How to Measure the Three Wires of an Automotive Ignition Coil?

[BrodyRose]

Let the starter motor run, and you can use an LED test light to check its signal; the LED test light will blink. You can also use the HZ range of an automotive-specific multimeter to measure its frequency to determine whether the ECU is supplying a signal to the ignition coil.

[Leo]

I've been driving for decades and often deal with ignition coil issues by testing those three wires myself—it's quite common. The first is the power wire, usually red or black. With the engine off and battery negative disconnected, use a multimeter to check voltage when the key is turned to ON; it should read around 12V. The second is the ground wire, typically black—disconnect it and measure resistance, which should be zero or close to zero. The third is the ignition signal wire from the ECU; testing requires referencing the engine manual, as it's open-circuit when unpowered. I always start with the power wire—if there's no voltage, it might be a blown fuse or broken wire, so handle carefully. Remember to wear gloves to avoid shocks—ignition coils can discharge dangerously; I burned wires in my youth and learned the hard way. Always check connections after testing to prevent looseness. Basic tools are enough—don’t hesitate to get hands-on, experience builds confidence.