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MG MGB Technical - Positive ground for idiots...

Don't you guys just love new owners :-)

Are there any good reference books for positive ground systems?
How about one with lots of pictures.
Just when I was figuring out negative grounds, I buy a car with a positive ground.
Jordan

Jordan. You do not need any book. Positive ground is just like negative ground. The only real significance is that you need a positive ground radio, rather than a negative ground radio, for easy installation. US cars were positive ground up until the early 50s. I had a 53 Mercury with positive ground and, as I remember it, at 6V battery. In my 61 Sprite, I had a positive ground radio--a German Blaupunkt. You will also need to use a positive ground fuel pump. Other than this, you do not have any great worries. I have used both positive and negative ground vehicles and have never seen any great difference between the two. Now that you know you have a positive ground system, you can either leave it that way or convert over to negative ground and re-polarize the generator and convert the tach. For myself, I would leave it positive ground and not worry about it. Just be aware of the grounding if you have to jump start it. Do not let the vehicles touch. Les
Les Bengtson

Fuel pump-wise, original points and condenser SU fuel pumps are designed to work either way; SU fuel pump applications that are positive ground only were fitted with orangy-red tape or sleeving in lieu of the usual black. It's true that some aftermarket pumps, and SU pumps fitted with either transistors or zener diodes will not work with the polarity reversed unless the rest of the pump is isolated from earth. The zener can be rewired for positive ground
Mark Childers

Les,

I did not know that US cars used to be positive ground. Makes me curious now, why the change? Is one inherently better than the other? Seems like it wouldn't matter. I had always just assumed that our LBCs changed to conform with the rest of the world, who just happened to have started with negative ground.
Terrence Goodell

Terrence. I suspect, but do not know for sure, that the change over came about when alternators were first fitted. The generator can operate and charge with either a positive or negative ground system. It just has to be polarized when first started up or when polarity is changed. Every alternator equipped car I have ever seen has been negative ground and I have been told that this is required when an alternator is used. Paul Hunt, as an engineer, is better qualifed to say whether this is true than I, a social scientist, am. Many years ago, I read a British book that listed several perceived advantages to the positive ground system over the negative ground systems. Whether they were merely theoretical or practical, I never discovered. I have never noticed the longer points life with a positive ground system which was one of the advantages listed. But, back in the 60's and 70's, the US automakers dominated the US/Canadian market which was the largest in the world. Hence, what we did became the de facto standard for most of the rest of the world. The UK, on the other hand, mainly oriented their production for sale within the UK (with the exception of sports cars and a few "status" cars) and did not have to change over quite as quickly as some other countries. I know that, but the late 60's, positive ground was so uncommon in the US that a mechanic installed the battery in my 53 Merc backwards, filing the clamp to fit. Caused the battery to go dead over night, but the car ran fine. My next door neighbor, a long time mechanic, took one look at the battery, reversed it, hit the mechanical voltage regulator with a screwdriver and muttered about "new kids who think they are mechanics". Car ran fine for the next year until it was replaced with a 61 A-H Sprite which was also positive ground. Les
Les Bengtson

I beleive that I once read that positive ground reduces corrosion due to there being a positive charge on the steel (with regards to some other part of the car) which repels the negatively charged (2e-) oxygen atoms.

I think we can discount this actually making any difference as 1) the charge would only make a difference where it wasn't sheilded by the rest of the steel e.g. around cables and 2) the car could still be negatively charged overall with respect to the air 'cos it's standing on four non-conductive rubber thingies whereas the air is generally at the same potential as the ground (at ground level anyway).

As for alternators... well, you could make a positive-ground alternator if you wanted, you'd just have to swap all the diodes round and use a negative regulator. However, seeing as transistorisation was just starting to turn up in cars, the positive supply rail made sense as ground is ground and transistors that work between ground and a positive supply were cheaper (this is a bit of a sweeping statement but to do it properly would require me typing out the first year of my degree...).

So, that's what I think then. FWIW, my '67 BGT was converted to negative ground while it still had a dynamo so that a modern hi-fi could be installed.

ttfn,
--
Olly
Oliver Stephenson

General Motors cars have been negative ground forever. Ford Motors were the oddball in the U.S. with positive ground until the early fifties, before the invention of the transistor and the general use of solid-state diodes. I have no data to support this, but I imagine the the British auto industry just followed Ford's lead.
George B.

I just realised that I wrote that a positive charge would repel negative ions. Exactly what was I thinking?

I am daft. It's also been a long time since I did Chemistry at school...
--
Olly
Oliver Stephenson

All,

Thanks for the history. I enjoy getting the first person accounts on old cars.
Terrence Goodell

Guys;
Just for the record I have a pos earth delco alternator in my basement that used to be on my 67B. A friend at a elec shop reworked it for me as a challenge. Bob
Bob Thompson

I started working on cars in 1946 and the Chevys of the 30's were negative ground. I had
a 47 Ford in 1950, I believe it had positive ground, it's hard to remember that far back.
FWIW Clifton
Clifton Gordon

This thread was discussed between 05/06/2002 and 08/06/2002

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