Alternator System Circuitry Test - Mercury 200 OptiMax Jet Drive Service Manual

Charging and Starting System
6. Inspect the 100 amp fusible link located on the starboard side of the engine. If the fusible link is blown, check the battery leads
for reversed polarity connection before replacing the fusible link.

Alternator System Circuitry Test

1. Check the belt condition and tension.
2. Check the wire connections at the alternator for tightness and corrosion.
3. Check the alternator ground wire for tightness at the alternator and cylinder block.
4. Check the wire connections at the battery for tightness and corrosion.
5. Check the battery condition. The battery should be fully charged.
Output Circuit
Perform the following tests with a DMT 2004 digital multimeter to ensure the circuits between the alternator and all components
within the charging system are in good condition.
DMT 2004 Digital Multimeter
1. Connect the DMT positive (+) lead to the positive (+) battery terminal.
2. Connect the DMT negative (–) lead to the negative (–) battery terminal.
3. Supply cooling water to the engine.
4. Start the engine and increase the engine speed to 1300 RPM.
5. Observe the voltage reading.
6. If the reading is between 13.5 and 14.8 volts, switch the DMT to the AC volt position. A reading of 0.25 AC volts or less
indicates the alternator diodes are functional. A reading above 0.25 AC volts indicates the diodes are faulty and the alternator
must be replaced.
NOTE: A voltage reading between 13.5 and 14.8 volts are for starting systems without a battery isolator installed. A battery
isolator will have a parasitic voltage drop and the alternator will compensate for the voltage drop. The output voltage may be
as high as 19 volts.
A very high voltage level measured at the output terminal of the alternator may be an indication of an open circuit between
the output terminal and the battery. A blown fusible link is the most likely cause for an open circuit.
The reason for the high voltage reading is the alternator sense circuit is indicating reduced battery voltage and causes the
alternator to increase voltage output due to the blown fusible link. The voltage output could be as high as 27 volts. This higher
voltage is sometimes interpreted as a failed regulator and the complete alternator is mistakenly replaced.
7. If the reading is below 13.5 volts:
a. Connect the positive (+) DMT lead to the alternator output stud.
b. Connect the negative (–) DMT lead to a ground on the alternator.
c. 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.
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a - 100 amp fusible link
a
32034
91‑892647A01
90-8M0050731 MAY 2011