View Full Version : TR2/3/3A TR3A coil. Correct resistance?

05-26-2014, 07:26 PM
Today my coil failed while on a trip.
A year or so ago, I replaced my Pertronix unit with stock points and condenser. I didn't replace the 3.0 ohm Pertronix coil. The car ran fine. Today the coil simply quit. No warning, no leakage etc. I was miles from any help, but my ohm meter told me the primary windings were open.
I got a ride to a parts store and bought an Accel "universal" coil. Nowhere on the box was a resistance rating but it was my only choice... I bought it. The car stared and ran perfect with the new lower resistance coil. When I got home I searched and found it to have 1.4 ohms resistance.
Educate me on the role of coil resistance in a breaker ignition.

05-26-2014, 07:46 PM
A good overview of the traditional ignition coil function:


I'm pretty sure the Pertronix system needs a specific coil, so you might want to start with replacing your "generic" coil. Oops - just realized you already replaced the Pertronix system. So replacing your coil *and* condenser might be a good next step.

Less resistance = more current hitting the points. Higher resistance gives longer point life.


05-26-2014, 07:49 PM
You need to either add a resistance for that coil (they are readily available at FLAPS), or get a proper 3 ohm coil.

The resistance limits the current through the coil (and points) with the engine running (mostly at lower rpm). Without it, the engine runs fine for awhile (as you've noticed), but the extra current will eventually damage the points, coil or both. (Also hard on the Pertronix and may eventually burn it up.)

The reason for moving the resistance outside on the later coils, was so that it could be shorted out during (cold) starts, when the battery voltage is much lower than usual and some extra current helps produce a hotter spark. But once the engine is running, the coil saturates anyway, so the extra current does nothing except overheat the coil & points. I have seen a plastic rubbing block literally melted from the heat.

Interesting that you had a failure with the Pertronix coil. Mine has been working fine (with points) for about 4 years now, which is much better than the previous Lucas Sports (which lasted less than a year).

05-27-2014, 10:26 AM
So let me see if I understand this:
The points in my stock distributor will eventually be "ate up" if the coil's primary output voltage is a full 12 volts? Okay, I guess I buy that.
My wiring diagram shows no external resistance so I assume the stock coil had 3.0 ohms built into it (how they arrived at that figure must have been through experience?). Now I've got a coil with 1.4 ohm resistance and I'll need to add a 1.6 ohm external resistor. I can do that.
My confusion comes from the secondary voltage. Won't cutting the primary voltage and current in half, likewise, reduce the secondary voltage by the same factor. The windings ratio hasn't changed. My car ran the same with both coils. Are the plugs that forgiving?

As always I appreciate the education this forum gives me. Thanks for all input.

05-27-2014, 10:40 AM
The ratio is designed for the voltage the coil is going to be fed when the engine is running.
The 1.4 ohm "6 volt" coil is designed to produce the same voltage as the 3.0 ohm "12 volt" coil.

05-27-2014, 12:53 PM
Won't cutting the primary voltage and current in half, likewise, reduce the secondary voltage by the same factor. The windings ratio hasn't changed.
Ignition coils are funny, they don't really work like transformers. When the points are closed, the primary current builds up a magnetic field inside the coil. Then when the points open, the field collapses suddenly. It is the sudden field collapse that causes the high voltage at the secondary.

Like most magnetic cores, the coil exhibits "saturation", meaning that more current does not increase the energy stored in the magnetic field. This is on purpose, so you get about the same spark energy regardless of engine rpm. See https://en.wikipedia.org/wiki/Saturation_%28magnetic%29 for more info.

So, running 8 amps into the primary (12 volts / 1.5 ohms) only saturates the coil faster and wastes more energy as heat. It does practically nothing for the spark intensity.

"Secondary voltage" is another misconception. In spite of all the advertising hype, the secondary voltage rises just high enough to fire the plugs, and no higher.

On top of all that, the plugs are "that forgiving". They are just lighting a fire; the fire is the same size regardless of how big the match (spark) is. As long as the voltage gets high enough to jump the gap, it will run. The only tricky part here is that the voltage required to jump the gap depends on pressure, takes more to jump the gap under compression. So what appears to be a "weak" spark in free air will usually indicate no spark under compression.

I "experimented" with this some years ago, with an MSD 6 ignition box. The MSD doesn't use the magnetic collapse of the coil, instead it drives the primary with enough voltage (about 400 volts) to create the spark. And it will generate enough secondary voltage to ruin practically anything (the coil, the cap, the wires, etc.) IF the spark doesn't keep the voltage down. Spark current is much higher as well; all that did was eat up the center electrodes. Engine still ran pretty much the same as before.

I should have saved some of those ruined parts, they were kind of impressive. If all else failed, the MSD would throw a spark over the coil tower and out to a side terminal. After it burned out the resistor in the distributor cap, the engine was still running (poorly) with a light show going on at the coil tower. I tried to limp home that way, but the spark worked it's way down until it was going directly through the coil tower and there was no longer enough left to jump the ruined resistor (which was white by then). Believe it or not, I did get home, by wrapping a paper clip around the ruined resistor and letting the tip drag on the rotor.

05-27-2014, 03:18 PM
As always, thanks for the education.