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MG MGB Technical - Pos to neg earth - is the change worth it?

Having fallen victim to the dreaded fuel pump woes recently I thought I would upgrade to an electronic SU pump. These pumps I gatehr are polarity sensitive. My car is a standard 66 BGT and is positive earth. Is there any benefit to be gained from converting to negative earth? It would give me the option of changing from a dynamo to an alternator but are there any other benefits?

Cheers
Richard
Richard Atkinson

Richard,
There is no advantage in my simple minded opinion. Most people make that conversion so that they may use modern electronic devices in their cars such as modern radios/cd player/etc. I'd just as soon listen to the music of that cute little engine. I'm not certain but isn't the fuel pump non-polarity sensitive?

Cheers - Dennis
D L Rainey

Richard.

One specific reason. In today's world, cars are negative earth. One brief moment of brain fatigue,forget your car is positive earth or better yet, have to have it jumped by someone else and and your generator and potentially your electrical system is toast.

If you want to add a radio or other modern accessories, negative earth makes it a whole lot easier. I also have a 67 and just added in a power plug for my cell phone charger. Having Neg earth and an alternator makes it a lot nicer.

If you go Neg. Go with an alternator. Very easy to switch over and as easy to switch back.
Bruce-C

I belive Burlen makes positive ground S.U. transistirized fuel pumps for earlier cars. That said, I too own a '67 and have converted to negative ground so that I can use an alternator as well as modern electronics in my car. I recently had to rewire a '72 GT because the owner had left his headlights on all day. For some reason he was under the impression that all British cars of that era were positive ground. He jump started it backwards and killed his alternator as well as some heavy wires in the loom. It was a real joy to repair. RAY
RAY

Richard - There is no compelling reason to run either negative or positive ground. Either one will make the car run properly. Ignition can be set up to operate properly with either polarity and all electronic fuel pumps ae available in either flavor. That said, there is also no compelling reason to stick with positive ground unless you happen to be a charter member of the positive earth society (in which case you will be promptly excommunicated upon changing the polarity of your car) and there are a lot of advantages to making the change. The biggest being what Bruce points out - we live in a negative earth society today and if you need to have your car serviced somewhere other than the garages of the MGOC, their eyes tend to roll back into their heads when you tell them that the car is positive ground (I once had a worker at the Washington State Terminal, who came over to jump start our car when the battery failed while waiting in line, say 'sorry I can't help you" and walk away with his jump start cart when I said the car was positive ground). Also as stated earlier, if the car is negative earth, you can install all present day electronic devices without the fear of releasing all the smoke from you car's wiring harness. You can also install a modern day alternator to supply the necessary current to power all those devices. One caveat if you do change the car to negative earth, place a Negative earth in a conspicuous place under the bonnet to alert all who ever work on the car that it is no longer wired positive earth (you will inevitably run into the odd individual servicing cars, who will recognize your car as being originally positive earth and will install the battery as it was originally intended). See the article, Negative Earth Stickers in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ for part numbers on these stickers. Cheers - Dave
David DuBois

I converted my 67B to negative earth for the simple reason that I wanted a modern alternator and to be able to charge my cell phone and GPS. The car ran fine with positive earth except in the rain at night with the lights on, wipers on and the heater on, stopped at a intersection. Nice to have a alternator than spins up to full output at idle.
JML Jim Lema

Hello Richard,

All has beed said in previous post. But it is a matter of convention, like driving on the left side of the road. It is easy to live with your car as it is and it retains its originality and its British mentality....

Cheers,

Jean G.
Jean Guy Catford

"All has beed said in previous post. But it is a matter of convention, like driving on the left side of the road. It is easy to live with your car as it is and it retains its originality and its British mentality...."

Ah, but prior to WWII, British cars (at least MGs) were all negative ground :)
Cheers - Dave
David DuBois

not that it seems to do very much good, but I am sure I recall at school being taught that -ve earth is also a corrosion protection issue - is that right, or was this a school day many years ago that i wasnt listening properly...?
mick

The only real advantage is when converting from dynamo to alternator, or possibly if you want a modern audio system. Someone elsewhere was talking about changing polarity simply to have an accessory socket he could plug a radar detector and mobile phone charger into. But for that all you need to do is mount the socket on an insulated panel and wire it appropriately. The devices being discussed would be very unlikely to have exposed metal surfaces that might short out on car metal work.

Electronic pumps are available in both polarities, but having been bitten once by one I would never have another. Having to tickle up the points once every 20 years or so doesn't seem much of a hardship.
Paul Hunt

Thanks for the advice. I had in mind a Kenlowe fan but that's pretty much it. It seemed a fundamental decision before stumping up for a -ve earth fuel pump, rather than a +ve earth one. The points on the last fuel pump only lasted 20k miles. Having said that replacement points are relatively cheap to buy and easy enough to fit.
Richard Atkinson

Mick, you are correct in that a positive earth car acts like a sacrificial anode and in theory at least rusts away much quicker than a negative earthed one. That is the main reason that I remember given when British cars changed from positive to negative earth in the mid 60's. However at the rate that some of them continued to rust I wonder if it made all that much difference.
Iain MacKintosh

You can run any fuel pump you want and just isolate it from the body and wire apprpraitly.

Steve
Steve

If you leave the car stock, there is no compelling reason to change the polarity. I did after thinking about driving in a Texas summer rainstorm where I would need to run an auxiliary electric fan for the radiator and air conditioner condenser, the a/c compressor, the a/c evaporator fan, the headlights and wipers simultaneously. The alternator I sourced from a later model MGB puts out enough power to run all these things. I wasn't sure the generator was up to the task.
Glenn G

Forgot to mention that the alternator is lighter than the generator. -G.
Glenn G

"You can run any fuel pump you want and just isolate it from the body and wire apprpraitly."

Don't do that! That just sets you up for a copious amount of smoke issuing forth from the wires the first time a wrench slips and completes the circuit between th body of the pump to ground. It is very easy to convert a positive ground pump to negative ground so there is no reason to set yourself up for an electrical disaster. Cheers - Dave
David DuBois

"Don't do that!"

Exactly! Although you can protect against harness destruction hy fusing the 12v supply where the main harness joins the rear harness by the fusebox. This is a good idea in any event, both my cars came to me with harness damage from a shorted pump.

You can convert between negative and positive ground on *points* type pumps relatively easily i.e. ones with diode quenching, but can you do so 'easily' with pointless electronic pumps, Dave?

Paul Hunt

Paul - "You can convert between negative and positive ground on *points* type pumps relatively easily i.e. ones with diode quenching, but can you do so 'easily' with pointless electronic pumps, Dave?"

Yes, it is very easy for someone with the skill to solder printed circuit boards. Two solder connections on the circuit board is all it takes (see photos below).
Cheers - Dave


David DuBois

Photo 2 (Positive earth)


David DuBois

Ah, the 'if' word!
Paul Hunt

In SC we had a eletrical shop that sold pos ground Alternators. If you just need more power that way, maybe you can find one of those.

Steve
Steve

I have a +ve earth solid state fuel pump. been on 3 or 4 years and so far so good. -ve earth is supposed to reduce body corrosion, however most of my car has survived from 1967. I like the period dynamo and +ve earth on a fun roadster, for an everday driver GT, hmmm.
Stan Best

I'm positive the change wouldn't be negative.
Curtis Walker

Does the heater fan turn backwards?
Tom

Tom - tongue in cheek or not, if the motor has different coloured wires (black and green/brown) going into the casing then yes it does turn backwards when reversing polarity. I understand early MGBs had a shunt-wound or wired stator motor which *doesn't* reverse (like the starter motor), and has two black wires. Later negative ground cars have permanent magnet motors which do turn backwards and have the different coloured wires, I don't know if the change in motor coincided with the change in polarity. Easy to determine, whilst air is blown out of the heater vents no matter which way the reversible motor runs, it is much more effective one way than the other. The wrong way you don't get much more than a tinny whine from the fan, the right way the sound of air blowing drowns out most of the motor noise. It was only because I already had a roadster wired the correct way round that I realised the one on my V8 was connected the wrong way when I bought it, otherwise I would probably still be moaning at how useless the MGB heater is like so many other people do.

The wiper motor also reverses, but then you are unlikely to notice that except for possibly a slightly different parking position.
Paul Hunt

Oddly, When I converted to -ve earth, niether the fan or the wiper motor ran backwards. To this day they run the same as when I bought my '67 in '72. RAY
RAY

This thread was discussed between 15/02/2009 and 20/02/2009

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