/phile/1/159920/RVIblock.JPG

and this

http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/tach3.jpg

so there seem to be two types but they work on the same principal.

And I'm obviously not remembering the flat strip correctly!!! Grey matter failure!!!

Rob
Rob aka MG Moneypit

As Rob says the RVI is current sensing, and the RVC is voltage sensing. As far as I know any of the RVC tacho's should work.

I sourced mine through Andy Jennings:

http://www.mg-cars.org.uk/andyjennings/mg-midget-interior.htm

when I switched from points to a pertronix Ignitor. Prior to the change to an RVC unit I saw exactly the same as you.

Here's a picture of my tacho so at least you have one reference number.

As I said earlier, you will also need to run a wire from the -ve terminal on your coil back to the tacho. You can see this wire on the wiring diagram for GAN5-128263 onwards, coloured White/Black, from the coil to the tacho.


TonyJH

Richard,

Did you convert to -ve earth to fit the Accuspark unit? If the rev counter is for +ve earth, it will not work on -ve. Can it be converted anyone know?

Richard
Richard Wale

I'm gonna have to be dumb here - I'm not the best at electrics and no idea what -ve or +ve is - the loom and everything is new so followed the wifi g diagram to fit and followed the accuspark instructions the fit the electronic ignitionso forgive me
richard weaver

Since your car is a '73, it will be negative earth. The negative terminal on your coil should be marked, and it will not be the terminal to which you currently have the white wire connected to!


TonyJH

This thread was discussed between 18/06/2015 and 24/06/2015

MG Midget and Sprite Technical index

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