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MG Midget and Sprite Technical - Convert back to positive from negative earth


I have a 1965 AHS Mk3, which I converted to negative to positive earth a few years ago, reasoning at the time, the tacho is broke and have one from a neg earth MGB and I want a radio. Since then had the original tacho fixed and see no point in a radio, the car is fun with it.

Is swapping back to positive just the reverse of swapping to negative ?

M Gilliver

Norm Kerr

....... but aren't there serious advantages retaining negative earth compared to positive?
G Williams (Graeme)

There is no compelling reasons for having a car either positive or negative ground. One can argue that positive ground was original in the year of their car (good enough reason to stay that way). The other argument is that all motor vehicles made today are negative ground (to which my 93 year old mother in law would counter "if everyone jumped off the bridge, would you"). If you want a modern radio or stereo in your car, it is necessary to have a negative ground electrical system in the car or get a polarity converter (which can get quite expensive for one that will support some of the auto stereo systems available today. If you want to plug in a GPS, I pod or charger for you cell phone, just install an auxiliary power receptacle that is isolated from ground - just make sure that the device you plug into said receptacle is completely isolated from any contact with ground (which most of today's portable devices are). You pays the price and takes your choice). Cheers - Dave
Daved DuBois

The change to Negative earthed vehicles was supposedly to reduce the cathodic reaction with the bodyshell as it was thought this increased corrosion in damp conditions. I am not really convinced of how significant that effect is, they all rust anyway, but that was the theory.
Guy W

There may have been some basis to that Guy.

Conventionally, electric current was thought to be from +ve to -ve. It was sometime later it was discovered only that the electrons move, and really, for the way we think about it, we should have called the the -ve terminal the "+ve" terminal, to denote that it was the "hot" terminal

"In metals, which make up the wires and other conductors in most electrical circuits, the positive charges are immobile, and the charge carriers are electrons. Because the electron carries negative charge, the electron motion in a metal conductor is in the direction opposite to that of conventional (or electric) current."

My physics is a bit weak on electrolitic action, but I suspect having a +ve earth car body might well contribute to accelerated corrosion.

It might also have to do with the adoption of active polarity conscious devices, -- semi-conductors -- in cars in the '60s. Semi conductors can be NPN or PNP for example, and that makes a difference to the circuit. NPN as far as I recall, was easier to make more efficient.

Wasn't the move to -ve earth, also coincident with the adoption of alternators over dynamos? These contain solid state rectifiers, i.e transistors and diodes.

Anyway, even if it doesn't much matter on an older car, I don't think the move from +ve earth to -ve earth was entirely arbitrary.

A bit of googling on the history of car electrics should find the definitive answer I reckon.

Lawrence Slater

There you go, read this.

Lawrence Slater

"Wasn't the move to -ve earth, also coincident with the adoption of alternators over dynamos?" - No. My '71 Sprite came with a dynamo not an alternator. And the switch to Neg earth was much earlier - late '67 I think.

I wasn't decrying the physics of the reason for the change to Neg earth. I know about that, which is why I mentioned it! What I was getting at is that in a damp climate they rust anyway, and the many other factors which contribute to rusting probably override and mask the corrosion due solely to the cathodic action.
Guy W

Need to change the polarity of the LT connections on the coil, so the spark jumps the right way (from centre to outer electrode).

Anthony Cutler

There were some galvanic reasons to convert to neg earth. My late '66 has pos earth and to my befuddled old brain it makes more sense to have the live feed coming down the wire and the body shell acting as the sink [earth]. But them I am very old.
David Cox

"My '71 Sprite came with a dynamo not an alternator. And the switch to Neg earth was much earlier - late '67 I think." -------- Yup just looked that up Guy. "Alternators were fitted instead of dynamos from 1973 onwards." So that can't have been the reason for polarity change on Sprigets.

But maybe other makers and other BMC stuff had alternators earlier, so that could still have been part of why it was necessary to change. Maybe BMC were just following industry trends by then.

I only changed my Sprite because I changed from a dynamo to an alternator when the dynamo packed up. I got the alternator for free of a "breaking" mini, left abandoned in the street, as they used to be back in the good old days, and the vultures would swoop and pick them bare lol.

But at that time I don't remember anyone ever having told me, that apart from things like fitting rev counters and newer radios, there was any reason at all to change polarity.

Quite interesting.
Lawrence Slater

The MGB had an alternator from 1968.
Yes, it was NPN semiconductors which drove the change to negative earth. Being a young physics student an petrolhead at the time I took a particular interest.
I'm very dubious about any galvanic action occuring as there is no net postive charge on the chassis (electrons leave and enter the chassis on a one-for-one basis), and what would be the cathode for current to flow to? But I am open to persuasion if someone can explain it.
Paul Walbran

Thanks for all the comments...

re-read my original post and while clearly crap English everybody got 'to negative from positive' although I fumbled that, and 'car is fun without it' again not what I typed !!

I had heard that pos earth was worse for rust and aware that any new bits would be a pain with the pos earth, but it's strangely nice having positive earth on the tacho....

On a related note, why doesn't my SU fuel pump have any spark suppressor fitted - no diode or capacitor ?

Malc Gilliver

Dave Dubois will be along shortly to tell you exactly why. But it may be internal I think.
Lawrence Slater

>>why doesn't my SU fuel pump have any spark suppressor fitted - no diode or capacitor ?

Two things, I guess:

- one is the points are bigger and doubled up compared with ignition points
- there are no-where near as many 'breaks' per second as there would be in an ignition system (100 times a second means 3K rpm in a 4-cylinder)

... they hoped they would last Ok without a cap...

Anthony Cutler

This thread was discussed between 20/01/2013 and 22/01/2013

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