Forum » EFI Wiring Fundamentals » Not actually an EFI question - Ignition Coils URGENT

Not actually an EFI question - Ignition Coils URGENT

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OK, bear with me here, I'm a mechanical design engineer and tuner NOT an electrical engineer and I'm confused.

Looking at rewiring an old ignition system; the car is negative earth.

I'm told the voltage at the spark plug centre electrode should always be negative on any negative earth vehicle.

If the centre electrode should be connected to the negative side of the HT coil, and the spark plug tip is connected to earth through the head, how come current flows and you get a spark? To my mind, you're connecting two negatives together?

What else is confusing me is this guy:

He says the capacitor (condenser) should be on the positive side of the coil but I've always seen it on the negative with the points, is he right and everyone else wrong or is the capacitor he's talking about installing on older ignition coils not the same as the condenser used to protect the points from arcing?

Importantly, if it's a second capacitor/condenser, what value should I be looking at?

He's saying "what's normally done", done by who, the OEM?

Many thanks, Chris.

When you get a big spark, the electricity is sort of running backwards through the coil, and the coil is actually turned off.

When the coil is turned ON, you have 12v power to the + terminal. And the -Neg is grounded via closed points (or a solid state transistor circuit). When the coil is turned on, you have no spark. When the coil is turned on, we are just waiting for the coil to be charged up, also called dwel.

When the points open (or transistor circuit turns off) the current in the coil turns off... or so you would think. However the magnetic field caused by the coil collapses, and now induces a voltage in the secondary coil. The secondary coil has far more windings that the primary (like 1000:1) so the voltage is far higher. Enough to jump the spark plug gap.

The capacitor is between the -Neg terminal on the coil, and chassis ground. Another way to think about it, the capacitor run parallel to the points. It stops some of the arcing when the points open and close and reduces wear.

In the video Andy is talking about modern systems with computer controlled ignition, and multiple coils. He is suggesting a capacitor on the positive side of the coil(s) to fix 'noise' that may play havoc with crank angle sensors and the like.

If you have a old system with points, you can ignore that video.

Jonny, thanks. I have a grasp of how the coil functions but thanks for your first post.

In the video though he talks about coil on plug and older type ignition systems and it is with regard to the older systems he specifically mentions the cap. Now it may not be essential but less noise and shorter ground paths have to be a good thing but I don't believe it is as "normally done" as he suggests as I've never seen or heard of it before.

So you don't know what value would be appropriate for the cap.

Still don't really get how current flows negative to negative earth but perhaps like epicyclic torque split it will just be one of those things I never really grasp.


I googled it, "around 0.22 microfarad with a voltage rating of over 450 volts". But pretty much any ignition condenser designed for points ignition would do the trick I think. One lead to chassis ground, the other to coil neg. But you could also have one lead to ground and the other to coil POS, and it would probably work in the same way.

I think the shorter ground paths is implemented buy the coils (modern coil on plug) being earthed right there on the head of the engine, which is normal for OEM.

All older ignitions with points need to have a cap, it's essential. I think what Andy is talking about with a cap placed in a modern ignition is more of an option to smooth out bugs.

But, no one on this forum should be running points. I used a Dick Smith (Australia) hobby kit to replace points with a transistor back in the 90's, never touched points since. You can probably buy the kit through Jaycar Australia now.

It's a Ford 2.1 Pinto on Webber carbs.

I will be using Luminition Optronic or Magnetronic so no points and no traditional condenser.

It's not the points condenser I'm interested in, it's this other cap he's talking about.

I wouldn't usually get involved in old school engines but I'm helping a friend and if there is a better way to do something I want to know about it.

I have a feeling that the answer to my 'negative to negative' question lies in the potential difference. In a DC circuit the negative can still have potential?? So current can flow anywhere there is a potential difference...?

It would be good if someone who understands electrical better than me could explain this in layman's terms.


I'm not sure what your 'negative to negative' is. Best use a diagram, or load up the diagram you're most comfortable with.

At battery negative (and chassis ground), there is no potential. That is the main reference point.

The negative terminal of the coil is not the same as battery negative. It is only connected to chassis ground momentarily, and weird dynamic things happen as it is switched on and off!

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