How to Troubleshoot a Johnson 150 GT Power Pack

by Will Charpentier

On the Johnson 150 GT, the current that runs the ignition system and the timing advance are managed by the power pack. The power pack is mounted on the right side of the engine block. Wiring from the sensor assembly and the power coil runs to the power pack and wires from the power pack are connected to the primary side of the ignition coil. Troubleshooting includes the testing of the spark at the engine, the connections to the key switch, stop button, emergency stop lanyard and clip.

Inspect the exterior of the power pack for physical damage. Unplug the 5-pin Amphenol connector that has a black wire with yellow stripes protruding from the power pack; this effectively disconnects half of the wiring harness. The plug is located between the power pack and the ignition coil mounting plate beneath the flywheel.

Plug the four jumper wires into terminals A, B, C and D on both of the parts of the 5-pin Amphenol connector to isolate the terminal E. Tag the spark plug wires with pre-wired paper tags, to indicate which wire serves each plug. Twist and disconnect the spark plug leads.

Connect the alligator clip of a spark tester to the engine block. Connect the the plug wire from the no. 1 plug to the spark tester. Crank the motor while watching the spark tester. A spark indicates the problem is in the stop circuit.

Remove the jumper wires. Set the multimeter to "ohms." Place one multimeter probe in terminal E and the other on the engine ground; with the emergency stop clip in place and the key switch in the run position, the multimeter must show low or zero resistance. Remove the clip and press the stop button. If the multimeter does not show low or zero resistance, replace the stop button.

Turn the key switch to the "On" position. Remove the black and yellow wire from the key switch. If the meter shows high or infinite resistance, replace the key switch. Reconnect all wiring.

Look for an open circuit along the black and yellow lead or an open circuit on the black and white lead from the power pack, if the engine fails to shut down after normal operation. Reconnect or replace the wires as necessary. If there are no open circuits inspect the key switch and the power pack for hidden physical damage and replace as required.

Items you will need

  • 4 Jumper wires
  • Spark tester
  • Digital multimeter


  • "Johnson Outboard Repair Manual -- 1973-1991"; Seloc Marine; 2007

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.