Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 44Get this manual
Diagnosis of Alternator System on Engine Perform the following checks to eliminate possible problem areasObserve preceding, to prevent
damage to alternator system
1If problem is an undercharged battery, verify condition
has nol been caused by excessive accessory current draw or by accessories which have accidentally left onCheck physical condition and state of charge of bat teryBattery must be at least 75 (1.230 specific gravity) of fully charged to obtain valid results in the following testsIf not, charge battery before testing systemInspect entire alternator system wiring for defectsCheck all connections for tightness and cleanliness,