Classic cars forum & vehicle restoration.

[Albioninoz]

I have a 1957 Rover P4 90 that is wired as positive earth, This is the first car I have had that is wired in this way and I am totally unsure as to what the effects are. Apart from the battery having the positive lead to earth and the negative to power, I have no idea how this affects other electrical units.

I would really appreciate an explanation on all this or a pointer to an article or similar that will explain it for me.

Thanks, Ray

[goneps]

Without going into technicalities, which others are far better equipped to do, it seems to have been a matter of fashion as much as anything. In terms of British cars, pre-war they were usually negative earth, but changed to positive around 1940, then back to negative again in the mid-'sixties. Our 1964 Herald was +ve earth, the 1968 Vitesse -ve.

It really makes no practical difference. My Morris Eight was changed to +ve earth during restoration in the 'seventies, but I changed it back to the correct -ve earth by the simple expedient of swapping the battery leads and re-polarising the dynamo. The chap who restored it claimed the contact breaker points arced less with +ve earth, but I've seen no evidence of that whatsoever.

Richard

[ka]

I also look forward to a technical reply, but from my experience, dynamos were positive earth, alternators were negative.
I wonder if it is anything to do with 'charging' the body of the car, and inducing an electrolytic action that leads to premature corrosion?
_________________
KA

Better three than four.

[ukdave2002]

Positive earth was introduced as there was some evidence that it offered better corrosion protection. From an engineering perspective it's odd to have a positive earth/ ground, I suspect any corrosion protection was minimal. Hence the change back to -ve Earth. There are no electical benefits either way.

Dave

[pigtin]

[47Jag]

Pigtin,

Great to see you back on the forum.

One other thing I've noticed on negative earthed cars is that the wiring corrodes instead of the bodies as Pigtin observed.

Art

[Penguin45]

There's a big "BUT" - Volta got it wrong. +ve as we understand it is really -ve, so all these negative earth cars are really positive earth and all the above comments are entirely correct, just the wrong way round!

I suspect that it was really too late to make the correction.

P45.
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'67 Wolseley MkI 18/85, '70 Austin MkII 1800 The Landcrab Forum.

[trampintransit]

my understanding has always been that, since oxidation is the result of the loss of electrons, then a positive earthed car,on a wet day, will set up a feint electrolytic path of electrons from the body of the car to the ground ( earth ) via the conductive film of water covering the car and tyres...this loss of electrons IS oxidation. Thus, a positive earthed car will rust faster.

That's my dose of pub grade chemistry anyway
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'60 Armstrong Siddeley Star Sapphire
'80 Reliant Scimitar
'47 Jaguar MKIV --- in bits!!!!

[lowdrag]

As a complete failure at physics etc. I just understood +ive cars rusted and -ive ones didn't but not why, not that I would have understood probably. But my Jaguar has been +ive earth since I've owned it and has never had rust problems. And that's over 30 years now. Does it really matter or is it really down to how the car is looked after and stored?

[trampintransit]

Oh, I think we all know that how and where a car is kept has the biggest single influence of it's longevity. That's why I personally am not a fan of the 'drive it all year round' theory. The minute salt hits the road old cars start rotting, even if undersealed to death, salt spray will still make it's way into doors and screen seals and all over the place...rot..rot...rot. ..mind you, I'm in Scotland, the roads are salty for about 4months!!!

I moved garage one year...wasn't sure why but I realised that if I got under a car in the garage in winter, despite everything topside appearing dry, the underside of the car looked....sweaty. There was some kind of micro climate under the car in that garage...even on a concrete floor. Spent a fortune on the dehumidifier!
But...if the car is out and about, and gets wet, then a positive earth car , in theory rusts faster...but I'd love to hear from somebody properly educated in such matters? 'cos it sure aint me.
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'60 Armstrong Siddeley Star Sapphire
'80 Reliant Scimitar
'47 Jaguar MKIV --- in bits!!!!

[peter scott]

[Bitumen Boy]

[jp928]

Nobody seems to have mentioned the preference that solid state devices like diodes in alternators have for negative earth - at least from the point of view that its MUCH easier to build an alternator with its diodes (rectifying the AC to DC) negative sides mounted to the body of the unit.
Swapping from pos to neg earth isnt always as simple as flashing the generator to suit. I once sold a P4 Rover to a man. A week or two later he called to say it wasnt running very well. What have you done? says I. Nothing, nothing, well not much.....Just swapped it to neg earth so I could fit a modern radio. Oh, have at look at the signs on the coil turret - they read + and - so the coil is polarity sensitive, swap its leads. All ok after that.

jp

[goneps]

Now you mention it, I recall someone else saying that electronics dislike +ve earth. The change to -ve earth in the mid-'sixties may have been prompted by the gradual emergence of radios, alternators, and other electronic gadgets on mass-produced vehicles.

The effect of the coil being wired the wrong way around might be why my man claimed that +ve earth meant less arcing of the points—weaker spark, thus less arc, although the car ran perfectly well. I didn't need to swap the coil LT wires because the CB terminal was already wired to the condenser.

Richard

[Bitumen Boy]