Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 77Get this manual
Alternator System Circuitry Test Using 0-20 volt DC voltmeter, perform the following tests:
OUTPUT CIRCUIT
3Voltmeter should indicate battery voltageIf correct voltage is not present, check sensing circuit (RED lead) for loose or dirty connections or damaged wiring
1Connect POSITIVE (+) voltmeter lead to alternator terminal (output terminal)Connect NEGATIVE (-) lead to case ground on alternator2Shake alternator wiring harnessMeter should indicate battery voltage and should not varyIf proper reading is not obtained, check for loose or dirty connections or damaged wiring