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Most boaters don't have the equipment a professional has to test the coils on a Johnson outboard motor. What most boaters do have is a digital multimeter. When you talk about "the coils," you are talking about the primary and secondary ignition coils and you have to test both, using the multimeter. If the readings on the digital multimeter are outside of the limits established for the particular motor, take steps to ensure that the test was conducted correctly and retest the coil.
Items you will need
Disconnect the primary wire from the ignition coil. Select the "resistance" scale on you digital multimeter. Connect the black meter probe to a good ground, one that is on the engine if the coil is installed or to a tab on the coil itself, if the coil isn't installed on the motor. Connect the red meter probe to the primary terminal, the small terminal to which the wire coming from the power pack normally connects.
Read the multimeter. Resistance should be between 0.05 ohms and 0.15 ohms for two-stroke engines, except the 25-horsepower and 35-horsepower three-cylinder motors built before 2000 and on all carbureted four-stroke motors; the resistance on those motors should be between 0.23 ohms and 0.15 ohms. On breaker point ignition two-stroke engines the primary coil resistance should be between 0.7 ohms and 1.1 ohms.
Retest the primary coil if tests are out of specification, making sure that only the wires or connectors were probed. Make sure the ground was unpainted. If you can locate no outside cause for the readings being wrong, replace the coil.
Select the "resistance" scale on the digital multimeter. Disconnect the secondary coil wiring and connect one multimeter probe to the coil primary connector tower and the other to the adjacent secondary tower.
Note the multimeter's reading. If there is a second spark plug tower for coils that fire two spark plugs, move both meter leads to the primary and secondary tower pair for the other spark plug. Resistance should be between 225 ohms and 325 ohms for all motors except for 25-horsepower and 35-horsepower two-stroke motors built between 1995 and 1999. On these motors, the resistance should be between 2,000 ohms and 2,600 ohms.
Confirm all connections and readings if the coil tests out of specification. Replace the ignition coil if either the primary coil readings or secondary coil readings are well out of specification and all test connections and conditions were conducted properly.
- Disconnect the negative cable of your battery before performing any maintenance work on your outboard motor, to prevent electrical shock or accidental starting. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from your battery, move it outside of the battery box and close the lid of the battery box.
- On most carbureted models (except those with breaker point ignitions) it is best to remove the spark plug leads before checking the ignition coils. Spark plug leads on these models should have little or no resistance when in proper condition, but age, deterioration or problems with the connectors can cause high readings that can make an otherwise good coil test out of range. On breaker point models, test the coil with the secondary lead connected, so it may have been taken into consideration in the specifications.
- In contrast, EFI motors are tested with the secondary leads (70-horsepower motors) or connector (40-horsepower/50-horsepower motors) installed. In both cases, any resistance of the secondary lead or connector has been taken into consideration with the resistance specification. The secondary leads for 70-horsepower motors are resistor leads and should produce between 2,500 and 4,100 ohms resistance when in proper condition. The leads may be checked individually as well to ensure they are within specification.
- "Johnson Repair Manual 2.5 to 250 HP Models, 2002-2007"; Seloc Marine; 2007