Mercury MerCruiser MCM 454 Mag MPI Service Manual page 376

SERVICE MANUAL NUMBER 23
2. Test rotor field circuit for grounds (Test 2), using an ohmmeter (set on R x1 scale) as
IMPORTANT: If alternator has output at low speeds, but no output at high speeds,
rotor field winding may be shorting or grounding out, due to centrifugal force.
Replace rotor if all other electrical components test good.
STATOR
IMPORTANT: Stator leads MUST BE disconnected from rectifier for this test.
1. Test stator for grounds (Test 1), using an ohmmeter (set on R x1 scale) as follows:
2. Test for opens in stator (Test 2), using an ohmmeter (set on R x1 scale) as follows:
3. A short in the stator is difficult to detect without special equipment, because of the low
Index
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follows:
a. Connect one lead of ohmmeter to either slip ring and the other lead to rotor shaft or
pole pieces.
b. Meter should indicate no continuity (meter should not move).
c. If continuity does exist, complete rotor assembly must be replaced.
a. Connect one lead of ohmmeter to one of the stator leads and the other lead to stator
frame. Be sure that lead makes good contact with frame.
b. Meter should indicate no continuity (meter should not move). If continuity does exist,
stator is grounded and must be replaced.
a. Connect ohmmeter between each pair of stator windings (three different ways).
b. Continuity should be present in all three cases (meter should move). If it does not,
one or more of the windings are open and stator must be replaced.
Testing Stator
a - Test 1
b - Test 2
windings resistance. If all other electrical components test out good, and alternator fails
to produce rated output, stator probably is shorted and should be replaced. Also,
examine stator for heat discoloration, as this usually is a sure sign of a short.
a b
b
CHARGING SYSTEM
b
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