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 - Interesting Facts From Rolls Royce


Retirement with too much spare time on your hands unearths too much knowledge.


Quote from the RREC 1978 Technical Manual:

"...it has been found that cars wired positive earth tend to suffer from chassis and body corrosion more readily than those wired negative earth. The reason is perfectly simple, since metallic corrosion is an electrolytic process where the anode or positive electrode corrodes sacrificially to the cathode. The phenomenon is made use of in the "Cathodic Protection" of steel-hulled ships and underground pipelines where a less 'noble' or more electro-negative metal such as magnesium or aluminium is allowed to corrode sacrificially to the steel thus inhibiting its corrosion. Also the radiators of post war cars wired positive earth have a tendency to become blocked due to electrolytic deposition of metallic salts, particularly in hard water areas. For this reason it is important to bond these radiators to the chassis and bodywork with flexible copper braiding so that they are at 'earth' potential"

The RR Service Sheets dated 1960 detail the problems of radiators being choked by silt (mostly iron oxide from the block) being deposited by electrolytic action. A system of braided bonding straps was developed from the generator to the radiator to try and reduce this problem and finally the generator was electrically insulated. The cause of these problems was, of course, the positive frame so that is why the company changed polarity to negative frame.

This document also detailed a step by step procedure to reverse a vehicles earthing polarity.

During my working life I have been exposed to many cathodic protection systems on water reservoirs and naturally I have a sacrificial anode on my boat. Never would I have put this into context with a motor car.

Off to reverse the earthing on the TF now.


Graeme
G Evans

Your mention of cathodic protection on water reservoirs stimulated some of my dormant gray matter from the days of my corrosion class in college. The early demise of iron water mains in the vicinity of electrified railroad tracks caused a lot of grief, then came the cathodic and anodic protection measures.
JRN JIM

Graeme: I am pretty sure that the flow of electrons is opposite in the southern hemisphere so you have nothing to worry about :-)
Christopher Couper

Christopher, you observation of the flow of electrons 'down under' is interesting. When I changed the polarity of my TF and turned the lights on....they shone black.
Don TF 4887
D J Walker

I find that quote to be interesting in the fact that Rolls Royce and Bentleys have always been negative ground, even though other (but not all) British cars went through a period when they were positive ground. MGs were negative ground through the early years, converting to positive ground around the time that the TA first came out and remained that way until mid 1967, when they reverted to negative ground. Cheers - Dave
David DuBois

... i think there may be about a dozen (or hundreds) of other factors determining corrosion / rust / etc...

My '53 TD wired positive is still around and my '74 Mazda GLC has now bonded with the earth (dust to dust/rust to rust) ... in fact, every negative ground vehicle i've owned is now a stove....

gblawson(gordon- TD27667)

Electrical engineers view. Ok, electrons move from negative to positive. This is not a theory it is a known fact. It is also known that opposites attract. To get a better understanding of why a negative ground system was chosen we need to look at the molecular level.

In an atom there are three parts:neutron,proton, and electron. The electron orbits the nucleus(protons and neutrons). The property or willingness of an atom to give their electrons ultimately determines the materials conductivity(way more to it than that; this is a simple description). By natural law materials are or tend to stabilize in an equal state(once again there are exceptions and I'm just generalizing). We know that an atom has an equal number of protons and electrons. Therefore an atom doesn't have a charge. Another important property we need to know about atoms is that they can gain or lose electrons and stay the same chemical makeup(ie copper is copper even when it is an ion) When an atom gives or gains an electron it becomes an ion and can be considered to have a charge.(This is important). The movement of these electrons is what creates a charge or difference/potential. The movement of these electrons is electricity! If the electrons flow in a uniform direction we have a current. That is a lay description of what electricity is.
Since we have addressed the molecular level lets get some vernacular out of the way. Voltage is the potential of electron flow. When you have a battery sitting on a shelf not in a circuit it is said to have 12 volts. Think of this statement as there is a difference of electrons equal to 12 volts. Or there is a potential of electrons to flow from the negative side to the positive side(greater the potential the greater the voltage). This is important when you try to justify a 1 foot section of wiring during a voltage drop test. When we perform a voltage drop we measure a voltage. If there is a resistance we will see a voltage(for example maybe 1 or 2 volts). But wait! How can a piece of wire create a voltage?!! Only batteries or generators and stuff can do that right? Wrong! Voltage is not something we create it is a measurement of potential Now, the easiest property of electricity to define is current(in lay terms). Current is the amount of electrons flowing(quantity).

Also, it is important to understand how a battery works. The chemical reaction of the lead plates(calcium, antimony, and lead) and sulfuric acid creates a solution of ions. These ions donate their electrons and that is where the original electricity starts. Without the chemical process there would be no potential flow of electrons(voltage). It is not just electrons flowing from a negative plate to a positive plate. It is the gaining and losing of electrons of the plate. When a battery discharges the chemical process sulfates the plates(plates lose electrons). When a battery is charged the chemical process is the reverse and oxides the plates(electrons are gained).

Ok who is lost?
Alright so you are asking yourself what does this have to do with negative and positive ground systems. Quite simply it is a matter of effort. Every conductor in that vehicle that connects to ground essential becomes eligible to give up electrons.There is a potential huge imbalance of negative ions. Another way to comprehend what I am saying is to think of a negative ground system as allowing a chassis to be an amplifier. This is also the bane of negative ground systems! In a positive system the positive side will receive the electrons. Therefore only the negative feed wires will be donating the electrons on the current side of the load. This requires larger wires when a large amperage is required(this is true in ground systems but I'm referring more to the momentary jumps like ignition coils and injectors). Therefore a positive system in essence will require better quality components. So, from a manufacturers stand point it is a lot cheaper to go negative. Also, standardizing the system played an important role in the decision. When computers came along it pretty much sealed the deal for negative ground systems in autos.

Ultimately it comes down to the fact that: negative ground imitates natural electrical flow, the industry needed a standard, and it can be cheaper for manufacturers.
Paul Jennings

Mack Trucks maintained a positive ground system well into the mid 1980's.
It was Mack's contention that positive ground systems caused LESS corosion to the FRAME of the truck.
A truck may use, in its lifetime, many engines so the frame is the more important piece of the truck to maintain.

For many years I built and designed Radio Frequency (RF) semiconductors of the Bipolar (BJT) type. These perform much better if the common terminal is directly connected to ground. That would mean that for a positive ground system you would need a PNP type of transistor. But in the PNP transistors are mush slower than the NPN which would be used with a negative ground.

Our factory thus needed to produce both totally isolated NPN transistors, where the entire RF circuit could be floated above DC ground (but have an RF ground at earth potential) and very high performance PNP transistors just because of MACK.

(Transistors were used in mobile radios)


Jim B.
JA Benjamin

Jim, That's very interesting. So what your saying is, Mack is using the best of both worlds in their systems. That's a unique idea, never heard of that, but with modern electronics, anything is possible. PJ
Paul S Jennings

Jim B, on a MUCH less sophisticated level, those of us who have electrically isolated a standard cigar lighter socket to make the case negative and the center point positive so that we can use eletronics (GPS and the like) in our positive ground cars have done functionally the same thing - floated the DC ground.

Jud
J K Chapin

Absolutely Jud.

I myself have been looking at doing in the rear lights for independent directionals with non positive ground LED's i.e. a lamp for running, a lamp for stop and a lamp for directional all above ground.

Jim B.
JA Benjamin

Jim, I did my high brake light/left turn signal with non-positive ground LEDs from the local auto-part-R-us store with no problems. The housing is insulated from the electricity stuff. There are two wires so you just have to connect the correct wire to the correct polarity. Being trained in chemical engineering and having had only two semesters of electrical stuff and that before transisters were commonplace, I was quite proud of my success.

Jud

J K Chapin

There are actually some sacrificial anode systems for cars. Not sure they really work. Seems if they did, everyone would have them.

http://www.ruststop.net/ruststop%C2%AE-rs-5/
Bruce Cunha

"There are actually some sacrificial anode systems for cars. Not sure they really work."

Sacrificial anodes work well on ships and boats, not so much on autos. Cheers - Dave
David DuBois

Dave,

My understanding (going back a long time to my physics & chemistry studies) is that for a sacrificial anode to work it has to be electrically connected to the metal it's going to save AND be immersed in the electrolyte that's trying to do the damage.

For boats, which are immersed in the sea, that's easy but it's not for cars, where there are patches/pockets of dampness or salty water etc., but often no electrical continuity through the electrolyte.

Best wishes to you all for the Xmas & New Year break BTW. I've ben preoccupied on other things for much of this year so I haven't posted, but I am still looking in from time-to-time.
Tom Bennett - 53TD 24232

My son just bought a Triumph Bonneville, a 1979 model. All Bonneville bikes were positive ground but in 1979 they finally turned it around to negative ground.

Since it's so easy to do and there seems no downside and make so many things easier, I have turned my TF to negative ground.
JE Carroll

Since it's so easy to do and there seems no downside and make so many things easier, I have turned my TF to negative ground.

I'm thinking of doing the same with mine, as long as I don't have to modify the new original style harness. PJ
Paul S Jennings

I understand the issue of Negative ground for use with modern equipment, and I know others have installed grounded plugs with reversed neg positive, so you can hook up things like chargers and gps, but I have not yet found a good reason to convert the TD. I did convert the 67 BGT, but then I wanted an alternator on this car anyway.
Bruce Cunha

"but I have not yet found a good reason to convert the T"

There is no compelling reason to convert the TD other than to eliminate the need to stand behind the mechanic with a club in hand if he/she is going to anything with the electrical system in the car. A member of our local 'T' Register called Triple A to jump start his TD. He told the guy that he needed to be careful when hooking up the jumper cables to the battery, but the guy said he knew what he was doing, proceeded to hook up the cables up backward and ruined our member's battery.

Since I purchased a Lifetime battery from Penny's back in the 70s and whenever the batteries give out, I take it back to Firestone so they can test it and give me a new battery, I stand right over them and watch them like a hawk.
Cheers - Dave
David DuBois

zinc/sacrificial anode on steel sheetmetal= galvanized steel

Is it too expensive? No, it is called "planned obsolescence!"

Remember seeing all those Cadillacs & Lincons running around with holes in the doors and fenders conspicuously under the chrome trim? They studwelded stainless steel studs to the bodies, so the studs didn't rust away. DUH! The stainless is cathodic to the carbon steel and ate the surrounding sheetmetal away. I guess you'd say, the body served as anodic protection to the studs.

Reminds me that they came out with "zincrometal," like zinc rich primers. Zinc rich paint coatings are just as good as galvanized, right? Nope. It's only a paint coating. The zinc metal must be in intimate contact with the basemetal, electrically conductive, to behave as an anode. The paint effectively insulates the great majority of the zinc powder from the substrate. There was a treatment proposed to bestow some anodic protection, but I doubt anyone ever put it into use... a very high voltage electric discharge played over the surface could effectively create short circuit paths linking some % of the zinc grains and grounding them to the steel.
So we're left with sheetmetal that the paint peels off of and rusts sets in.

JRN JIM

Good point Dave.

When I bought the B, it had already been converted and my brother, knowing British cars of that age are positive ground, jumped it. Of course he blew out the alternator and the petronix ignition. It is now well marked as NEGATIVE GROUND.

Bruce Cunha

"It is now well marked as NEGATIVE GROUND."

I forgot to add in my previous posting that whether the car is left the original positive ground or converted to negative ground, there should be a conspicuous "THE VEHICLE IS WIRED NEGATIVE (POSITIVE) GROUND close to the battery. These stickers are available from Moss Motors and British Wiring for a reasonable price and will at least help a bit in keeping people from attaching jumper cables incorrectly.
Cheers - Dave
David DuBois

This thread was discussed between 23/12/2013 and 25/12/2013

MG TD TF 1500 index

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