Another coil question.

[persiflage]

The attached picture shows the original coil removed from my Sprint many years ago. So long ago that I can not remember the reason why.
I'm confused by the third terminal. Has anyone ever come across one of these before or have an explanation for the third terminal? Switch and contact breaker terminals are clearly identified, the 3rd has no ident. I spent some time yesterday searching the web but failed to find any information.

[HCA]

I have a vague memory that these coils were part of a particular tachometer set-up. Not completely sure of detail at all. I could be wrong.

[nigelrbfurness]

Built-in ballast resistor perhaps?

[persiflage]

Built-in ballast resistor perhaps?

Having read a couple of lines somewhere about interior ballast coils, that had crossed my mind. It would have been sometime in the early '90s that I changed it and I can't for the life of me remember how it was originally wired.

[HCA]

If it is an internal ballast, then quite easy to determine: with an accurate meter, measure the resistance between the negative terminal and the other two.

However, i would have thought there would be a mark on the bakelite differentiating the two terminals as it would be quite important to wire these terminals correctly...

[2cams70]

Given that the Elan has a fibreglass body might it be an earth connection?? It should be shown in the workshop manual wiring manual if it's an original coil.

[elansprint]

Check for continuity between that tag and the metal case
Ian

[HCA]

There is a manufacturer's logo on the coil body - post a close up piucture of this? Also a close up of the bakelite top.

If this 'third' terminal is a vital part of the primary windings, such as ballast or earth (but I cannot see why an earth is needed at this point as the whole idea of a primary winding is to earth at the condensor/points...) it would have signicant casting markings as to its function.

[2cams70]

(but I cannot see why an earth is needed at this point as the whole idea of a primary winding is to earth at the condensor/points...)

It needs a ground because that is the return pathway for the high tension supply to the spark plug. I ask the question because many years ago I once came across a misfire issue in a friend's old Datsun 180B. After much searching the cause eventually turned out to be that the clamp screw securing the ignition coil to it's mounting bracket had come loose thereby interrupting the return earth pathway.

[Craven]

(but I cannot see why an earth is needed at this point as the whole idea of a primary winding is to earth at the condensor/points...)

It needs a ground because that is the return pathway for the high tension supply to the spark plug. I ask the question because many years ago I once came across a misfire issue in a friend's old Datsun 180B. After much searching the cause eventually turned out to be that the clamp screw securing the ignition coil to it's mounting bracket had come loose thereby interrupting the return earth pathway.

Always exception to the rule but the other end of the secondary winding (HT) is internally connected to the – neg terminal of coil. You would be looking for trouble introducing an earth connection inside a coil. Metal case is usually connected to chassis for interference suppression.
Re diagram, It looks like a tapped winding, the third terminal being connected to the SW terminal only, the 8 ohms being the 4 + 4.6 path being SW to CB to third terminal ?? ps never seen one before.

[2cams70]

[quote=Always exception to the rule but the other end of the secondary winding (HT) is internally connected to the – neg terminal of coil. You would be looking for trouble introducing an earth connection inside a coil.[/quote]

That doesn't make sense to me. The spark plug fires when the points open. When that happens there is no pathway to ground for the HT spark because at that instant the negative terminal of the coil is not grounded.

[HCA]

Exactly - the earth is the threaded part of the plug. The voltage is desperate to find an earth, it jumps the plug electrode, finds an earth that then gives the spark.

Polarity and connection of windings is well explained here:
https://mgaguru.com/mgtech/ignition/ig104.htm

And if you really want to read about sparks being produced, try this:
http://gpzweb.s3-website-us-east-1.amaz ... iring.html

[Craven]

Hope you guys now understand how the coil operates having read the links provided by HCA.
There’s a lot more to Kettering Ignition than meets the eye

[2cams70]

Yes exactly. See the diagram of the Kettering system in the link. HT side of the ignition coil - one side connected to the spark plug and the other side connected to ground. In the case of a metal can type ignition coil fitted to a metal bodied car the earth pathway for the HT supply is through the coil's metal can housing and not the negative terminal on the coil that is connected to the points.

[Craven]

2cams70
You are just so wrong. Please read it fully, the primary winding provides the HT return path in a practical operating system not as in an illustrated schematic.