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Coils. How to tell what I've got?
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Mon May 21, 2018 3:39 pm    Post subject: Coils. How to tell what I've got? Reply with quote

Can anyone tell me, what I need to do to test [measure?] a box full of different ignition coils I have?

Most had no markings on them, other than either + or -...or SW & CB.

I would like to be able to tell what their 'operating voltage' would be [12 volts, or less, ie, 9 volts, 6 volts, etc]

The ones with a resistor piggy backed onto the can are easy.....but some may work with a remotely mounted resistor.

So far, I have isolated one or two which are resin-filled, rather than oil-filled.

I would also like to find out what number of ohms the primary coils are rated at.[lower gnomes, better for use with electronic ignition....and a higher voltage HT output.]

I have a Gunson dwelling meter, which is so old, the scales have become almost opaque....and another meter that has instructions written in Shakespearean English, it's that old. [the battery contact panels match no known batteries currently on sale, it's that old!.....I've had to bend up some shim brass, to fit a modern battery!!]

Once I know what I've got, I can label them!!

[I was able to donate a coil to a chum, for use with electronic ignition he had fitted...simply because the Unipart label was still intact!!]

Someone must know?? Smile
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Mon May 21, 2018 4:28 pm    Post subject: Reply with quote

There is no hard and fast formula unfortunately, its a bit like having a load of unmarked bulbs and trying to determine the voltage and wattage by measuring the resistance!
Firstly I'd nip to Maplin and purchase a cheap multimeter (they are closing so everything is massively discounted at the moment) Then I'd measure the resistance of a known 12v and 6v coil and use this as your benchmark.

For interest I have attached an article I wrote for the MR about coils:

A Bit About Ignition Coils

Lucas coils fitted to Morris cars were intended for the negative earth electrical systems that were standard practice until around 1939, when a change to positive earth was made. As a general rule the terminals were identified by the letters SW (switch) and CB (contact breaker) rather than + (positive) and – (negative).

It was an odd idea to mark coils SW & CB, because many must be fitted to vehicles the wrong way round! A coil will work either way round, but with about 20% less power if the polarity is incorrect.
An incorrectly fitted coil will not necessarily present adverse symptoms since its full ‘capacity’ is not required most of the time. Where it can present problems typically will be during starting, when the low tension voltage to the coil is lower and so less energy is ‘absorbed’ in the coil during the period when the points are closed, and at higher engine speeds when the dwell time is shorter and the coil has less time to ‘charge’.

A bit about how coils work

Contrary to popular belief coils don’t have a fixed HT output; when the points open the magnetic field created in the coil collapses and induces a voltage into both the primary (LT) and secondary (HT) winding. On the primary LT side the induced voltage is a couple of hundred volts, and since there are around 100 times more turns on the secondary than the primary; the secondary voltage is several thousand volts.

When the points open, the HT side voltage rises until it can find its way to earth (the car’s ground)—hopefully via the spark plug! But if it finds it easier to escape via a damaged ignition lead or distributor cap it will do so!

With the points open the condenser is now in parallel with the LT (primary) side of the coil; a coil and a condenser in parallel creates a electrical tuned circuit, the couple of hundred volts on the LT side goes back into the coil now as alternating current (AC) which in turn generates a voltage on the secondary side, extending the duration of the spark. As the energy in the coil diminishes this voltage reduces. All of this occurs in a matter of milliseconds before the points close and the process starts again.

The condenser plays a far more important part in the ignition than simply preventing the points from burning out.

What influences the HT voltage?

If the plug gap is opened up the coil voltage has to rise proportionally higher as the spark has a larger gap to jump, but the spark will be better, and a bigger spark at the plug is good. However, if the plug gap is too large there may not be enough energy in the coil to create the spark. If the fuel mixture is too rich the electrical resistance in the cylinder air reduces, consequently the voltage required for the spark to jump the gap will be lower, which means less energy was required and the spark will be weak. The higher the compression in a cylinder, the harder it is for the spark to jump; thus an engine with poor compression will also result in a weak spark.

Spark Polarity

Regardless of whether a vehicle is negative or positive earth, ignition systems are designed to have negative polarity at the plug’s centre (or core) electrode, and positive at the earth electrode (the one that’s bent to get the correct plug gap).
There are a few things that we need to understand (or just accept!) about electrons. Electrons are the things that are flying around to create the spark:
Firstly and most importantly, electrons only jump from negative to positive. Secondly, they prefer jumping from certain metals to others, which affects the materials used in the spark plug. Finally, electrons prefer to jump away from a hot surface and fly toward a cooler surface; in an engine the centre core of the plug is hotter than the outer part. All these things combined compromise the spark if the coil is reversed.

Testing whether the coil is reversed

The traditional method used a lead pencil interposed between the HT cable and the plug; the flare should be on the plug side and the spark between the cable and the pencil:



If you do have a coil with terminals marked CB & SW, and it’s not also labelled as to whether it’s for a positive or negative earthed vehicle, it’s worth performing the test and, if required, simply swap the CB and SW connections round.

Electronic Ignition; pros and cons on an older vehicle

Over the last 40 years after-market electronic ignition systems have become available; some people swear by them, others curse them! What do they really offer?

First of all let’s go back to the coil; the coil effectively charges up when the points are closed, so the longer the points are closed (the dwell) and the greater the current, the more energy will be available as the points open and the magnetic field collapses. The dwell is adjusted by adjusting the points; a larger gap will result in shorter dwell resulting in less energy in the coil. Any electrical resistance in the points (and there will be some even on a perfect set of brand new points) will reduce the current flow, again resulting in less energy in the coil. A limitation of contact breakers used in conventional ignition systems is the amount of current they can switch. There are coils available with a lower impedance (lower impedance means we can get a greater primary-to-secondary-turns ratio in the coil, consequently higher HT in the secondary), that demand higher current and ultimately offer more energy; however, the price would be a significant increase in points wear. The dwell could also be increased by reducing the point’s gap; the downside of this would be potential points bounce at higher engine speeds.

In the 60’s the Japanese’s manufacturers introduced Transistor Assisted Contact (TAC), sometimes also called Ignition Amplifier. Essentially TAC systems maintain the points, but they only serve to switch a power transistor on and off. It’s the transistor that is now switching the current to the coil, offering the following advantages:

1) In a conventional points system the time it takes for the current to rise to its maximum is some 15 milliseconds (a big chunk of the dwell period). In a TAC system it’s around 5 milliseconds. More current for a longer period means more energy in the coil.
2) The transistor can be specified to switch a higher current than would have been practical with a conventional points system, meaning that a coil with a lower impedance can be fitted.
3) As we previously discussed, when the points open the magnetic field in the coil collapses, inducing a voltage into the windings; the quicker this collapse occurs the greater the initial punch of energy from the coil. An exaggerated analogy would be a tyre blowing out compered to a slow puncture; the tyre will release the same amount of stored energy in both examples but in very different ways. The transistor provides a much cleaner and quicker disconnect than the mechanical points would when switching the coil current off.
4) With the increase in energy being available we can open up the plug gap, forcing the ignition system to fire the plug at a higher voltage. To benefit from this the rest of the HT side of ignition must be in good fettle, this will improve both starting and higher engine speed running.

Contactless systems

Replacing the points with an electronic switch can make further enhancements. There have been a number of variants; optical switches, magnetic, infrared. The advantages are that it may be possible to increase the dwell slightly and that points maintenance will be non existent, although where points are retained in a TAC, adjustment will be limited to every 12,000 miles and then only to compensate for the heel wear, and of course where the points are maintained it’s a simple job to return to conventional ignition in the event of electronic failures.

© Dave Lawton—April 2016

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Rick
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Joined: 27 Apr 2005
Posts: 22429
Location: UK

PostPosted: Tue May 22, 2018 7:52 pm    Post subject: Reply with quote

I'd just like to add, that I very much appreciate the informative replies that many queries such as Alastairq's receive from numerous posters on here. It makes keeping the place going all the more worthwhile to see, so thanks to everyone who contributes their time and knowledge.

Now, back to coils Smile

RJ
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Wed May 23, 2018 9:30 am    Post subject: Reply with quote

I'd like to echo Rick's words..thanks everybody.....I like the explanation of the TAC system....I have often wondered about the boxes of tricks one used to be able to buy, labelled 'ignition amplifiers'...?
I like Japanese electrical stuff...I used to wander around local scrappies with a carrier bag, snipping off bulb connectors, plugs & stuff...to replace rubbish Lucas and PAL connectors...I still have my suzuki supercarry's alternator...sucha compact wee thing..cleaned up, almost a thing of beauty? [Compared to the hulking Lucas items I was used to?]

Now.....back to this box of coils?

Maplins are no longer online!!!
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MikeEdwards



Joined: 25 May 2011
Posts: 2464
Location: South Cheshire

PostPosted: Wed May 23, 2018 10:22 am    Post subject: Reply with quote

Yes, the TAC explanation is interesting. I picked up an "ignition amplifier" kit from the Maplin liquidation sale the other day for a couple of pounds, so I might actually get around to building and fitting it, now I've more idea of what it does.
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