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MG Midget and Sprite Technical - Rev counter swap

I intend changing polarity of my 68 Sprite to -ve earth. The existing instrument is marked RVI and positive earth. Rather than fiddle with the induction loop, is it possible to just swap to a negative earth tacho, or are the connections different as well?
Ta, Mark
M Crossley

I run a negative earth tacho on my 66 car

wiring is straightforward

live off ignition circuit (green wire usually)

sensing wires (my car has a male and female "in" and "out" to work the needle) on white circuit as directed by Haynes's manual

plenty more advice will be along shortly

(I dont advise here anymore these days but this is plain vanilla stuff for you)

tell us how many wires you have at the back of your gauge and we can advise what comes next

(give colours please)
bill sdgpm

You can also convert your existing tacho to -ve earth, info available here: http://mgaguru.com/mgtech/electric/et202a.htm
I did mine with no problem.
Graham
Graham P 1330 Frogeye

Thanks for the help chaps.
I have aquired a tacho marked "RVI 2430/00A" and "negative earth".
I haven't removed the existing instrument from the car yet. (I presume that you just reach up behind the dash and unscrew the knurled nuts from the mounting studs?)

From my wiring diagram there should be a white ignition feed that goes to and from the tacho and then on to the coil.
Looking at the 'new' meter there is a male and also a female bullet connector. I expect those are the ignition in and out.
There is a male spade connector which I assume is power in, and I assume that the case should have good chassis contact as an earth return?

Does all this make sense?

I was hoping to just replace the existing tacho, thereby avoiding the removal of the bezel and faffing about with the induction loop. The needle was also bent and the scaling wrong on the current one. My intention is to try and fix it at my leisure.
Is there a simple way of calibrating a rev counter? I don't think my sig gen is up to driving the induction loop. I thought of calculating revs for a road speed, but I don't trust the speedo either!

I have long since learned that if I don't quickly finish jobs on my car or bike that I get sidetracked and a weekend job takes six months!


Thanks again,
Mark
M Crossley

there is a lot of info in the archive about tachos and changing earth polarity...
David (davidDOTsmithAT stonesDOTcom)

It's the 2 knurled nuts as you say; one should have a back earth cable attached; these secure sleeves that hold the instrument against the dash; sometimes the rubber mount on the instrument sticks it to the dash, so needs a small amount of force to free.

You can calibrate the 'counter with a battery charger (the signal generator in your garage).

For current-type/coil connection 'counters:

Connect rev-counter -ve to -ve on battery. Then take the white-black lead off the dissy; make sure the points are closed and so conducting. Then +ve from the battery charger to headlight bulb; other side of bulb to black-white cable you've just removed.

The bulb should be bright - then you have current flowing around the 'counter loop. The counter should show 3K rpm (freq of full-wave rectified mains). Adjust to suit.

(Note to curious: the battery charger output is full-wave rectified but not smoothed, so the 'mains' sine-wave has the -ve peaks turned +ve to give a 100 Hz signal. This operates the rev-counter in a similar manner to points opening and closing.)

There's a slightly different method to do the same for the volt-pulse later 'counters.

BTW - leave the ign on a few mins to warm up the 'counter as it may drift a bit.

HTH

A
Anthony Cutler

I converted my +ve earth tacho - there are links to show how to reverse the pick up loop and swap a wire connection inside to convert. I calibrated my tacho by drilling a small hole in the back of the casing to access a "pot", then ran the engine up with another meter attached and tweaked the pot with a small screwdriver to get the best readout across the rev range. I'm not sure but I guess the -ve earth tach will have a similar pot in the back... cheers
N Broadbent

OK Mark

you have the same tacho as mine

the two bullet connectors (one male-one female) take the ignition feed from the ignition switch in and out of the tacho which acts as the counting circuit when it reaches the coil and then through the coil to the contact breaker.

at the points that flow of electrickery is switched on/off and this count is measured as the r.p.m in the instrument

the male bullet on the back of the gauge is supplied by the switch via a rubber insulated white wire (insulation is so any inadvertent disconnection behind the dash wont have a dangerously live wire dangling around there)

the female bullet connection coming off the gauge takes the current off to the + (plus) side of the coil. (This is a ordinary male bullet end and isnt insulated, 'cos if it comes off the back of the gauge or even off the other end at the coil it is a dead wire without connection to the gauge, where the volts live!)

If at any time the gauge packs in you can reinstate the ignition feed without the gauge by connecting the two wires via their rubber protected bullet ends.

The other wire going to the gauge is as you say, the power feed to turn on the gauge's internal electronics


sort of like the picture-ish


bill sdgpm

This thread was discussed between 10/12/2009 and 11/12/2009

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.