Mercury Mariner V-250 V-275 Outboard Service Shop Manual 1990, Page 328Get this manual
Problem: Warning Horn Soundwhen Engine'RunningOil Level In Engine Reservoir is FullPottlble Cause Correclive Action
Faulty eng ine igniti on system (incorrect voltage being sent to the warning module)
Look at the ignition coi lead connections on the ignition switch box and determine what coi lead has the green wi re from the warni ng module connected to itCheck that coi for co rrect voltage using the OVAIf the voltage to the coi is correctthen the voltage to the warning modu le is correctDo not remove cap from oil reserviorDisconnect both tow oi sensor leads from terminal connectorsConnect an ohmmeter between leadsThere should be NO continuity thru sensor, If continuity existssensor is faulty