In most ignition coils, there are two windings: a low‑voltage primary and a high‑voltage secondary. The primary typically reads about 0.4–2 ohms, while the secondary usually measures around 6,000–15,000 ohms. Exact figures depend on the coil type and vehicle.
There is no universal single value. Distinct coil designs—distributor-based systems, coil‑on‑plug assemblies, and high‑output variants—have different ranges. For accurate testing, consult the vehicle’s service manual and test the coil out of the circuit with a proper ohmmeter.
Typical resistance ranges
Note: the ranges below cover common passenger-car coils. Always verify with the OEM spec for your vehicle.
- Primary (low‑voltage winding): typically 0.4–2.0 ohms. Some high‑output or older coils can be as low as ~0.2–0.5 ohms.
- Secondary (high‑voltage winding): typically 6,000–15,000 ohms. Some designs read higher (up to 18,000–20,000 ohms) depending on the coil.
- Coil‑on‑plug and modern coil packs: often primary ~0.4–1.0 ohms and secondary ~8,000–14,000 ohms.
In practice, readings that fall well outside these ranges may indicate a failing coil, but measurement method, temperature, and the manufacturer’s specifications can influence the numbers. Always compare to the official spec for your particular vehicle and coil.
Testing tips and safety
Before testing, take precautions to avoid shocks and damage to the electrical system. Always disconnect the battery and remove the coil from the engine wiring to test in isolation.
- Disconnect the coil’s electrical connector and remove the coil from its mounting, ensuring the engine is cool and the battery is fully disconnected.
- Set your digital multimeter to the lowest resistance (ohms) range.
- Measure the primary winding by placing the probes on the primary terminals (the low‑voltage side and the coil’s ground or mounting frame). Record the reading.
- To measure the secondary winding, place one probe on the coil’s high‑voltage output terminal (spark plug boot side) and the other on the coil’s primary ground terminal or a known good ground. Record the reading.
- Compare the readings to the manufacturer’s specification for your coil type. If values are far outside tolerance or readings are inconsistent, consider replacing the coil or testing related ignition components (plugs, wires, or ignition module).
Note: Some coils are designed to operate with specific circuit conditions, and measuring in‑circuit can yield misleading results. When in doubt, consult your repair manual or a professional technician.
Summary
Ignition coil resistance varies by design, but a practical guideline is: primary winding around 0.4–2 ohms and secondary winding around 6,000–15,000 ohms. Always test out of the circuit and check your vehicle’s official specs. Readings outside the expected ranges can indicate a faulty coil, but consider the entire ignition system and testing conditions before deciding on replacement.