Mercury V6 175 Diagnostic Manual page 222

Charging and Starting System
iii. Wiggle the engine wiring harness while observing the voltmeter. The meter should indicate the approximate
battery voltage and should not vary. If no reading is obtained or if the reading varies, inspect the wiring harness
for loose connections, corrosion, breaks, or shorts. Repair or replace the harness as required.
iv. If the reading is 18 volts or above at the alternator output stud, test the fusible link for continuity. A failed fusible
link will prevent the alternator's charge from reaching the battery.
b. If the reading is between 13.5 and 14.8 volts, switch the DMT to the AC volt position. A reading of 0.25 VAC or less
indicates the alternator diodes are fully functional. A reading above 0.25 VAC indicates the diodes are faulty and the
alternator must be replaced.
c. If the reading is above 15 volts at the battery, the alternator is overcharging and must be replaced.
Sensing Circuit
1. Unplug the engine harness connector from the alternator.
2. Connect the positive (+) DMT lead to the red pin (pin D) and the negative (–) DMT lead to the alternator ground post.
3. The DMT should indicate the battery voltage. If battery voltage is not present, check the red lead for a loose or dirty
connection or damaged wiring.
b
Excitation Circuit
NOTE: The ignition key must be in the ON position (engine not running).
1. Unplug the engine harness connector from the alternator.
2. Connect the positive (+) DMT lead to the purple pin (pin C) and the negative (–) DMT lead to the alternator ground post.
Page 6B-8 © 2018 Mercury Marine
a - Sensing circuit red lead
b - Alternator ground post
a
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90-8M0146617 eng JULY 2018