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MG TD TF 1500 - Help for positive to negative ground change

I would like to convert from positive ground to negative ground on my 52 MG TD. I have scanned many threads on the subject that are disjointed and jump around without the necessary detail. I got smart enough to ask questions but not do it. Here’s my questions.
How do you repolarize my dynamo?
What wires have to be switched on my tach?
Does my clock have to be rewired?
Does my fuel pump need to be changed or altered?
Will my distributor need changes?
Are there any other concerns or changes that have to be made?
Photos of my equipment are included. I would appreciate some help on clarifying exactly what has to be done regarding the above questions.
Thanks guys,
Mike
TD15250



Mike I



How do you repolarize my dynamo?

After you have changed the battery leads to make the car negative ground, use a short insulated piece of wire and hold one end on the "A" terminal of the regulator and brush the other end of the wire on the "F" terminal. Do not hold the wire, but just brush it momentarily. You will see a little spark... job done.


What wires have to be switched on my tach?


None on a stock tach as it's driven mechanically

Does my clock have to be rewired?
No

Does my fuel pump need to be changed or altered?

Depends... if you have an original points type fuel pump then you do not need to do anything. If you have a newer type without the points then it will need to be converted to negative ground or you will need to purchase a pump with the correct polarity.

Will my distributor need changes?

If you have points you don't need to do anything with the distributor. If you have a Petronix, then you will need to purchase a new replacement with the correct polarity. I believe that the Crane type system can simply be rewired for negative ground.

Are there any other concerns or changes that have to be made?

You will need to swap the wires on the Amp meter terminals to get the correct reading of charge or discharge.... it will still work if you don't but it will show a discharge when it is charging and charging when it isn't.

You should swap the wires to the opposite terminals on the coil. Some don't and the car will run but the consences is that it would be better if you did.

Thats about all I can think of.

L E D LaVerne

LaVerne
Thanks for your info. Can you tell from my photos whether I have:
A MECHANICAL TACH? I see a wire attached in my photo (Ground?). Looks like a flex shaft also attached.
ORIGINAL POINTS TYPE FUEL PUMP? No markings visible.
A PETRONIX OR CRANE TYPE DISTRIBUTOR? It looks like a points type to me. Says "Lucas made in England". I don't know what the other types look like.
Almost there.
Mike
Mike I

That is a mechanical tachometer.

If you have points inside the cap then you do not have an electronic ignition and you don't need to worry about it.

I can't say what type of fuel pump you have as the they look the same from the outside with the exception of the color of the sealing tape on the outside of which yours is missing.

L E D LaVerne

One more VERY important step. Purchase "Negative Ground" stickers from Moss or VB and put it where you and others can see it.

I had my electronic points burnt out by a friend who knew early MG's were positive ground and that is how he hooked up the battery.
Bruce Cunha

Hi Mike,

You will have to check the connection to the ignition coil.

If they show SW (switch) and CB (contact breaker) you have an early version which was labeled for a positive earth. Hence when you change polarity you must change he connection so SW now goes to the points and CB is the wire from the ignition switch..

If the coil is labeled + and - for a negative earth car the - terminal goes to the distributor.

Good luck

Jan T
J Targosz

Do the ammeter another day, it will just indicate the wrong way until changed.

This should take about 5 minutes, hardest is re-positioning the battery so the leads reach.

I have done this on early MGBs and MY TF1500, easy.

Peter
P G Gilvarry

Good tipps - but why do you want to do it?
W_Mueller

Mike, you might want to hold off with the switch until you become a bit more familiar with the car's electrical system. For instance I note that you have a white wire attached to the case of your tachometer. The purpose of that wire is to provide a ground return for the lamps in the tachometer housing. Ground wires are normally black. White wires normally indicate lines that have 12 volts on them when the ignition switch is turned on, as is seen on your fuel pump. You might want to learn about the OEM points ignition.

I'd suggest getting familiar with the wiring diagram on page N.23 of the WSM. (Do you have a WorkShop Manual?) I see that you have a cutoff switch attached to the negative terminal of your battery. You might want to consider relocating that switch to the ground side of your electrical system. Opening the ground side is a much better way of protecting yourself from electrical accidents that, as they say, let the smoke out of the wires.


You have a beautiful car that you are very proud of. Getting familiar with its electrics is a good way of protecting it. Bud
Bud Krueger

Thanks for all your inputs. There couldn't be a better place to get expert advice on my MG TD.

The reason why I'm considering it is I wanted to add a usb/radio player and phone charger utility outlet. I don't want to get into isolating the player chassis and worrying about shorting to ground someday. I already installed a fused utility power lighter socket (rubber jacketed) using the correct polarity as instructed by Dave DuBois on another thread.

Bud, I have several wiring diagrams I acquired on this site and yours, and I do have the Workshop Manual. I will study it further before I decide to do anything. I just wanted to be reassured before I do anything that I'm covering all bases. I've been maintaining reversibility in all that I did, and in this case the same holds true as long as I don't have positive ground only parts. The wiring diagram was also necessary for the fog lamp wiring.

LaVerne, my only unknown so far is the fuel pump type. Anyone know how to ID it? It plinks awhile when ignition is turned on (pressurizing?). Is there anything else I should know?

Thanks again
Mike
Mike I

Just read about the operation of the original fuel pump. The points make and break contact wth current to a magnet which drives the armature/piston and is returned by a spring. Could that be the plinking you hear at ignition on? Would this ID the pump type?
Mike
Mike I

Mike,

Take the Bakelite cover off and look underneath. If there is a set of points that look like a flat blade on top and a brass mechanism under the Bakelite bridge, and nothing that looks like an electronic component, then the pump can fairly safely be assumed to be non-polarity sensitive. There is nothing further you would need to in that case.
If there is evidence of some electronic components, then best to send it off to a specialist such as Dave DuBois or myself.
Lew Palmer

What Lew says. Don't forget to reverse the wiring on the insulated plug you installed.
L E D LaVerne

Mike,

Your fuel pump, even if it has mechanical points my still be polarity sensitive. Across the points is an arc suppessor, this device was changed twice during the production. You could have one of the following suppessors on your unit:

1/Capacitor.
2/Diode.
3/Varistor.

1/ AND 3/ are non polarity sensitive 2/ IS. Your need to identify which component is installed on you pump.

Some guidelines which may help you:

1/ is button shaped, and may be red/brown in colour. 2/ is cylinderical, black with a sliver ring around one end.
3/ is a button, light blue in colour.

John
J Scragg

Yes, Mike, the 'plinking' that you hear when you first turn on the ignition is the sound of fuel pump filling the float bowls. Bud
Bud Krueger

Mike, after spending some time recently messing about with my clock, my research suggested that the clock's coil/solenoid MAY be polarity sensitive, although this probably just applies to clocks with a rectifier bridge e.g. Jags etc. LaVerne believes that it isn't an issue & as nobody has disagreed with him, then he is almost certainly correct. Just curious is all as I'm writing up some notes on the restoration of my clock & I wanted to get my facts straight. I had somehow got the erroneous? impression that applying the incorrect polarity to the clock could damage the coil. Cheers
Peter TD 5801
P Hehir

Peter,

The purpose of a rectifier is to take AC as an input and give DC output. A single rectifier gives half wave rectification, 4 rectifiers in a bridge circuit, will give full wave rectification. However in the case of our clock, the input is DC, The only reason I can think of using a bridge in this context, is to make it polarity insensitive. Reversing the polarity will not burn out the coil, nor will it harm the bridge.

John
J Scragg

Thanks John. I'll add this info to my clock restoration notes. Cheers
Peter TD 5801
P Hehir

Peter, sometime in the past I ran across something saying that late clocks had a solid state device installed, most likely a diode. I think that it may have been across the coil for suppression purposes. Bud
Bud Krueger

Peter, a bit of archive digging located a thread entitled
"Clock wiring inside - who knows?" It talks about this diode. Bud
Bud Krueger

This thread is the most comprehensive I've read on the subject of P to N conversion on an MG TD. I have put together all of your comments so they can be used by anyone who needs them in the future. I've researched some additional info and included a photo to clarify the fuel pump ID. I would like your opinion on it's accuracy. Bud, maybe subject for ttalk if not already there? Here it is.

MG TD Positive Ground Conversion to Negative Ground
This article deals with the requirements to convert a positive ground MG TD to negative ground. It is a compilation of information by some of the best informed owners around the world on what is necessary to make the conversion successfully. To do this all the affected components must be able to accept, or made to accept change in polarity without impact.
First you must determine what components may be affected by the change. They are the following:
Generator or Dynamo
Fuel Pump
Distributor
Ammeter
Ignition Coil
Clock
Tachometer
Windscreen Wiper Motor
Battery Cut-Off Switch
Second determine whether they are affected and how to proceed.
1. The Generator must be repolarized. Do this last, after you have changed the battery leads to make the car negative ground. Use a short insulated piece of wire and hold one end on the "A" terminal of the regulator and brush the other end of the wire on the "F" terminal. Do not keep the wire in contact long, but just brush it momentarily. You will see a little spark and the change will be made.
2. The Fuel Pump must be a simple mechanical points contact type without solid state components. This can be determined by removing the bakelite cover and looking underneath (see photo). If there is a set of points that look like a flat blade on top and a brass mechanism under the Bakelite bridge, and nothing that looks like an electronic component, then the pump can fairly safely be assumed to be non-polarity sensitive. The early pumps which had bare contact points or a capacitor across the points are not polarity sensitive. There is nothing further you would need to in that case. Do NOT switch the external wires at the fuel pump.
 There is a chance however, your fuel pump, even if it has mechanical points may still be polarity sensitive if it is an aftermarket electronic fuel pump. Across the points is an arc suppressor, this device was changed several times during the production. You could have one of the following suppressors on your unit:
Photo Note: Capacitor shown on left and diode shown on right

 a) Capacitor. (Button shaped, may be red/brown, or cylindrical, and yellow)

b) Varistor (Button shaped, blue in color)

c) Diode. (Cylindrical in shape, black with a silver ring around one end)

The a) and b) types are non-polarity sensitive and the c) type is.
All the newer SU pumps use a varister, which is not polarity sensitive . Originally SU just used a swamping resistor for arc suppression, later they used the resistor and a capacitor, then the resistor and a diode and now the varistor. You need to identify which component is installed on your pump. If there is evidence of some electronic components, then it’s best to send it off to a specialist.
3. The Distributor cap must be removed to verify the type. If you have points you don't need to do anything with the distributor. If you have an electronic ignition system like Petronix, then you will need to purchase a new replacement with the correct polarity. It may be possible to rewire the Crane type system for negative ground.
4. The Ammeter is wired for positive ground to function correctly. When changing polarity, you will need to swap the wires on the Ammeter terminals to get the correct reading of charge or discharge. It will still work if you don't but it will show a discharge when it is charging and charging when it isn’t.
5. The Ignition Coil connections should be checked. If they show IGN or SW (switch) and DIST or CB (contact breaker) you have an early version which was labeled for a positive earth. Hence when you change polarity you must change he connection so SW now goes to the points and CB is the wire from the ignition switch. If the coil is labeled + and - , for a negative earth car, connect the hot wire to + and the - terminal goes to the distributor. This will prolong the life of the spark plugs.
6. The Clock most likely will not be affected, especially the pre 1962 types. After that date the early clock modifications replaced the shunt, which formed the (normally green) block which made up the base of the clock supply terminal. This contained a resistor that was designed as a form of spark suppression across the clock coil. It was replaced with a rectifier (diode) which provided a somewhat improved solution to the wear caused by sparking, but made the clock polarity sensitive. 

7. The Tachometer will not be affected if it is a stock mechanical type driven by a flex shaft. Any electric type retrofits made for positive ground systems will not work.
8. The Windscreen Wiper Motor may run backward, but this would only make a minor difference in the parking location of the wiper blades. If necessary just relocate the wiper arms on the spindles to make them park where you like.
9. The Battery Cut-Off Switch should be installed on the new negative ground terminal side of the battery. Opening the ground side is a much better way of protecting yourself from electrical accidents.
Finally, purchase "Negative Ground" stickers and put them where you and others can see it. Others may assume positive ground when servicing and may cause electrical damage to the car.

Editor: Mike Izzi
Contributors:
LED LaVerne Lew Palmer
Jan Targosz John Scragg
Bud Krueger Bruce Cunha



Mike I

The wiper motor will run in one direction only, regardless of polarity.
Lew Palmer

Thanks Lew
Picked that up from MGAguru. Must apply to MGA and on.
Will delete.
Mike
Mike I

This thread was discussed between 13/10/2017 and 16/10/2017

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.