This is one of the simplest ignition components on an outboard motor. No matter which brand of outboard you have, the ignition coil performs the same basic job: it receives energy output from the power pack / switchbox and magnifies that energy into very high voltage for the spark plug.
That output can reach 15,000 volts or more at the spark plug. In other words, the coil takes the incoming ignition energy and boosts it to the much higher voltage needed to fire the fuel mixture in the cylinder.
Outboard ignition coils can fail in a couple of different ways. The outer housing can crack or break, allowing electrical leakage. Sometimes you can even see spark jumping outside the coil or wire at night. Internal windings can also burn out and stop the coil from working correctly.
One of the biggest killers of ignition coils is bad spark plug wires or bad spark plugs. When the coil creates that high voltage, the energy has to go somewhere. If the plug wire has too much resistance or the spark plug is not allowing the spark to discharge properly, the coil can overload itself.
Plug wire resistance can be checked with an ohmmeter. If a plug wire shows more than about 40 ohms resistance, it should be replaced. Spark plugs, on the other hand, either fire correctly or they do not. If you have proper spark reaching the plugs according to spec, but very little or nothing comes out of them, it is time to replace the plugs.
Misfiring spark plugs from heavy carbon buildup or other issues can also create ignition problems that look like a bad coil. So when diagnosing ignition trouble, check the plugs and wires before assuming the coil itself is the only problem.
If you need to test your coil or coils, use the Outboard Troubleshooter section for guidance.
