How to Change Spark Plug Wires in a Mercury Outboard

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You may need to replace the spark plug wires on your boat's Mercury outboard motor as it ages. Running from the capacitor discharge module to the spark plugs, the spark plug wires carry the electrical charge from your boat's coil to the plugs that are found in the top of the motor's cylinders. A bad spark plug wire will make your motor misfire, waste fuel or, if there are several bad wires, refuse to start.

Items you will need

  • Open-end wrenches

Turn the ignition key "Off" and remove it from the switch. Remove the 5/16-inch nut from the negative post of the battery with the appropriate open-end wrench. Remove the negative/ground cable from the battery.

Trace the spark plug wire from the plug to the capacitor discharge module associated with that plug. There will be one module per plug.

Pull the spark plug wire from its push-on/pull-off connection on the capacitor discharge module. Pull the other end of the wire from the top of the spark plug.

Push the straight end of the new plug wire onto the connection on the capacitor discharge module and push the end with the 90-degree bend on the top of the spark plug. Repeat for all plug wires.

Replace the ground cable onto the negative post of the battery. Thread the 5/16-inch nut onto the post and tighten with the appropriate wrench.


  • Always disconnect the ground cable from the battery before doing electrical work on the boat.
  • Removing the ground cable from the battery that powers the motor will keep the motor from trying to start while you're trying to change the spark plug wires.


  • Keep a spare spark plug wire in your maintenance kit for each plug.


  • "Mercury Mariner Outboards 2.5-275HP 1990-2000 Repair Manual 2 Stroke, Including Fuel Injection & Jet Drives"; Seloc; 2010

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.