GMC T Series 1994 Repair Manual page 609

ELECTRICAL TESTS
Except as stated, make the following tests with an
ohmmeter on the low range scale.
ROTOR FIELD WINDING CHECKS
The rotor may be checked electrically with a self-pow­
ered test lamp or an ohmmeter (figure 10).
Open Winding
To check for opens, connect the test lamp or ohmme­
ter to each slip ring. If the lamp fails to light, or if the
ohmmeter reading is high (infinite), the winding is open.
Replace the rotor.
Grounded Winding
Connect a test lamp or ohmmeter from one slip ring
to the shaft. If the lamp lights, or if the reading is low,
the rotor winding is grounded. Replace the rotor.
Short Circuit or Resistance Check
Check the winding for short circuits or excessive
resistance by connecting a battery and ammeter in
series with the edge of the two slip rings. Note the
ammeter reading and refer to " Specifications" later in
this section. An ammeter reading above the specified
value indicates shorted windings.
An alternate method is to check the resistance of the
field by connecting an ohmmeter to the two slip rings. If
the resistance reading is above the specified value, the
winding has excessive resistance; if below the specified
value, the winding is shorted. The specified resistance
value can be determined by dividing the voltage by the
current.
Remember that the winding resistance and ammeter
readings will vary slightly with winding temperature
changes. If the rotor is not defective, but the generator
fails to supply rated output, the problem is in the rectifi­
er bridge, stator, or regulator.
If the rotor fails any of the above checks, replace it.
STATOR CHECKS
Check the stator with a self-powered test lamp or an
ohmmeter (figure 11). If the ohmmeter reads low, or if
the lamp lights when connected from any stator lead to
bare metal on the stator frame, the stator is grounded.
An ohmmeter can not be used to test the delta stator
for shorts or opens. Noticeable discoloration at any
place on the assembly usually indicates a problem in
the stator windings.
RECTIFIER BRIDGE CHECK
To check the rectifier bridge, connect an ohmmeter to
the grounded heat sink and one of the three terminals,
pressing down firmly on the flat metal clip (figure 12).
Observe the ohmmeter reading and reverse the lead
connectors to the grounded heat sink and the same
metal clip. If both readings are the same, replace the
rectifier bridge. A good rectifier bridge will give one high
and one low reading. Repeat this same test between
the grounded heat sink and the other two terminals, and
between the insulated heat sink and each of the three
terminals. This makes a total of six checks, with two
readings taken for each check.
Some digital ohmmeters cannot be used to check
diodes in the rectifier bridge. Consult the ohmmeter
manufacturer to determine your ohmmeter capabilities.
REGULATOR
The regulator cannot be checked. If the rotor, stator,
and rectifier bridge are OK, but the generator still pro­
duces more than 16 volts or does not produce within 15
amperes of the rated output during a generator output
test, replace the regulator.