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Back in the day it was believed, for a while, that positive-earth would result in less body corrosion. I don't think anybody thinks so now.

Manufacturers switched to the marvellous new method somewhere between 1956 and 1965. I can remember em doing it.

 

Negative earth is now so universal that audio and other fancy stuff only works with this.

 

Nothing that works by heat such as lights and gauges cares which way the current flows.

DC motors dont care if both the armature and field are switched so the original Dynamo and Starter would work if you switched.

 

Stuff that may need changing is: semicondiuctor-based instrument regulator, alternator.

 

Basically you can take your choice if you are prepared to fiddle about a bit. Battery got to turn around maybe and have different leads, minor stuff like that.

 

Al.

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Also, if you intend keeping the dynamo you need to polarise it for negative earth. I think there are instructions on how to do that on this forum. Try a search or google it. It's quite straightforward.

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When converting to negative earth, the ammeter connections need to be reversed, as do the low tension connections (CB & SW) at the coil.

See Section J4 of the Technicalities CD, but don't worry about the fuel gauge as it is a hot-wire device in the TR4 (and a moving iron device in the TR2/3/3A) and hence is not polarity sensitive in any of the cars (thanks, Tony Thomson for correcting me on that!).

 

Ian Cornish

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The ammeter doe not really matter. The needle will go the wrong way thats all. Swap it any, way anytime.

 

I am surprised about the coil. The only reason I can think you need to swap this is that there may be a bit less erosion of the sparking-plugs.

 

I'd expect it to spark and run fine either way. Unless there is something about the coils I don't know. Maybe somebody will enlighten us both.

 

Al.

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Coils only work properly one way thats why they are marked CB(Contact breaker) and SW(Switch) or + and - ;)

Stuart.

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Regarding the coil, Tony Thomson sent me this useful explanation about 3 years ago:

 

According to Tony Tranter in Haynes "Automobile Electrical Manual", ignition systems are designed to have negative polarity at the centre electrode and if reversed can be up to 40% less efficient.

 

Coils for both Positive and Negative earth vehicles were produced, but who knows if anyone has changed a coil for the wrong type in the history of older vehicles, so it is wise to check polarity of the spark (see below for testing polarity using a lead pencil).

 

With early coils, swapping the CB and SW leads will switch polarity of the spark. Later coils use + and - but the requirements are the same.

 

To test the spark direction with a lead pencil, interpose the writing tip of a lead pencil between the spark plug top stud and the HT lead, leaving a small gap either side of the pencil tip. If the polarity is correct, a flare between the pencil and the spark plug will be seen (if the flare is between the pencil tip and the HT lead, then the polarity is reversed).

 

Since all modern vehicles use a negative earth system, almost all replacement coils will expect the vehicle to be negative earth. Almost all in-car entertainment systems are designed for negative earth, as are almost all in-car electronic devices.

 

I remember that someone who was unaware of the need to swap the coil connections was surprised how much better his TR ran once I had told him about this and he swapped the leads!

 

As far as wiper and heater motors are concerned, I don't believe anything needs to be done (in fact, I don't think one could do anything as the body of the device is always "earth" and there is just a lead supplying power).

 

The change to the wiring of coil and ammeter should be done at the same time as the change of polarisation of the dynamo (or the fitment of an alternator).

 

Ian Cornish

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The wiper motor, and heater motor IIRC, have field coils and they are not polarity conscious. If they had permanent magnet fields they would be polarity conscious, but they don't, so they're not.

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A long time ago I changed the polarity on an Austin Healey I owned so that I could fit a cassette player. It was a very quick exercise. Reverse the battery connections, strike the dynamo on the battery to remove any residual polarity in the core of the field coils. Reverse the ammeter and coil connections and I think that was about it. If you search the web you are sure to find some more detailed instructions.

 

 

I guess the only downside is non-originality, the upside is that you can fit a modern radio without having to insulate it from the vehicle.

 

 

Rgds Ian

 

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brilliant and thanks.

Q1: What do you mean 'strike the battery'. Do I bash it with the dynamo :P .

 

Q2: I am currently +ve earth. IF my coil reads + and - which of the se terminals should supply current to the CB or SW?

 

I'm keeping the dynamo by the way, I enjoy watching the ammeter needle doing a 'jig' !

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Most important:

 

YOU DONT SWAP THE CONNECTIONS ON THE WIPER-MOTOR.

 

If you do it will go up in a puff of smoke because the self-parker will short the battery. I know. I did one wrong last week.

 

These will always rotate the right way as will the heater fan.

 

Interesting about the coil. Must try it.

 

Al.

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You're welcome :wub:

Don't forget to have the battery - connected to earth when you dab the field wire on the + terminal. Do it a few times to make sure and don't be surprised if you get sparks and hear the dynamo make a noise.

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