Piper COMANCHE PA-24-180 Service Manual page 571

FIGURE 9-279. TESTING ROTOR
FOR GROUNDS
FIGURE 9-2h. TESTING ROTOR
FOR SHORTS
a lead from one of the rectifiers in thit heat sink. Then reverse the position of the leads. The
test bulb should light in one direction and not light in the other direction. If the test bulb lights
in both directions, one or more of the rectifiers in thet heat sink is shorted. To pinpoint the de-
fective rectifier, the stator leads must be disconnected and the above test repeated on each
rectifier. Open rectifiers can only be detected, when using the test bulb, by disconnecting the
stator leads. The test bulb will fail to light in either direction if the rectifier is open.
c. Stator: The stator can be tested for open or grounded windings with a 12-volt test bulb,
described in the rectifier section, or an ohmmeter, in the following manner. Separate the stator
from the slip ring end head Just far enough to insert a fold of rags or blocks of wood. In other
words, insulate the stator from the end head. To test for grounded widings,
touch one test bulb
or ohmmeter probe to the stator frame. If the test bulb lights, or the ohmmeter indicates con-
tinuity. the stator is grounded. To test for open windings, connect one test probe to the auxiliary
terminal or the stator winding center connection and touch each of the three stator leads. The
rest bulb must light, or the ohmmeter must show continuity. Due to the low resistance in the
stator windings, shorted windings are almost impasible
to locate. However, shorted stator
windings will usually cause the alternator to "growl" or be noisy during operation and will
usually show some signs of overheating. If all other electrical checks are normal and the al-
ternator fails to supply its rated output, the stator should be replaced to determine whether or
not it is the faulty component.
d. Bearings and Setls Whenever the alternator is overhauled, new bearings and oilor grease
seals are recommended, even though the bearings and seals appear to be in good condiuon. A
faulty seal can cause an alternator to fail within a very short period of time.
ELECTRICAL SYSTEM
ISSUED: 8/18/2
3 C1