2005 Yamaha F90TR 90HP Outboards Factory Service Manual, Page 204Get this manual
Electrical systems NOTE: The ignition spark can also be checked using the "Stationary test" of the Yamaha Diagnostic System
Ignition and ignition control system
Checking the ignition spark
1Remove the spark plug wire cover
2Disconnect the spark plug caps from the spark plugs3Connect spark plug cap to the special service tool
Checking the spark plug wires
1Remove the spark plug wires from the spark plugs2Remove the spark plug wires from the ignition coils3Measure the spark plug wire resistanceReplace if out of specification
S6D88050
Spark checker: YM-34487 4Crank the engine and observe the spark through the discharge window of the special service toolCheck the ignition system if the spark is weakS60C8100