1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 35Get this manual
Optional Voltage Regulator Test 1Check battery voltage at battery with engine running2If battery voltage is above 14.5 volts, replace voltage regulatorCheck condition of battery as overcharging may have damaged battery3If battery voltage is below 14.5 volts, charge battery; refer to "Charging Discharged Battery", precedingIf battery can NOT be satisfactorily charged, replace battery4If battery accepts satisfactory charge, check battery voltage while cranking engine; refer to "Charging Discharged Battery", precedingIf cranking voltage is not acceptable, replace battery5If cranking voltage is acceptable, disconnect end of RED wire (located between rectifier (+) terminal and starter solenoid) from rectifierSecure RED wire (from voltage regulator) to rectifier (+) terminal with hex nut6Connect RED (+) ammeter lead to (+) terminal of rectifier and BLACK () ammeter lead to RED wire (disconnected in last step)IMPORTANT: For accurate test results the voltage at battery with engine running, in next step, must be 13.5 volts or lessIt may be necessary to operate electrical accessories to drop voltage to 13.5 volts or less7Run engine at 3000 RPM8Meter should read betweenamperes9If meter readsamperes, this indicates the charging system is functioning properly and the battery is being discharged because the amperage draw on the system is greater than the amperage output of the system