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MG TD TF 1500 - Starter & generator + to - Earth?

When converting from positive to negative earth, does the generator have to be modified? I understand that the starter will work either way. What has to be changed and will I have to modify my new original type harness, which I hope is not necessary? PJ
Paul S Jennings

Paul - See the article, Polarity Change in the Other Tech Articles section of my Homepage at: for information on what has to be done to change the polarity of your car. Cheers - Dave
David DuBois

Neither the starter nor generator have to be modified. Basically, the only components that need to be changed are the battery connections and the wires on the ammeter. This is not considering other possible polarized devices, e.g., fuel pumps, etc. Bud
Bud Krueger

If you have pertronix or some other solid state stuff that was made for positive ground be warned that converting will fry them right now, especially the pertronix.
Tom Maine

I see in Daves site that the fuel pump might have to be changed. My fairly new point operated pump is for positive ground, unless there is a way around this I guess I'll have to have it converted or buy a new pump. Sound right? PJ
Paul S Jennings

Dave, You have an email. PJ
Paul S Jennings


Check with Dave Dubois. He is the fuel pump specialist. May be able to tell you how to convert it.

SUfuelpumps at donobi dot net
Bruce Cunha

Paul - I just e-mailed you regarding the pump. I hadn't read your latest posting on here prior to sending the e-mail. If your pump says positive ground on it (and has red tape around the end cover to coil housing junction), then you do have a polarity sensitive pump. Go ahead and send me a picture of the area under the end cover so I can see what is under there. If it is one of the diode/resistor assemblies, you might be ahead of the game to send the pump to me for conversion.

Bud - "Neither the starter nor generator have to be modified"
I have to disagree with this statement. The generator will have to be flashed to change the polarity on it. While this doesn't entail changing any components, it does require doing the flashing procedure to it before starting the engine up after changing the battery polarity.
Cheers - Dave
David DuBois

Just a note... Any LED items may have to be rewired.
Geoff Baker

But LED items do not self destruct if wired in reverse, they just don't work, right?
John Quilter (TD8986)

"But LED items do not self destruct if wired in reverse, they just don't work, right?"

David DuBois

One other component that needs "modifying" if you want to call it that, is the coil. Just swap the thin primary wires around. This is brought out in Dave's good dissertation on polarity swap.

The ignition will still function, but not quite as well.
The "sparking plug" (we are talking British cars!) center electrode is long, narrow and insulated to achieve some high temperature, defining the heat range, anywhere from "cold" to "hot." The center electrode metal wire is selected for its high temperature thermal integrity, and also its functions as a cathode. Hot cathodes emit electrons much easier than cold; if the coil primary wires are not switched when the polarity is changed, the electrons are forced to jump from what was the colder ground wire underneath the electrode. Not only is the temperature cold, but the larger area & shape are not conducive to "boil off electrons" (a phrase welding engineers toss around with respect to arc physics). "Arc density" is a reference to the area of the discharge, where sharp points or corners better focus smaller cathode spots; large, flat surfaces don't get it.

The engine may run, BUT starting, idling and high power can suffer.

Dave, just remembered it's Christmas day. After all the hoopla is over with, I'll go out to the shop and get a couple photos and email them to you. Thanks! PJ
Paul S Jennings

"boil off electrons"
That is a phrase I had nearly forgotten. When I went through Electronics 'A' school in the Navy (about a hundred years ago), everything was based on the old, forgotten vacuum tubes and we were taught that the filament heated the cathode to 'boil' off electrons...
Thanks for a trip down memory lane Jim.
Cheers - Dave
David DuBois


John Twist has a good You Tube video on polarizing (flashing) the generator.

Merry Christmas,

Kirk Trigg

Well, I really missed the mark identifying my fuel pump! It's not a point pump, it's electronic! I just sounds like a point pump. Sent photos for Dave to look at. PJ

Paul S Jennings

This thread was discussed between 24/12/2013 and 27/12/2013

MG TD TF 1500 index

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