A direct measuring of the primary resistance is not possible in many cases because the integrated ignition module does not allow to directly access the primary coil.
The connector may look the same, but the signal on the "Signal 1" wire is different. This pictures show the difference:
Separate ignition module (controller): Ignition signal can be measured on "Signal 1" (Trig) connector.
In this configuration, primary resistance can be measured and secondary circuit can be checked indirectly by measuring the primary signal between control unit and Coil with an oscilloscope.
Integrated ignition module (controller): On the "Signal1" connector only the "switch" signal can be measured. Primary coil cannot be checked.
Measurement of secondary resistance:
Note: Some ignition coils contain diodes to suppress unwanted sparks. If diodes are installed, resistance measuring is not possible.
- Using the ohmmeter, check the secondary resistance in between both high voltage output connectors.
- A resistance should be detected. If "open line" is detected, it can be concluded that the coil is defective.
- If a resistance is measured, it does not clearly prove that the coil is OK. The primary coil might be damaged. The coil might fail under vibration or when it heats up.