The resistance of the secondary coil (which is connected to the spark plug) can be measured with a DMM (digital multimeter). If you do not know how to use one, read How to use a digital multimeter (DMM) to measure voltage, current and resistance.
- First, you must connect the sensor probes between the two ends of the coil. One end of the primary and secondary coils is grounded, so connect one probe to the motor and the other to the end of the spark wire. This should give a reading between 2.5 and 5 Kiloohmios. However, this does not necessarily mean that the coil is fine, as the high voltage could be being traced to ground through an insulation failure. However, an open-circuit reading indicates a fault.
- Check the grounding from the end of the coil to the engine block. It is likely to be a ring crimping terminal, so make sure that it is properly connected and that it is not an open circuit at this point. The resistance of the primary coil must be approximately one ohm, decreasing to less than one ohm as the flywheel is turned, causing the points to close.
- If the resistance does not change when the steering wheel is turned, the points may be missing. Make sure the motor is in the start position (which opens the off switch) during the measurement. Otherwise, the kill switch cortocircuitará the coil, giving a false reading.