Pre-war ignition coil

Tony Fearn · Post by **Tony Fearn** » Mon Jul 09, 2007 11:39 pm

Hello again.

I'm just about to start re-wiring the 1933 'Flying Fox' and have studied a blow-up of the wiring diagram.

This shows the coil to have two electrical terminals, one marked as CB (contact breaker), the other as SW (switch).

Modern 12 volt coil terminals are marked +ve and -ve.

How do they equate to CB/SW on a negative earth vehicle??

Tony.

Keith Andrews · Post by **Keith Andrews** » Mon Jul 09, 2007 11:50 pm

With the modern +ve / -ve
Follow the circuit in each direction to where it goes
ie from coil to dizzy then to earth...is earth + or -ve?
or going the other way, from coil to idn to battery...is it the + or 1ve terminal?

Old vechicles generally are +ve earth, modern are -ve earth

This can be swapped around...how to is described in older threads...I think in the Bradford section.

A further note, couls also come as 6v / 12v ballast and non ballast
Ballest coils are normally 6v on a 12v system...with either a ballast resistor or resitance wire between the ign and coil.
Gwnerally most coils have the voltage rating stamped on the bottom

Tony Fearn · Post by **Tony Fearn** » Tue Jul 10, 2007 12:52 am

Hi Keith, and thanks for the reply.

As I mentioned, the car is NEGATIVE earth, so I want to know:-

Does the old type CB terminal equate to the positive, or negative terminal on a modern coil.

Tony.

P.S. - Going to bed now.

ian Howell · Post by **ian Howell** » Tue Jul 10, 2007 9:55 am

Tony:-

I have recently gone into this for my Long Four and I found some notes on a Ford website. http://www.lotus-cortina.com/electric/convert.htm

ELECTRICALLY it doesn't seem to matter that much (I / my son used a modern Peugeot coil to 'get things going' and there were no problems with it connected either way round), BUT

The spark at the plug is stronger if it 'strikes' FROM the centre electrode to the casing.

This is because hot metal releases electrons more easily than cold metal.

Now the way to find out which way the spark goes sounds dodgy (I have no intention of trying it!), but here goes: -

With the engine warm, remove one (HOT) spark plug and start up again (see what I mean! I suppose you could bung up the hole with a spare plug).

Then CAREFULLY put the sharpened lead of a pencil in the path of the spark. There will be a 'flare' in the direction of current flow. (and possibly up the lead of the pencil, through your fingers, arm and legs, to the ground!).

If the spark goes the 'wrong' way, simply reverse the +ve and -ve.

Sounds simple, but will the first survivor please let us know how effective it is?

Alastair Gregg · Post by **Alastair Gregg** » Tue Jul 10, 2007 10:14 am

I suggest if anyone takes up the challenge they don their wellies and make sure the path to earth (don't have the car on axle stands and cling to it) is high resistance.

Tony Fearn · Post by **Tony Fearn** » Tue Jul 10, 2007 11:51 pm

Thanks to Ian, Keith and 'Big Al' for their replies, but it should be a simple matter to say for a negative earth vehicle whether the CB contact on a pre-war coil equates to either the positive or the negative terminal on a modern coil. I will await your answers.

I don't want a congflagration on connecting the battery.

Come on George, help me out here.

A.J.

ian Howell · Post by **ian Howell** » Wed Jul 11, 2007 6:07 am

Tony: -

On the pre-war cars the CB terminal goes to the contact breaker - and thus to 'earth' (actually chassis) via the 'points', and the +ve terminal goes to the supply from the ignition switch.

As the car is negative earth (chassis), then the wire that went to the CB terminal should go to the negative terminal, and the one that went to the SW terminal to the +ve terminal.

As I said earlier, it doesn't matter electrically (no risk of conflagration), but the spark may be less effective.

Is the Flying Fox 12 volt? (See previous posts).

Post by **Forumadmin** » Wed Jul 11, 2007 1:08 pm

You may also have problems with capacitor which can be polarised (electolytic) but paper and polypropelene are usually OK.

Tony Fearn · Post by **Tony Fearn** » Wed Jul 11, 2007 1:11 pm

Thanks Ian, That's very clear now.

Yes the FF is negative earth.

Regards, Tony.

ian Howell · Post by **ian Howell** » Thu Jul 12, 2007 10:06 am

Yersssse: - Forumadmin has a point, and the ELECTRICAL size of the capacitor also matters (a bit).

As well as reducing the spark at the 'points' and so prolonging their life, the capacitor forms a 'tuned' circuit with the coil which gives a much more powerful spark. (There is a similar circuit in fluorescent lamps to give a very high starting voltage to 'strike' the tube).

It would be reasonable to expect that the capacitor that was fitted to whatever car the coil was originally designed for, would be the 'right' one.

Are all modern coils designed with similar inductance? Perhaps someone with more knowledge of automotive electrics can give us a further insight?

ian Howell · Post by **ian Howell** » Thu Jul 12, 2007 3:55 pm

Yes O.K. Pat, but not everyone has access to, or can use, a 'scope (I have / can), and anyway stuffing 25kV up it might not be a good idea! (Not on mine anyway!).

ian Howell · Post by **ian Howell** » Thu Jul 12, 2007 10:34 pm

I guess you are talking about the back emf after the points 'break'.

I had visions of putting the test leads on the plug leads - pencil or no pencil, not a good idea!

My 'scope doesn't get much exercise these days, so a warm up might help it.

ian Howell · Post by **ian Howell** » Fri Jul 13, 2007 10:41 am

We are opposite sides of the same coin here!

When you add the back emf in the primary, to the induced voltage in the secondary (as they each have one end in common), the total is quite a bit more - so even more reason to take care of your 'scope!

Do you have a 'feel' for the back emf of the primary?

I have recently had cause to investigate this on a quite small 12 volt (non automotive) relay, and it was well over 300V!

AlanBartlett · Post by **AlanBartlett** » Sat Jul 14, 2007 11:24 am

Speaking of the capacitor how would one test it for working properly

Bryan Walker · Post by **Bryan Walker** » Sat Jul 14, 2007 1:23 pm

Hi Alan,
There is no REAL easy and cheep way to test a capacitor to see if it is working properly-
You can test it to see if its failed and gone open or short circuit with a digital voltmeter but then you need to know its original design specs - Inductance,Equivalent Series Resistance, designed working voltage range etc and then test it for them. But capacitors do age and change fast from their design spec. (shelf life of about 5years is normally quoted) so if you are wanting to check the condenser in the distributor you are best to replace it with a new one.

Bryan

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Pre-war ignition coil

Pre-war ignition coil

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