1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 280Get this manual
Electrical System Troubleshooting GENERAL CHECKS Before troubleshooting the Power Trim electrical system, check the following: 1Check for disconnected wires
2Make certain all conditions are tight and corrosion free3Check that plug-in connectors are fully engaged4Make certain battery is fully chargedRefer to the preceding four wiring diagrams for connection points when troubleshooting the electrical systems (Connection points are specified by number)
Troubleshooting the "Down" Circuit (When "Up" Circuit is OK)
Connect Voltmeter RED lead to Point and BLACK lead to groundDepress the "Down" trim button
Battery Voltage Indicated: Connect Voltmeter RED lead to Point 9
No Voltage Indicated: Connect Voltmeter RED lead to Point and BLACK lead to groundDepress "Down" trim buttonIf battery voltage is indicatied, wire is open between Points and 1