When you check the coil on an outboard motor, you're looking for a certain level of electrical resistance on the circuit. If the resistance is too high or too low outside the ranges for the coil's primary and secondary resistance, you may have either a break in the circuit between the coil and the spark plug or a busted coil. If the resistance is within the acceptable range though, your coil and spark plug wires are in working order.
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Things you need
- Digital multimeter
Turn the digital multimeter's dial to the "omega" symbol, for ohms, the unit in which resistance is calculated. Connect the multimeter's red positive lead to the positive coil terminal, usually the terminal sprouting a coloured, rather than a black wire.
Push the black negative multimeter lead onto the negative terminal of the coil which is, of course, sporting a black ground wire.
Read the multimeter digital readout. The resistance showing should be less than 1 ohm; more specifically, it should lie somewhere between 0.02 ohm and 0.04 ohm. This is the coil's primary resistance.
Move the red meter lead to the spark plug lead to read the secondary resistance. The meter should show resistance of between 8,000 ohm and 11,000 ohms.
Remove the black multimeter lead from the negative terminal of the coil and the red lead from the spark plug. If the primary and secondary resistances were within the ranges specified, the coil is fine; if not, replace the coil.
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