Actually I want to disconnect it to eliminate or verify that this is where I'm getting interference on a new sonar unit I recently installed.
Kevin, The tach sender wire is grey. You will find it on the motor, connected to the water cooled voltage rectifier on top of the cylinders, behind the flywheel. You may be able to pull it out of the connector, to run your test. Alternatively you can disconnect the whole V/R and run your test.