1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 236Get this manual
Troubleshooting the "Up" Circuit (When "Trailer" Circuit Is OK) Remote Control Equipped with Trailer Button Use an Ohmmeter (R scale) and check continuity of wire between Point 10 on "Up" solenoid and Point on trim button
Continuity: Connect Voltmeter RED lead to Point (push button trim switch) or Point (toggle trim switch) and BLACK lead to ground
No Continuity: Wire is open between Point 10 and Point 7
Battery Voltage Indicated: Connect Voltmeter red lead to Point 7D Depress "Up" trim button
No Voltage Indicated: Wire is open between trailer button Point 11 and trim switch Point (push button trim switch) or Point (toggle trim switch)
Battery Voltage Indicated: Check for poor connection at Point 7
No Voltage Indicated: Trim switch is defective