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MG Midget and Sprite Technical - Swapping out a tacho?

Hi all,
Ok, my car midget Mk III 1972 and I have the Powerspark points replacement unit in the dizzy.
The tacho that came with the car is a Smiths RVI 2430/01 with NO external coil on the back of the instrument case.
It WAS working, but has given up the ghost again, so I was wondering as I have access to a Smiths RVC1414/00F (From a late model Rubber Bumper MGBGT)
Can I just simply swap them over?
I have checked the web and found the following from Spiyda
From the Spiyda Website:
“If you are fitting an RVC tachometer where there was previously an RVI, ignore the old signal wire and run a new wire from the coil / ecu / electronic ignition as appropriate
(if the old signal wire (usually white) connected to the tacho with a male and female bullet connector, connect them to each other.”

Has anyone done this?
Will it work with my proposed RVC 1414/00F tacho?
I've attached a photo of the new tacho id that helps or if anyone can suggest what goes where!
Cheers all ( and Seasons Greetings too_
Col



colin frowen

I had thought about doing this recently.

The original two white wires at the tacho are supplying a pulse to the tacho from the ignition feed to the positive side of the coil. One white wire comes from the ignition switch, loops through the tacho - either internally, or externally - and the other white wire goes to the coil +ve terminal.

When fitting the RVC tacho, you can join these two white wires together, so the ignition coil still gets a feed from the ignition switch. You then need to add a separate pulse wire from the switched (-ve) side of the coil to the pulse connection on the RVC tacho.

The other alternative is to add a new white wire from the ignition switched circuit to the +ve terminal of the ignition coil, then move the existing white wire from the +ve terminal to the -ve terminal of the coil and connect the other end of it to the RVC tacho. The original white wire from ignition switch to tacho would be redundant.

I hope that makes sense.
Dave O'Neill 2

Hi Dave,
Many thanks.
Sadly, I'm a bit thick- so do you know which wire goes to what on the image I posted?
Could I check out the RVC tacho by taking a 12 V + feed and a wire from the coil neg?
Told you I was thick...
Cheers col
colin frowen

'A' is the pulse wire form coil -ve.

'B' is 12v ignition switched feed (green).

'C' is ground/earth.

"Could I check out the RVC tacho by taking a 12 V + feed and a wire from the coil neg?"

Yes, along with an earth wire. You could just hook it up inside the engine bay and see if it works.
Dave O'Neill 2

Hi Dave,
Thanks very much indeed.
Got a few quiet days soon , so will have a crack at it. I'll post a report here .
Thanks again
cheers col
colin frowen

This thread was discussed on 20/12/2019

MG Midget and Sprite Technical index

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