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MG Midget and Sprite Technical - Polarity

What are the things to think about with car polarity? My 1960 Frogeye should be positive earth. I've put off deciding whether to switch (note the pun) to negative earth, though I understand that many do. So what are the considerations?
I don't plan having a radio, and I've not (yet) bought any led bulbs.
GuyW

Why change? If you don't want to fit a radio there is little point.

If you do then the dynamo needs to be "flashed" to change the polarity and the fuel pump needs to be swapped for a neg earth one or the internal wires swapped over.

Unless you have an electric tacho, that's it.
Chris at Octarine Services

Why change? Quite!
Heater fan wires also need swapping over.
Anything needed doing with the voltage regulator, Chris?
GuyW

Guy

Mine is still positive as apart from the fuel pump (facet) there are no other considerations.
Bob Beaumont

Coil LT leads need swapping.

Spridget (SU) fuel pumps aren’t polarity sensitive, unlike MGBs. That’s assuming you’re not using a mechanical one.
Dave O'Neill 2

Guy, I think you're using a mechanical tach? Then, unless you intend fitting a -ve earth electronic ignition or cigarette lighter charging socket in the future, may as well stick with +ve earth. I think it should be clearly marked in case someone tries jump starting it without thinking (in the unlikely event you might be indisposed at the time).
Bill Bretherton

Yes, mechanical tach.
Fuel pump is a points type su, mounted at the back like later cars, so not original to a frog. Not sure if it's polarity sensitive though, but will no doubt find out! It's an AUF214. From my SU guide book the later versions of this pump had a diode which obviously affects the polarity, but this doesn't apply to all of the production of this pump.
GuyW

The regulator should work on either polarity (because it's basically a couple of solenoids which are not polarity dependent) as will the wiper and heater motors. It is only small motors with a permanent magnet field that reverse direction (not used on a Sprite). Yes, if the pump has a diode that matters. Electronic accessories are polarity dependent.

There is a theory that +ve earth cars corrode more quickly but I think it is unproven.
Bill Bretherton

Depending on which heater fan you have you may not need to swap wiring, mine worked better wired opposite (unless the wiring to it was already opposite).

LED bulbs can be dual.

Do you want to add any other modern stuff electronic stuff, TwatNav, (I don't know any others) - fridge, ect.
Nigel Atkins

Nigel, the fan motor will run, and blow air, either way. But is more efficient when rotating the correct way.
No, I've no wish for any new fangled electrical additions on this car.

I am currently (another pun!) leaning towards retaining it as positive earth.
GuyW

Blimey my brain is already tired again, I was looking for the pun with the word leaning!

Yeah I know the fan is Timmy-two-ways I watch JT vids and discovered for myself but I meant my fan is more efficient when wired opposite to it's wiring code (unless the loom wires are wrong where I can't see them, they're correct at the switch).

What no new gadgets but yet you talk of, what I'm told are totally illegal, LED bulbs.

Oh, of course, that'd be the other modern thing, phone/tablet/laptop charger.

Electronic speedo in case you stray too far south?

I think you can get positive earth 7" record players, though I don't know if you'd get Caruso to remind you of your youth, perhaps there were 7" EPs of him.
Nigel Atkins

I changed mine to negative. I find electrics baffling enough without complicating things and as was starting from scratch with a new loom I just thought It would be easier.
John Payne

I'm surprised the heater motor runs both ways as it has a wound field (well the older one does at least) which, in theory, will only turn one way, regardless of polarity.
Bill Bretherton

It was a modern replacement uprated blower motor so possibly originally for a different or multiple applications.

It didn't seem to push that much more air even wired in the wrong right way, it doesn't blow your trilby off or pipe out when set to windscreen but it can be noisy until the exhaust gets upset, I tell passengers it's the engine turbo.

Of course it's not needed anyway over about 25mph so isn't used that much.
Nigel Atkins

Just checked a stripped motor and I was wrong - looks to have a permanent magnet field which accounts for it being a Timmy-two-ways Nigel. I suppose it's only a little thing so they kept it simple. Model train dc motors are Timmy-two-ways for obvious reasons as will be many of the small dc motors on moderns e.g. mirror motors.
Bill Bretherton

I thought that was the case but with my memory I'm never sure.

Perhaps my motor is uprated for an MGB which is why its ar*e-about-face in the midget but I don't remember there being a massive difference running either way and I don't remember it coming with a filler plate as I had on the B heaterbox.

See here - https://www.youtube.com/watch?v=2gCN3hbtVgQ
Nigel Atkins

Guy

John E Davies does a really nice job in refurbing dynamos and voltage regulators if you want to keep the car positive earth and making sure that the system is in good order.

If you want to go negative earth and an alternator while keeping your mechanical tacho you can get an alternator that has the rear rev counter drive (and still looks like a dynamo - 'Dynamator' - http://www.bbclassics.co.uk/dynamator.html) If you are happy with an electric tacho then use a later gauge and a standard alternator from a later car.

Need an alternator bracket: http://www.minispares.com/product/Classic/Electrics/Alternators/12G1053.aspx?0902&ReturnUrl=/shop/classic/Electrics/Alternators.aspxBack%20to%20shop Plus a few other bits and bobs (such as alternator plug, adjusting bracket IIRC etc)

Alternators: http://www.minispares.com/shop/classic/Electrics/Alternators.aspx?0902

Cheers
Mike
M Wood

Hi Mike,
No I am not going to use an alternator on this car.
I already have a dynmo and regulator from John Davies. As you say, he does a really good job!
GuyW

Guy
I have to ask
If you're not going alternator and not having any modern polarity dependant electrical items fitted, I'm wondering what's the reason for thinking to change to neg. earth------------just curious
William Revit

Will, I was just asking should I?
I understood a lot of cars have been changed to Neg earth over the years and wondered why, and if I should too. There doesn't seem to be any strong argument in favour of changing so I have now made my decision. It will stay positive earth.🙂
GuyW

Some useful info here:
http://www.mg-cars.net/mgb-technical-bbs/positive-earth-to-negitive-earth-2007082317475326976.htm
&
http://www.mg-cars.net/mg-midget-sprite-technical-bbs/convert-back-to-positive-from-negative-earth-201301201507274182.htm

Cheers
Mike
M Wood

Thanks Mike. Useful links though not needed now as I have decided not to change, but interesting to read the discussion.

I even see that I took part in the second one about converting back! Odd though, as I don't recall ever coming across this MG-cars.net site and that was a BBS discussion. Looks like it's been lifted from the BBS, or cloned somehow.
GuyW

Guy, you appear quite a lot on mg-cars.net, even I'm in some threads! I've found that when searching for Sprite info using Google that a thread from here comes up on mg-cars.net a lot as if it's a sub-set of this site.
Bill Bretherton

Does anyone know if LED bulbs are available for positive earth systems? For those I have looked at, it either isn't mentioned, or they are listed negative earth only.

Nigel did mention they are available as dual polarity, but not where from. I am not sure how that works with a diode. Any more info on this?
GuyW

Guy,

http://www.dynamoregulatorconversions.com/online-shop-for-led-bulbs-and-light-boards-etc.php

and it says:
Our LED products are available in 6 or 12v to suit negative and positive earth systems.
David Smith

Thanks David. Easy when you know where to look!

I wouldn't have visted that site because of the name, and I have no interest in dynamo conversions.
But having browsed it a bit, there is a LOT of useful information there. I could get lost in there for several hours!!
GuyW

Guy,
I've put links up to that site many times for LEDs and the other stuff he does, would upset your Dr Davies with the electronic conversions.

Another place for you to look is Classic Car LEDs Ltd, lots of good reports from other BBSers.

I've bought from Dynamos and can recommend his service.

https://www.classiccarleds.co.uk/
Nigel Atkins

Thanks Nigel, but I wouldn't have remembered links you posted a while back,however frequently, when it wasn't relevant to me at the time. My only comment was I would not have thought of looking on a site apparently dedicated to dynamo conversions when looking for LEDs!

Rest assured, it is only 6 v LEDs that I am looking for. I am considering getting a pair of the dual function front sidelight/indicator bulbs so that I can keep the clear glass covers, but have orange repeater lights.
GuyW

Guy, I thought you hung on to me my every word, you disappoint me.

You've lost me on the 6v bit but if you mean the LED bulbs that go from (warm) white to amber for the front lights then Gary got some from Classic Car LEDs Ltd - https://www.classiccarleds.co.uk/collections/indicator-bulbs/products/2x-bay15d-warm-white-and-amber

Gary was happy with them, see his thread in S&M General titled 'Frogeye front indicator bulb colour shock!'.

His (FB!!!) video - https://www.facebook.com/plugins/video.php?href=https%3A%2F%2Fwww.facebook.com%2Fgary.lazarus.75%2Fvideos%2F10157335688885513%2F&show_text=0&width=560

If you've read Peter's bit in Classic Dynamo & Regulator Conversions then you'll know the bulbs are best used in full sets
. for indicators 4, 5 inc telltale,
. 4 or 5 including number plate (plus 3/4 more panel lights).

And you'll need a suitable flasher unit too - lord, the expense!

And then it depends on the rear lights you have, this is going to be a mortgage job, what with wires and relays, my head aches.
Nigel Atkins

Guy, you must keep the grey matter busy! It was Nigel's link that had me reading that site only a week or two ago, and I bookmarked it as I'm poor at electrics. And I remembered the LED connection too ;-)
David Smith

Guy's not so keen on books he'd need to benchmark the site.
Nigel Atkins

No, I think you probably do mean bookmark, not benchmark in this context.
GuyW

Guy – I understand that in the early days of motoring, most cars were wired negative earth, but that around the 1920s manufacturers changed over because it had been found that a positive earth system produced less corrosion around the battery terminals.

By the early Sixties, most cars were being produced with unitary or monocoque bodies. Around this time, I believe an influential paper was presented to the American Society of Automotive Engineers which showed that there was less cathodic corrosion (as you mentioned in your old post, Guy) in car bodywork with negative earth. As far as I understand this process – which is not necessarily that far – it means that any panel or component with (even slightly) dissimilar metallurgy can become a ‘sacrificial anode’ and attract to itself significant and even catastrophic corrosion.

I’ve never actually managed to track down this paper, though I haven’t really tried as it’s never been totally germane to my research. But the report was widely noted and reacted to by the US car makers and in this country Ford changed its entire range over sometime in the mid-Sixties. BMC/BL was tardier in its response (why are we not amazed??), and I think it was into the Seventies before all BL products were negative earth.
Tom Coulthard

Tom,
Yes I do think this is fairly well reported on, though probably with most more along the lines that "I read somewhere" than many people actually having evidence of it. As regards my questioning about whether to change my MK1 car over to Negative earth, I was looking for more immediate pros and cons to think about.
If the corrosion issue is true, then I do think that for cars with relatively low annual mileages, it probably is of fairly minimal relevance.

As it is, I am sticking with Positive earth, if only so as not to upset the sensibilities of Mr J E Davies, who occasionally drops by this way.
GuyW

This thread was discussed between 19/04/2020 and 16/05/2020

MG Midget and Sprite Technical index

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