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MG TD TF 1500 - Positive vs negative ground

While fixing all the quirks in my electrical system, resoldering bad wires, figuring out what is wrong with my turn signals, and fixing the wiper motor, I was given many useful links. One was to a site where the author discusses reversing the polarity of his MGA.

According to him, it's pretty easy.

If it's easy on an MGA it should be easy on a TD too, right?

Can anyone tell me the pitfalls and issues that might be different for an MGTD... or give me good reasons not to do it? Sounds like a simple changeover that would make installing radios, aftermarket products and modern electronic parts much easier...

Advice welcomed!
Geoff Baker

I can't see any reason for it....
Steve Wincze

It's easy and worth doing. Not only does it make installing a GPS, a radio or whatever else you want easier, it also reduces the chance of you (and especially someone else) screwing up when you have to jump start your car.
Gene Gillam

Geoff:

I'm also running a neg ground for all the reasons that Gene mentions. I also wanted a 60 amp alternator because I've got a heater, 60 watt quartz high beams, and 2 x 100 watt driving lights, so I wanted the output to be able to handle everything at once.

Dave


Dave Jorgensen

Dave, not to mention those horns! :)

Any issues coverting an MG TD 51 flavor that I need to know about that aren't covered in the MGA site polarity conversion page?

If I don't hear any real negatives, I think I may do this after Thanksgiving...

My justification is cost; ultimately, I'd like to go with a modern electronic ignition and electronic fuel pump, and both are cheaper for neg ground... it will also make it easier to run polarity specific LEDs which will lower my power needs, not to mention being able to add radio, run gps, all that stuff, later on...

Geoff
Geoff Baker

I'm going for 110V AC! (not)

Actually, I kinda like negative ground for all the above reasons, not only radio & LEDs, but also cigaret lighter for charging phones, etc.
jrn Northrup

There is no compelling reason for one polarity over the other, except that people go crazy about positive ground (had a Washington State ferry worker refuse to jump start our car once because it was positive ground). A radio would be the only real reason to go to negative ground, component wise (and you can't hear that when your are driving, so why bother). All of the rest of the electronic components you mentioned can be run from an auxiliary power outlet that is isolated from ground (see picture) and not cause a problem (they are all isolated in their cases). Electronic ignition is cheaper for negative ground (there is no reason for this other than the manufacture can get away with it). All electronic fuel pump is not a problem, get whichever one is cheapest and I can send you directions on how to change it over. Or, send me your old pump and I can restore it and convert it to solid state in either flavor you want for the same price. There are instructions on my web site at: http://homepages.donobi.net/sufuelpumps/ for converting to negative ground also. I don't know that they are any different than Barney's article, which is very good.
Cheers - Dave


David DuBois

Positive ground also reminds me of an episode years ago when a "know-it-all" wrecker driver went to jump start a friends old positive ground (might've been 6V, too)Cadillac. In spite of all warnings, he said he knew what he was doing and proceeded to blow up the battery in the Caddy. AW SH*T!
jrn Northrup

I just completed adding an auxiliary outlet as Dave shows in his photo, to power a GPS in our '65 MGB which is pos earth. Right after completing the install, my wife and I took that car on a 1250 miles around trip up the Blue Ridge Pkwy into virginia then back to Northern Alabama with zero trouble. It was a very easy installation and works great. This gives you all the advantages of neg earth without having to convert. The only exception being a modern FM radio but like Dave, ya can't hear them anyway with the hood down. Course I can't hear anything very well anyway.

Cheers - Dennis
D L Rainey

Geof The hardest part in changing to negative gnd. is getting under the dash (facia) to change the wires on the back of the ammeter. Old guys like me have a heck of a time getting under and then back out. Otherwise it's a piece of cake.

One other thing not mentioned above is that the battery terminals are further from the firewall and so there is less tendency for the lip to be eaten by corrosion.
Cheers,
Bob
R. K. Jeffers

Well, it sounds extremely easy to do, and does avoid the dangers of idiot tow truck drivers.

It's true I could run an isolated circuit for all power needs, and I could run polarity specific LEDs if I am willing to build isolated circuits for them (the dash acc lights would have to have insulated housings sitting in the metal brackets, same for LED turn signals, also for HID headlamps).

It just seems like more work to keep it positive ground... and the other thing I suppose is that if it is easy to convert to negative ground, it would be just as easy to switch back some day if I wanted to!

I'm interested in this because I'm betting I could build a negative ground MG that used about 80 watts max total draw on the system - high beams, accessories, all lamps added together... maybe half to one third the draw of conventional lighting circuits - with brighter bulbs throughout. To me that seems like a good improvement from the safety point of view. But not on a positive ground without a huge amount of rewiring.

I'm mulling it over.
Geoff Baker

Geoff,

80 watts requires less than 7 amps at 12 volts, and can be handled by the stock generator. Nothing wrong with converting to negative ground, it is hardly noticable and there is hardly anyone left from the Flat Earth/Positive Ground Society to critize you. That said, I left my car positive ground because I like the dim little lights (well, my sealed beam halogens are pretty darn bright), the quiet of no radio and if I really need a GPS I have my battery operated handheld. That said...

No matter how much lighting you put on your car, people will still pull out in front of you because you are 1) small, and look farther away. 2) your headlamps are close together, and you look farther away. 3) They know you are vulnerable and don't realize that older cars may not stop as fast as newer cars with discs (Although, my TD brake system seems to be excellent) and you look further away.

Regular drivers are dumb that way, especially in America.

No matter how much lighting you put on your car, they will still tailgate you because 1) if you are small and old you must be running slow, even when you are going 80 mph, so they want to pass you. 2) They just have to get by you so they can admire the entire car and the funny little British cap you wear, so they want to pass you. 3) The exhaust fumes are a tad obnoxious if someone who lacks skill tuned your carburetors, so they want to pass you. Luckily the tailgating part is not too bad as their brakes are excellent... if they aren't too busy texting.

warmly,
dave
Dave Braun

Have popped my ammeter out a few times...change the wires and pop it back...the little bracket can be put back by feel!
gblawson(gordon)

I have not found a need to change my TD to Neg. My MGB was changed by the PO so, I left it negative.

I did have one question. I know we discussed poliarity for LED's, but I purchased an inexpensive LED light that I was going to add to my luggage rack to act as a third brake light. I tested it on the TD system just to see if there was a polarity issue. It worked either with positive or negative ground.
Bruce-C

Bruce - The LED third brake light works on either polarity because it is a 2 wire light - red and black that can be hooked up either way. One just hook the red wire to positive and the black wire to negative and it works fine. Where there is a problem is when the LEDs are arranged in a standard light socket where the center terminal goes to the 'hot' wire and the brass shell goes to ground. Most LED units like that are designed for negative ground systems and won't work on our cars that are positive ground. Cheers - Dave
David DuBois

Bruce, LEDs and housings come in a huge variety of flavors, some polarity specific some not. Some are very very durable and others are surprisingly fragile. Some can handle wide voltage fluctuations, the ultrahigh power LEDs on the market now require both heat and voltage overprotection or they will blow easily. It all depends. I ended up building my own in some cases.

Dave, I agree, all the light in the world doesn't stop stupid.

Still, when I built my motorcycle, I went for brighter light because it IS safer, and a Nautilus airhorn because LOUD is good, too. And sure enough, there were times when I knew that my 3200 lumen headlamp made me more visible, and when my loud honk got a driver's attention when the weak little buzzer of the original horn never would have.

But I still had to always be on the lookout for Mr. Idiot, cellphone in ear and slurpee in hand, turning one-handed into traffic without looking...

The first rule of safety isn't about equipment... it's about planning your escape route from the moron just ahead of you...

But good light and loud horns help.

Going back to LEDs; MOST LEDs on sale at the average auto store are dimmer than conventional lamps in the same application (despite all the claims); in other words, the cheap plug in LED turn signals you can buy at Autozone aren't as bright as the most modern incandescents in the same housing. But you can get very bright ones. Go to www.superbrightleds.com or http://www.ultraleds.co.uk
It's easy - and fun - to build your own!

Because my walnut dash restricts the light from the panel lamps getting to the instruments somewhat, I'm thinking of using some flexible strip LEDs which I can "wrap" right around the speedo and tach housings to improve the readability of the instruments. And this would be an easily removeable mod that would 'plug' into the existing panel lamp bracket for power, so they could just be unplugged and returned to the original incandescent lamp with ease. But I'm pretty certain that switching to neg ground would make it much easier to do this...
Geoff Baker

This thread was discussed between 24/11/2009 and 26/11/2009

MG TD TF 1500 index

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