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MG TD TF 1500 - Positive vs. negative ground,

Is it cheaper or more expensive to maintain a TD when it is positive or negative ground? Does it matter?

There is no difference in price. Unless you try to jump start it backwards, or you have a major short from an accessory that's got a grounded case. I kept mine positive ground because I'm just that way. But they will run as well negative ground, even if you forget to switch the wires on the coil.

Dave Braun

Zero advantange for positive ground except it's period correct. Negative ground on the other hand allows you to safely use any modern electronic device...phones,GPS, radios, LED tail lights etc.
LaVerne Downey

Gotcha. She's set up positive anyway.

I converted my B to Negative as I added an alternator. For the TD. I did not see any real need to change it.
Plug is painted so I remember to connect it correctly (that is positive for power and negative for ground.)
Bruce E Cunha

LaVerne - I was going to answer this issue for you in my next e-mail, but now is as good a time. With modern day portable electronic devices (Phone, GPS, Laptops, etc.) there is no problem plugging into an auxiliary power outlet socket. All of these devices are completely isolated from ground by all the plastic they are buried in. You can get an auxiliary socket that is also isolated from ground at most auto parts stores and if you can't find one there, they are carried by marine supply houses (see the picture below of the one I installed in our TD). Radios and LED replacement bulbs are a different story. Ask Gordon what happens when one installs a negative ground radio in positive ground vehicle, even after going to great pains to keep it isolated from ground. LED third brake lights are not a problem as they are isolated from ground also. Cheers - Dave

David DuBois

I did the auxiliary power outlet just like the one in your photo in my '65 MGB which is positive earthed for using our GPS. It works beautifully. But I have to ask the question. Do you have to reach back to activate your turn signals or is that a battery cutoff switch?

Cheers - Dennis

D L Rainey

I think it was Gordon's radio short that convinced me that at some time I want to convert to negative ground. Yes, you can use gps and ipods and so on without much trouble; but if you ever want to permanently wire in a radio and speaker system, you are asking for trouble with positive ground.

Additionally, there are other possible benefits to switching. New HID lighting is brighter and safer (there was an earlier thread on this) and if you like HID (some hate the color etc) then you would almost certainly want to convert to negative ground, wiring in an isolated circuit for HID might be very problematic, and easily shorted. Additionally, most high output LEDs are polarity specific so that if you want nice bright dash lighting, negative ground might be the way to go.

I haven't converted yet. I'm thinking when I have everything else running perfect, and have money to blow on a sound system or HID lighting, I might convert at that point.

Lastly - if I remember correctly, when I last looked, it's a bit cheaper to buy negative ground replacement or backup fuel pumps.
Geoff Baker

Geoff - Backup fuel pumps can be had in dual polarity versions, such as the Facet. Yes, the positive ground pumps are considerably more expensive through suppliers here in the US, but purchasing them directly from Burlen Fuel Systems, they are less than $4.00 difference and there shouldn't be any difference considering that the only difference 'under the hood' is the position of two wires, which can be switched by someone with any expertise in circuit card soldering in less than a minute. Radios and HID lighting would be the only real reason to convert to negative ground. LED dash lighting can be accomplished without much difficulty with positive ground since, to my knowledge they would not have to go into a grounded socket. The biggest reason to switch to negative ground is that the car would then match modern day convention and be less of a problem if it is serviced in any shop that is not British car specific.

The one thing I would highly recommend if the polarity on a T series, MGA or early MGB is switched to negative ground is to get one of the tags that state "This Vehicle is Negative Earth" and install it next to the battery and under the hood where it is readily seen by service personnel, see the article Negative Earth Stickers in the Other Tech Articles on my web site at: In fact, since Moss also carries stickers that say Vehicle is Positive Earth, it would probably be a good idea to attach one of those next to the battery on those cars that are still positive ground. Cheers - Dave
David DuBois

I saw this on another site and thought it might be of interest.

The strength of the flow mentioned must not be that strong - if it can be redirected with such ease - must have been done originally in part to help promote clean terminals

Electron flow theory says that electricity flows from negative to positive, so perhaps some engineers preferred to wire it up that way? Think of it this way, electrons, not protons move during current flow. Electrons are attracted to protons, and repelled by one another, so they'd be quite inclined to travel from the area of highest concentration (negative terminal) to the area of lowest concentration (positive battery terminal). So from an electrical veiwpoint, positive ground technically makes more sense.
Jeff Delk

Dave, your "This Vehicle is Negative Earth" sticker is a great idea.

Who knows, one day the mechanic working on my MG might actually know something about the car :)

Geoff Baker

Here is some information from Lucas, regarding Positive Earth Systems :
Negative earth systems were actually used on British Vehicles up to about 1936.
It was then found that certain specific advantages were obtained by earthing the positive pole of the battery instead of the negative.
One of the main advantages is that the polarity of the spark plug central electrode is made negative, which results in improved spark plug performance and longer service life of the spark plugs and the H.T. cables.
At the battery itself, the formation of electolytic sulfation at the positive lug is reduced, and also the marked tendency to electrical leakage from the cells to earth, the result of the presence of acidulated moisture. The corrosion effects at switch contacts, cable connectors, soldered joints, etc. are also reduced, especially under conditions of excessive humidity.
Phil Atrill

"One of the main advantages is that the polarity of the spark plug central electrode is made negative, which results in improved spark plug performance and longer service life of the spark plugs and the H.T. cables."

This one statement I can comment on. All one has to do to insure a negative pulse at the center electrode of the spark plug is to change the primary connections on the coil (a primary step in changing polarity on a car). I have to assume that the other points are also equally in error or all cars today (including MG and the rest of the British cars) would still be positive ground. Cheers - Dave
David DuBois

david, good point! regards, tom
tom peterson

Besides Lucas, Ford used pos earth until the 50's, fighting the dominant GM, and finally gave up. There are other arcane and arguable theories regarding polarity, but none of them make enough (if any) sense to fight the reality of all electronics today being neg earth. There is no reason other than simple obstinate insistence on "originality" to do anything but convert to neg earth, and a lot of reasons why it should be done, namely the number of people who know nothing else, can't read even if it is labeled, don't know what the label means anyway. The only good reason for pos earth I see is if you have a period radio that can't be converted, otherwise you are just inviting expensive disaster.

The best label by far was the black and yellow Jaguar XKE one, which I don't think has been repopped, but should be. It's about 2x3", rigid phenolic plastic, screwed on with 4 screws. I have one stashed someplace, waiting until I get something finished that deserves it.

Fletcher R Millmore

This thread was discussed between 03/03/2010 and 04/03/2010

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