Some U4 category coils have a two-pin primary connector (Example U4002). In this case the resistance of the primary coil can be measured in between these two pins.
Most coils of the U4 category are equipped with integrated ignition modules (controllers). Therefore, in this case the primary resistance cannot be measured. On the "Signal1" connector only the "switch" signal coming from the engine control module can be measured.
Measurement of secondary resistance
Note: Some ignition coils like NGK U4009 contain diodes to suppress unwanted sparks. If diodes are installed, resistance measuring is not possible.
- With the coil removed, using the ohmmeter, check the secondary resistance between both high voltage outputs (to the spark plug and to the ignition wire)
- 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 coil might fail under vibration or when it heats up.