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MG TD TF 1500 - Moment of truth approaches

They say the road to hell is paved with good intentions. Well,I've been intending for years to covert to negative ground. Everybody makes it sound so easy, but I'm easily intimidated. "you just ... Its so simple" Yeah ... right!

"Flashing" the dynamo scares the hell out of me. I expect to see flames coming out of it! And what about changing th polarity of the ammeter? It all sounds excessively simple. What about the voltage relay, my pertronix?

Where can I find some text that describes then entire process, without having to have a spare pair of underwear hanndy?

Gord Clark
Rockburn, Qu.

Gord Clark

Your Pertronix is almost certainly polarity sensitive so that would have to be changed. Flashing the genny is painless and is something I'll have to do after lying dormant for nearly 50 years. Plenty in the archive on this process. Takes less than 10 seconds. I guess the ammeter adjustment simply means swapping the leads from one terminal to the other. Some contortionist movements involved though unless the dash is on the bench. Others will no doubt chime in. I'm keeping the positive earth as I don't see the need to make the change. Cheers
Peter TD 5801
P Hehir

I can guarantee you Perronix is voltage sensative. (Been there, screwed that up)

I guess my question is unless you are changing to an alternator (as I did with my 67 BGt), Why change polarity?

It has worked of a lot of years just fine with positive ground.
Bruce Cunha

If you have the Pertronix II it is reverse polarity protected but not dual polarity.

Timothy Burchfield

1. Remove the ground cable from the battery

2. Remove the hot cable from the battery

3. Turn battery 180 degrees....the size of your battery may require removing the bonnet to accomplish this task. The battery post arrangement should be examined to be sure it will not cause shorting issues with the chassis

4. Reverse the leads on the back of the amp meter

5. Reverse the leads on the coil

6. Replace the Pertronix with a negative ground attention to the wiring instructions as the connections are different than the positive ground units.

7. fuel pump.... if you have an original points type pump with any modifications, then no action is needed. If however you have a replacement that has been modified or a fully electronic unit, then it will most likely need to be replaced with a suitable pump.

8. Accessory ports...if you have added any ports for plugging in cell phones or GPS devices, the wiring will need to be reversed.

9. Any other electric devices (other than lights) will most likely need to be changed for a unit with the correct polarity. If you have converted to any LED lighting, they will need to be changed out as well.

10. Connect the hot cable to the positive battery terminal.....depending on your cable ends, you may have issues with fit. The positive battery terminal is larger than the negative terminal, so just a warning.

11. Connect the ground cable to the battery...same cable end warning applies here.

12. Polarize the dynamo..... my prefered method is to use a piece of insulated wire and touch one end to the A terminal on the control box and briefly touching the other end to the F terminal on the control box. Just touch the wire and remove quickly. You should see a spark. Do it a couple of times and it's done. There are other methods that involve removing the lead from the dynamo but that isn't really required and is just more work.

13. Start the car and observe the amp meter shows charge when the engine is reved up.

Thats about it.
L E D LaVerne

It's so simple that even I can do it.

For the sake of completeness I would add this information on point 7. (fuel pump).
During the production life of the points type pump, SU used three types of arc suppressors.
1/ Capacitor.
2/ Diode
3/ Varsitor.

1/and 3/ are not polarity sensitive.
2/ IS polarity sensitive.

If your pump has a diode. it should be identified by a red (positive ground) or black (negative ground) tape around the points cap.
It is best to check this, as if you have the polarity reversed, the diode will be conducting permanently and may burn out the coil of the pump.


J Scragg


That's what I've been looking for. Many thanks.

The only down-side, is that it involves bending over or doing things that my back can no longer do ... obviously have to get some younger help.

Thanks everybody for your excellent comments. To Peter Hehir ... the main reason for upgrading is twofold a) to convert to LEDs, and not all kits can be used on positive ground, headlamps in particular, and b) to eventually switch to an alternator.

John Scragg, your inputs good too. But its much less expensive to install a Yank fuel pump like those on Amazon ... I've put these on my MGB, midget and my daughters BugEye and will be putting one on the PA. They're insensitive to polarity and are considerably less expensive. I just connected them, and the cars started right up.

Still cold up here, and the TF slumbers in an unheated garage for another couple of weeks.

Gord Clark
Rockburn, Qu.
Gord Clark

Gord, I'm still operating with positive earth on TD 25009. I just purchased and installed a pair of dual polarity LED headlights from LBC. Installation was plug and play and the light output appears to be nicely enhanced. I'll get them aimed today and try a wee bit of night driving soon to see what the illumination pattern on the road is.

I also bought a pair of dual element pos. earth tail light bulbs for my TD and a friends '51 TD. Installation in the '51 with rectangular lenses was easy but the length of the bulbs interfered with mounting my round lenses. I don't know which lenses your TD has but you may want to consider this.

I also have several reverse wired accessory outlets for a GPS, a plug-in volt meter, electric heated driving gloves and the like. These work fine with reverse wiring.

My point is that I see no need to change from the original pos. earth. My car is far from concours or pure original but it runs good and suits my Blue Ridge Mountain driving style. I wouldn't change to Neg. earth just for lighting but an alternator may be a different kettle of fish.

J. K. Chapin

I also second the fact that the Pertronix will fry in a heartbeat if you change polarity. They are very much specific on plus or minus ground.
TRM Maine

This thread was discussed between 24/04/2019 and 25/04/2019

MG TD TF 1500 index

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