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MG Midget and Sprite Technical - Coil question

hello all. I have a bog-standard as-it-left-the-works 1967 pos earth midget. I bought a new coil. The old one is marked SW and CB which I understand. The new one is simply (+) and (-)....which I don't! Which one is which, please? Thanks...
David Cox

"SW" or Ign..... and ...."CB" or dist are how positive earth coils are marked

the SW is (+) the CB is (-)

BUT

I dont know if you can use a modern day coil on a positive earth system....(My guess, and only a guess) is yes you can, but OTOH your running backwards thur the coil, so not sure thats a good thing.


Prop
Prop

Prop is right.
Here is how Barney Gaylord puts it on his mgaguru.com
tech page for positive to negative ground conversion:
"To be nice to the spark plugs, you should switch the two small wires on the ignition coil to make the spark jump the right direction across the plug gap. The engine will run as well either way, but the plugs should have a longer life with correct polarity. See ignition circuit. Original positive earth coils may be marked "SW" and "CB" or "IGN" and "DIST". These are the terminals that will be switched. The white power wire will end up on the CB or DIST terminal, while the wire running to the distributor will end up on the SW or IGN terminal. Newer coils may be marked "+" and "-", which makes it easy. Just connect the hot white wire to "+" and the other wire to "-". "

So just keep your '+' coil terminal to wherever the battery positive is connected. In your case through the
distributor and on to positive ground (earth).
Bob
R.W. Davis

Actually, Prop is not right. With a positive ground system, the SW (switch) terminal is negative and the CB (Contact Breaker--points) is positive.

This is how the system needs to be hooked up. If a positive ground system, the +, positive, terminal of the coil needs to connect to the distributor while the -, negative, terminal of the coil needs to hook to the ignition switch.

Thus, the effective flow of current is from the negative terminal of the battery, through the ignition switch, to the input terminal (-) the coil, through the coil, to the out put terminal of the coil (+) to the engine, to the engine to chassis ground strap, to the ground terminal of the battery.

With a positive earth system, the negative (-) terminal of the coil goes to the ignition switch (input) and the positive terminal (output) is attached to the distributor.

Swap the terminals--i.e. rotate the coil 180 degrees and reconnect the wires, if you do a negative ground conversion.

Les
Les Bengtson

I certainly apologies,


My old notes must be wrong...I had one of those old coils on my car when I bought it and it was marked (SW) and (CB) even though its a neg. earth car


so to clarify and make the correction on a coil marked (SW) and (CB)

SW/Ign post of the coil is (-) and the CB/dist post side of the coil is (+)

("sw" = neg.) ("cb" = pos.)


That could have been a real mess


Thanks Les

Prop
Prop

Les,

I certianly dont want to argue or call you out, And I have nothing but respect for your insight, knowledge and experiance

....BUT.....


Im almost certian on that old coil I had that was marked SW and CB, I was hooking the white ignition wire to the (SW) post/terminal/side of the coil, and was connecting the (CB) terminal/post/ side of the coil to the black wire coming out of the dist.

meaning SW=(+) and CB=(-)



Maybe thats why the coil burnt out in the 1st year.


Prop...Confusion is a normal state of mind for me...Prop
Prop

Prop - With the old style coils marked SW and CB, it is necessary to know what polarity the coil was made for. If the coil was made for negative ground, the SW = + and CB = -. If, on the other hand, the coil was made for positive ground, then SW = - and CB = +. That is probably why Lucas started marking the terminals + and -, to avoid confusion (that and be in step with the rest of the world). For a coil marked SW and CB, either you or Les could be right, depending on what polarity the coil was designed for (which, without looking up the numbers stamped on the coil, one would never know.

Finally, if the coil were hooked up backward, one would only loose a slight amount of efficiency for producing a spark - not enough to be noticeable unless one was using old worn out plugs to begin with. It would certainly not cause a coil to burn out prematurely. Cheers - Dave
David DuBois

Well that is inresting....I was under the impression that coils marked SW and CB, where concidard to be positive grounded coils.

I have to admit, I find this thread funny if not humorous. Its amasing what you think you know, only to find you really dont know much at all, LOL


So I guess in the end it really dosnt matter which post david cox puts the pos.and neg. wires on the 2 post?

Its amazing what you can learn in a day.

Prop
Prop

thanks for all that trans-atlantic chat!
I think I know what I am doing now!
Cheers.
Dave.
David Cox

Prop - "I was under the impression that coils marked SW and CB, where concidard to be positive grounded coils."

You may be right. I was going to say that in my last post, but then I go to thinking about the fact that prewar MGs were all negative ground vehicles and thought that I would approach it as I did. There is a test that can be run to determine if the coil is hooked up correctly, that I will put on here if I can find it. Cheers - Dave
David DuBois

David Cox and Prop - I found the article I was looking for on how to test for the proper polarity of ignition coils. If you will e-mail me at SUfuelpumps@donobi.net I will send you copies of it (I can't post it on the BBS as it is too large and contains pictures). Cheers - Dave
David DuBois

This thread was discussed between 23/05/2009 and 26/05/2009

MG Midget and Sprite Technical index

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