How to Test the Ignition Parts for a Johnson Outboard
A properly charged battery produces at least 12 volts. When the electricity passes through a component, its internal resistance drops voltage a bit, and damaged parts stop the show altogether. If that 12-volt charge drops below 11.5 volts, your motor might not start. You're looking for three things: the sufficiency of voltage in the system, a power loss in a particular component that affects the system's performance, and high resistance, which indicates an internal wire break or a short circuit.
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Instructions
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Select the 20 V position on the digital multimeter's selector switch. Clip the alligator clip of the multimeter's black ground lead to the battery's negative post and touch the red probe to the battery's positive post. Turn the ignition key to the "On" position. A "12-volt" reading tells you that the battery is fully charged and in working order; if the reading is lower than 11.5 volts, install a new battery.
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Select the 20-volt position on the multimeter and connect the multimeter's black ground lead to a handy ground to test for a mismatched part while the part is in place in the ignition. Turn the ignition switch to "On" and set the red positive probe on the component's power lead -- a power drop on the multimeter means bad wiring or bad connectors that you need to replace. Move the probe to the positive electrical connection of the next component on the same circuit and look for a change in the voltage. A small change is acceptable, but if the multimeter's reading drops to 11.5, replace the part you first checked.
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Probe the negative side of the component with the positive multimeter lead and note the voltage readings. You should see a small voltage drop, caused by the resistance of the component. Repeat this test for each component in sequence. If a large voltage drop occurs, the component, the connector or the wire is suspect.
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Remove the cables from the battery, turn the key switch off and remove the key. Turn the multimeter to the "Ohms" scale and touch the positive probe to the the power lead of the component and the negative lead to the ground side of the component. High resistance is often an indicator of a short circuit or an open circuit in the ignition system. If you have a spare part of the same kind that you know to be good, check its resistance and compare those readings to the readings you get from the part you're testing.
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Tips & Warnings
If you are checking the components of the ignition system without removing them from your outboard motor, remember that when current flows through a component, the voltage on the circuit beyond the component drops slightly. This voltage drop is due to the resistance created by the small amount of resistance created by corrosion at the wire junctions in the component or by damaged insulation.
