Alternator.

Rory has brought this to us :

The original reason for changing the polarity of a vehicle from positive to negative earth was enable more modern electronic equipment to work. As you suggest the change is easy enough, just a matter of swapping the battery terminals over, a quick splash of the correct polarity on the generator coil and making sure the electronic equipment is negative earth.

So thats what needs to be done if you install a negative earth alternator, except for needing to do the generator. A separate problem is that the generator will have some sort of charge control system, if an alternator is fitted, then this will need to be bypassed/removed. Modern alternators include a built in charge rate control, though I understand the very early ones did not and needed a separate controller.

Snip

Thank you for the reply.

I don't want to change the polarity! That is why I am asking for advice on POSITIVE earth alternators.

ISTR that some early alternator-equipped cars (BMC, British Leyland?) had

Chris

They did make a few in the early days of alternators, but it is unlikely you would find one.

I don't see why, however if you were really desperate to retain the positive earth, then you might be able to find some means to insulate the body of a negative earth alternators you could reverse wire it to the system. It does of course carry the risk that someone might just short the body to the engine with a spanner. Much easier, simpler and cheaper to simply change the cars polarity.

They were made fitted to some cars. I'd check with an MG or Triumph specialist as they definitely had positive earth alternators at one time. However, they're likely to be expensive and low output.

FWIW, in PC this month someone is selling alternators that look exactly like period dynamos. At a price. ;-)

Any reason why not to? You'd then have a choice of decent alternators for small money.

;-)

The message from "Rory" contains these words:

It might be quicker to change the vehicle to negative earth.

Basicly I'm being pig headed, the car (1957 109 Series One) was built as posative earth and I would like to keep it that way for no other reason that it bugs the hell out of some of the "elders" of the Series One club that I don't do the normal thing and change it!

I've never seen a pos. earth alternator, although I believe the posters who have said they exist. I started working in a garage in 1966, and cars were changing to neg. earth and changing to alternators most wings had a sticker on warning of them being neg earth.

As a matter of interest, why don't you want to change to neg. earth, is it because you have an OE radio and want to keep the authenticity? If so, you may be lucky if you remove the radio and find it may have a switch to change to neg.

Err, what car is it btw?

Steve

Now you're talking! I can't think of a better reason to do something than to annoy people who think you should do it 'for your own good'.

As for a positive earth alternator... seems to me there's 12V AC being generated by it which gets rectified then regulated.

switching the polarity of a diode pack shouldn't be too difficult if your knowledge of electronics goes a bit beyond wiring a plug. The regulator is more tricky though - especially if it's built in and you can't get in between it and the rectifier.

This site looks quite handy for understanding the innards...

In article , shazzbat writes

Iv'e never seen one at all. Used to swap old positive earth minis over to neg earth many years ago and fitted a few cars with alternators and got rid of the dynamo and that 'oribble slap 'n tickle voltage regulator!...

I can relate to that. But there's an advantage of going to a negative-earth system you mightn't be aware of. And that's a reduction in the rate of body-work corrosion through a measure of electrolytic protection.

This was a consideration in the choice of negative-earth as the universal system.

See eg.

If you're being so pigheaded and determined to keep it original, then why are you fitting an alternator anyway?

You still used to get those regulators, with an alternator & electronic ignition! on Triumph Acclaims. So if you fancy resoldering 12 large diodes there's one solution.

On or around 27 Jun 2006 10:28:35 -0700, "Rory" enlightened us thusly:

Since the alternator produces AC, it's not positive or negative. However, almost all the modern ones have a built in regulator/rectifier. You'de have to rebuild that the other way around.

If you can find one, there are some with a separate reg/rect which would be easier. The regulator and rectifier are not that difficult to build, and there's no specific reason I can think of why you shouldn't build one that works the other way around and has positive earth. rebuilding the kind that's all built into the alternator housing might be more tricky though.

On or around 27 Jun 2006 11:55:49 -0700, "Rory" enlightened us thusly:

In that case, keep it with a dynamo. properly set up, they work well enough.

That's a good enough reason as any.

There are kits available in the USA, to convert a negative earth alternator to positive earth (new rectifier pack and regulator), but I'm not entirely sure if they'd work with british alternators. I read about them on some of the Vintage Tractor forums a while back, so I'd suggest trawling some of those sites.

moray

The message from Austin Shackles contains these words:

Provided you keep the revs up.

Here's the extract from the previous post-

Not true. There are specialty manufacturers now making positive ground diode packs and regulators for the Delco

I'm sure some googling will turn up some websites.

Some of the early Lucas ACR alternators were positive earth. I'd say they're extinct now, but there are plenty of negative earth ACR's around and it is possible to convert them to positive earth if you get one with the soldered diodes. It's not really necessary anyway if it's just to charge the battery. You'll get the same output from the dynamo if it's charging properly. The existing regulator can be adjusted to increase the dynamo output if it is a bit low. What voltage have you got at the battery when you start the car.

John