MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Info needed

My TF came to me Negative Earth but really what is the difference apart from the obvious, the chap that owned it before me according to the receipts he gave me he had a new battery put in some time back, now knowing the level of workmanship and knowledge (particularly of MG's) from my observations of other work they carried out it could be possible they just put the new battery in wrong not knowing the difference, knowing the previous owner I would say he wouldn't have had a clue.
So what does it all mean as far as the operation of the car, is it possible the car would just run as normal with the battery reversed or not.


Thanks

Regards

george

G Mills

George there's a thread called "Converting to Negative Earth" in the archive which lists what has to be done to make the change. I guess you would just undo what was supposedly done. If you search for "negative earth" in the archive there's a heap of info. Cheers
Peter TD 5801
P Hehir

Thanks, I am not that phased about it being Neg earth as long it works ok


Regards


george
G Mills

George - check the battery - the ground cable originally came on the positive side of the battery. On a car that has been converted to negative ground, the starter switch wire will be on the positive side.

When you rev up the engine, does the ammeter needle go to the positive or the negative side? If the needle goes positive you have no worries. If it goes in the opposite direction, look in the archives and change the two wires and polarize the generator.

Quite easy.

Tom Lange
MGT Repair
t lange

http://homepages.donobi.net/sufuelpumps/Other_Subjects/Electrical/General/Polarity_Change.pdf

This is a link to Dave Dubois' excellent article on changing polarity.

If someone changed your vehicle to negative ground, and then wired the battery in incorrectly, at the very least, your ammeter would operate in reverse.
Geoffrey M Baker

George, if the only thing done was to reverse the battery connections you would have an 'ungenerator'. If the generator had not been repolarized the thing that would be happening is that the generator's output would be trying to reverse your battery's voltage. And, that doesn't work. I seem to recall that your generator is in need of an armature, so I don't think that's happening. Bud
Bud Krueger

George, also if you have the original helmet style battery cables, the positive terminal cable will be a bit to large for the new negative ground battery post. When I converted to negative ground, it was difficult to get a good connection. I would up having to add a few wrappings of aluminum foil to the terminal. Been working for years. Larry
Larry Brown

George - With a T series car, there is a lot less things to worry about when reversing the polarity than in newer cars with a lot of polarity sensitive bits and pieces in them. First of all, if the battery was reversed accidentally (as in some nob in a service station putting the battery in backward). The first indication would be the ammeter reading backward. As for the generator needing to be flashed so that it will put out the correct polarity - that would be very obvious in that the battery would run down very quickly (and would probably be accompanied with a lot of smoke escaping from the generator and possibly the regulator). It is my understanding that the generator could well "self polarize" as soon as the regulator connected the generator to the battery. The bottom line here is that the car came to you as negative ground, so if done by accident or on purpose, the car is running correctly and as long as the battery lasts longer that a few weeks, things were either done correctly or they sorted things out by themselves.

I would suggest that you check the following things.

First of all check that the ammeter is reading in the correct directions - rev the engine up and observe which way the ammeter reads - it should be to the charge or plus side. If it reads in the discharge side, reverse the leads to it (or mark it temporarily so you are aware of what the generator is doing)

Second (this can be skipped if the ammeter reads correctly) disconnect the two leads from the generator and use a jumper wire to connect the two terminals together. Attach the positive lead of a multimeter to the terminals just jumped together and the negative lead to ground. Set the multimeter to ground (this test works best if you use an older, analog meter, but will also work with a digital meter). Start the engine and while watching the meter, rev the engine. What you want to see is a reading the progresses up scale (no - sign if using a digital meter). Do not rev the engine so far as to exceed a 20 volt reading on the meter. If this test works ok, then your generator is working fine and is polarized correctly.

Finally, look at the primary connections to the ignition coil. If the markings on the primary terminals are SW (ignition switch) and CB (contact breaker), the coil is most likely one for a positive ground vehicle and should be changed such that the terminal marked SW goes to the distributor and the terminal marked CB goes to the ignition switch (white wire). If the terminals are marked + and -, the terminal marked + should go to the ignition switch (white wire) and the terminal marked - should go to the distributor.

Unless you are a die hard originality person, I would suggest leaving the car wired negative ground, it will make your life much easier in this day and age. I would suggest that you get one of the stickers from Moss Motors, British Wiring or Triple C that says "This Vehicle Wired Negative Ground" and put it next to the battery so that no one makes a mistake when installing a new battery. Cheers - Dave
D W DuBois

Thank you all for your wonderful input, I am now at ease with this.

Regards


george
G Mills

This thread was discussed between 08/06/2014 and 09/06/2014

MG TD TF 1500 index

This thread is from the archives. Join this live forum now