Voltage Drop Test (Alternator Circuit) - Mercury MerCruiser Service Manual

DMT 2004 Digital Multimeter
4. Supply cooling water to the power package.
5. Start the engine and operate at 1300‑1500 RPM. Read the DMT in the VDC position. Most systems will read between 13.8
and 14.8 volts.
6. If the reading is between 13.5 and 14.8 volts, switch the DMT to the VAC position and observe. A reading of 0.25 VAC or
less indicates that the alternator diodes are fully functional. A reading above 0.25 VAC indicates that the diodes are faulty
and the alternator must be replaced.
7. If the reading is below 13.5 volts:
a. Connect the positive (+) voltmeter lead to the alternator output post.
b. Connect the negative (‑) lead to the ground post on the alternator.
c. Manipulate 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, refer to Voltage Drop Test
(Alternator Circuit).
d. If the reading is above 15 volts, the alternator is overcharging and must be replaced.
e. If the voltmeter reading is now between 13.8 and 14.8 volts, there is too much resistance between the alternator and
the battery.
f. If the reading is, or drops below 12.5 volts, the alternator may not be charging. Check all wiring leading to the
alternator.

Voltage Drop Test (Alternator Circuit)

1. Supply cooling water to the power package.
2. Disconnect the coil wire so that the engine does not start.
3. Remove the fuel injector fuse or otherwise disable the injectors.
4. Crank the engine for 15 seconds to slightly discharge the battery.
5. Reconnect the coil wire, insert the fuel injector fuse, and verify that all accessories are off.
6. Connect the DMT positive (+) lead to the alternator output terminal. Connect the DMT negative (–) lead to the battery
positive (+) post.
90-8M0099748 eng DECEMBER 2015
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DMT connection
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DMT alternator connections
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Charging System
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