Intermittent ignition coil failures are tough to diagnose because ignition coil windings and internal insulation tend to be sensitive to engine heat. Like all circuits, heat increases primary and secondary circuit resistance and that both windings expand with heat. An ignition coil might pass all bench tests, but will still fail when subjected to high operating temperatures and maximum loads.
To reproduce an ignition coil failure, it might be necessary to warm-up the engine and allow the coils to be heat soaked for 10-20 minutes. To test the ignition coil’s primary resistance, connect the ohmmeter’s two test leads to the coils primary terminals (+ and -). Most coils should read between 0.4 and 2 ohms. Zero resistance would indicate a shorted coil while a high resistance reading would indicate an open coil. Secondary resistance is measured between the positive (+) terminal and high voltage output terminal. Newer coils with segmented core construction typically read 6,000 to 8,000 ohms, while others can may read as high as 15,000 ohms.
On coil-on-plug setups, the primary terminals may be located in a connector. Refer to the vehicle manufacturer’s service information and wiring diagrams for the terminal locations and ignition coil test procedures for resistance values. If a coil doesn’t meet the manufacturer’s specifications, it should be considered defective. But, meeting primary and secondary resistance specifications on the bench is no guarantee that the coil will perform correctly under extreme heat and load. This video is sponsored by Blue Streak.
