Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 30Get this manual
IGNITION DIAGNOSTIC PROCEDURES TROUBLESHOOTING TIPWith engine runninguse ind uctive timing light to check spark advance of each cylinder as throttle is opened and closed If timing advances and retards smoothly on each cylinder, ignition system is MOST LIKELY functioning properlyPROBLEM 1No spark or weak spark on all cylinders
IMPORTANT: If outboard appears to have an ignition system failure, it is recommended that before begining in-depth troubleshooting1All grounds should be checked for proper continuity2Disconnect and reconnect ignition harness connectors to verify proper continuityIMPORTANT: The following corrections are listed in sequence of probable occurrenceRefer to "TESTING IGNITION COMPONENTS" to determine individual component failureVerify harness integrity BEFORE replacing componentsCORRECTION No SparkStop lead (BLKYEL) shorted to groundStator harness disconnected Weak SparkHigh resistance on sto lead (BLKYEL) resulting in voltage being bled off to ground
2No spark or weak spark on cylinder