Why this question matters
An ignition coil is a step-up transformer. A low-voltage primary winding with a small number of heavy turns sits around an iron core, and a secondary winding with thousands of fine turns is wound with it. While current flows through the primary, energy builds up in the magnetic field. When that current is switched off abruptly, the field collapses and the secondary produces the tens of thousands of volts needed to jump the spark plug gap.
The primary side needs real current to store a useful amount of energy, especially under boost, with rich or lean mixtures, or with worn plugs and leads. That current is far more than an ECU ignition output is designed to carry. The ECU output is a signal, not a power switch.
That's what an igniter (sometimes called an ignitor or ignition module) is for. Think of it as a very fast, heavy-duty relay: the ECU gives it a low-current trigger, and it switches the coil's primary current. Some coils have that igniter built in. Others need an external one. If you connect a coil without an igniter directly to an ECU ignition output, there is a very good chance you'll damage the ECU's ignition driver.
Never guess. A few minutes with a multimeter is cheaper than an ECU repair. If you are not sure what you have, ask us before you start the engine.
Typical coil resistances
Two numbers are useful for the tests below:
- Primary winding: usually around 1 ohm.
- Secondary winding: usually somewhere between 4,000 and 10,000 ohms (4k to 10k).
Method 1: count the terminals
Every coil has at least two low-voltage terminals besides the high-tension output: a positive that gets switched 12 V, and a negative that the trigger device pulls to ground. The number of terminals is a good first clue, but only if you read it correctly.
| What you have | Built-in igniter? | What to do |
|---|---|---|
| Single coil, 2 pins | No | Needs an external igniter |
| Single coil, 3 pins | Maybe | Test it; many 3-pin coils have none |
| Coil pack, 3 pins, one connector | Usually no | Treat as two 2-pin coils sharing a 12 V feed |
| Single coil, 4 or more pins | Almost certainly yes | Confirm with the pinout, then wire to the ECU output |
Two-pin coils
With only a 12 V feed and a negative, there is nowhere for an internal igniter's trigger or ground to connect. These always need an external igniter. The Bosch HEC coil is a common example.
Three-pin coils: the trap
It is easy to assume that a third pin means an igniter inside. Often it doesn't. A number of Nissan, BMW, Audi and Volkswagen coils have three pins and still rely on an external igniter. In those designs the third pin is usually a ground to the cylinder head, not a trigger input. The S13 and S14 Nissan coils, for example, use one pin for the igniter's output, one for switched 12 V, and one for a head ground. The VW 1.8T coil follows the same idea with a different pin order. Any three-pin coil has to be tested before you wire it.
Coil packs that share a connector
Some packs house two coils behind one plug. A common Subaru pack has three pins, but it is really two coils that share one 12 V supply pin, with one negative each (one firing cylinders 1 and 2, the other 3 and 4). Count it as two 2-pin coils. It has no internal igniter and is driven by an external two-channel igniter.
Four pins or more
We are not aware of any single coil with four or more pins that lacks an internal igniter. GM LS coils and the Haltech IGN-1A coil are good examples of coils with the driver built in. See our IGN-1A article for its pinout.
Method 2: primary resistance test
Set your multimeter to ohms and measure between the coil's 12 V supply pin and its signal pin.
- Roughly 1 ohm: you are reading straight across the primary winding. There is no igniter inside.
- Open circuit (no continuity): there is electronics between those pins, which means an internal igniter.
Method 3: signal to high-tension output
Now measure from the signal pin to the coil's spark output terminal.
- Open circuit: an igniter is present.
- A reading in the megohm range: also points to an igniter.
- A reading in the kilohm range that roughly matches what you measure from the 12 V pin to the output: you are reading through the secondary winding, and there is no igniter.
When the two tests agree, you have your answer. When they don't, stop and ask before connecting anything.
If you need an external igniter
Match the igniter's channel count to the number of coils you are firing. Haltech's HPI4, HPI6 and HPI8 high power igniters cover four, six and eight channels, and a two-coil pack like the Subaru one above is the job for a two-channel unit such as the dual channel OEM igniter. Mount igniters where they can shed heat, and give them a clean ground.
Also check the ECU's ignition output settings. A coil with a built-in igniter and a bare coil driven by an external igniter both want the correct dwell, and too much dwell overheats coils and igniters.
Troubleshooting
| Symptom | Usually |
|---|---|
| No spark on one cylinder after connecting a coil directly | The coil has no igniter and the ECU output may be damaged; stop and test |
| Coil or igniter very hot at idle | Dwell set too high for the coil |
| Test results disagree | Wrong pins identified; confirm the pinout before going further |
FAQ
Can I wire a 2-pin coil straight to my Haltech?
No. A 2-pin coil has no igniter, so it needs an external igniter between it and the ECU.
My coil has 3 pins. Does it have an igniter?
Maybe. Plenty of 3-pin coils use the third pin as a ground and have no igniter. Run both multimeter tests to be sure.
Which other coils have you covered?
See our notes on the LS1 coil and the Toyota Yaris coil-on-plug, or browse the knowledge base.
