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MG Midget and Sprite Technical - electronic ignition/tacho question

Hi,

Recently installed an electronic ignition unit from Simon BBC. As expected, my RVI tacho no longer works so have obtained an RVC type. Test rigged and works fine so now want to permanently install. Got the wiring diagram, but what size/rating of wire should i use for connecting the tacho to the coil?
Regards,

Andrew
aj robinson

Same wire as before...

should be no upgrades or changes needed to the wiring harness

Prop
Prop and the Blackhole Midget

Hmm. My understanding is that i have to wire the RVC tacho in parallel and need to run an extra wire direct to the coil so it senses the voltage.
aj robinson

Is the RVI type a current sensing tacho? I had this problem many years ago. If it is the current sensing type it will have the White ignition wire looped around a rigid square poking out the back of the tacho. This arrangement forms a transformer made from the White wire and the square loop. When on normal points there is a fair current down the White wire. When you fit electronic it cuts the current by quite a lot and your tacho stops working. I found if you loop the White wire as many times as the slack will allow it increases the turns ratio and as the square of the turns ratio determines the coupling it might just be enough. i.e. 2 turns instead of 1 give 4 times. 3 turns 9 times, 4 turns 16 times. Worth a try.
MG Moneypit

I fitted a BBC ignition unit to my '68 that I had previously converted from +ve to -ve earth. The polarity conversion included reversing the diredction of the white pick-up winding on the back of the RVI tach.
Fitting the BBC had no effect on the operation of the rev counter. It seemed to be as inaccurate afterwards as before!

Mark
M Crossley

Woooohhhaaa,

""Sparks out the exhauste""

....good catch guy, I glossed over that one...this thread just got a whole lot more interesting

My guess is a busted piston ...most likely the piston ring landing, Wow, ive been there, and done that....not fun

In my case I used the haynes manual spec.for the ring gap, which was way WAY differant from the new AE piston specs for ring gap...once the rings fuse, there goes the landings imediatly after

Granted...im just guessing at that, but for spaarks to.be shooting.out the exhauste something had to go wrong inside the cly. And burning oil just backs up my guess

If there is oil mist.shooting thur the vent hole in the oil filler cap like I asked above....then your 3 for 3

I had a 6 foot oil geyser when I broke the front 2 pistons

I feel for you buddy, I really do, hopefully the cly survived okay... if so its just a fast.hone, new piston set and.the rods re-rounded back to true, And a new head gasket

Good luck....ill see if I can find some pics of my adventure

Prop

Prop and the Blackhole Midget

Ermmm, we seem to be way off track here. I only need to know what rating/size of wire to use. And I understand that an RVI is current sensing. Got very little room to manoeuvre behind dash, and I know my replacement RVC works, so please, somebody, refer to my original question!
aj robinson

AJ,

I have had my RVI pos earth tacho refurbed by Speedograph who have changed the insides such that it needs to be connected to the coil, I took the view that it was sensing rather than carrying the power, so just fitted a piece of wire that looked and felt the same as the white ign wire and it works fine.


Malc
Malc Gilliver

Thanks Malc.
I was hoping someone who knows about electrickery would be able to tell me the size/rating
aj robinson

I fitted an RVC to mine too. From my understanding there is very little current passed down this wire as it is used to just 'sense' the spark.

In my case I fitted a length of 28/0.30mm, 2mm2, 17.5amp wire and have not had any problems.
TonyJH

thanks tony
aj robinson

This thread was discussed between 05/09/2013 and 11/09/2013

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