The ignition coil on a motorcycle serves to step up the relatively low battery voltage to the high voltage needed to fire a spark plug. Coils are generally a sealed, watertight component. As such, they are generally not a repairable item. Routine checks on a coil are not necessary. They should be tested as part of a troubleshooting procedure whenever the motorcycle becomes difficult to start or runs rough. Testing them tends to be relatively simple and involves the use of an ohmmeter.
Turn off the motorcycle ignition.
Disconnect the spark plug wires from the coil by hand. Disconnect the two smaller primary wires from the coil using an open-ended wrench.
Measure the resistance between the coil connections for the spark plugs with an ohmmeter. The resistance should be 0.5 to 3 ohms. A resistance outside of this range indicates a broken coil.
Measure the resistance between the two smaller primary wire connections on the coil with an ohmmeter. The resistance should be 6,000 to 12,500 ohms. Again, a resistance outside of this range indicates a broken coil.
- "Service Manual for Softail Models"; Harley-Davidson; 1997
- Dan's Motorcycle: Electrical Troubleshooting
- Try testing the coil after the motorcycle has been run and the coil is hot. The resistances measured across the terminals could change under heat.
- Find exact ranges for the resistance on a coil in the service manual for your particular make and model of motorcycle.
- Use caution when working around electricity. Remove all watches and rings to help guard against shock.