1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 262Get this manual
Trim Sender (Optional Accessory) Test Check trim sender BLACK lead for proper groundTrim engine to full "Down" positionPlace ignition switch in "Off" positionConnect Ohmmeter (R scale) leads between engine ground and Point 1Depress "Up" trim buttonOhmmeter needle should move as the engine is trimmed upIf needle does not move, the trim sender is defectiveBRNWHT
Trim Indicator Gauge (Optional Accessory) Needle Adjustment
Trim engine to full "Down" positionIf trim indicator needle is not at the bottom of the green arc on the gauge face, tilt engine "up" to gain access to the trim senderLoosen screws and reposition trim sender, as shownTrim engine to full "Down" position and recheck needle position