TRUCK SERVICE MANUAL
d. A grounded brush lead clip can result from
Rotor Field Winding Checks
omission of insulating washer, Fig. 6,
omission of insulating sleeve over screw, or
To check for opens, connect the test lamp or
damaged insulating sleeve. Remove screw
ohmmeter to each slip ring. If the lamp fails to light or if
to inspect sleeve. If satisfactory, replace
the ohmmeter reading is high (infinite), the winding is
regulator as covered under "Brush Holder
open, Fig. 7.
and Regulator Replacement. "
The winding is checked for short circuits or
excessive resistance by connecting a battery and
ammeter in series with the edges of the two slip rings.
An ammeter reading above the specified value indicates
shorted windings; a reading below the specified value
indicates excessive resistance. If the winding is shorted,
replace the rotor and the regulator as covered in "Brush
Holder and Regulator Replacement."
An alternate method is to check the resistance of the
field by connecting an ohmmeter to the two slip rings,
Fig. 7. If the resistance reading is below the specified
value, the winding is shorted; if above the specified
value, the winding has excessive resistance. The
1. Remove four thru-bolts from drive end frame.
specified resistance value can be determined by dividing
the voltage by the current. Remember that the winding
2. Separate slip ring end frame and stator assembly
resistance and ammeter reading will vary slightly with
from drive end frame and rotor assembly.
winding temperature changes.
3. Separate stator from end frame by removing three
Diode Trio Check
stator lead attaching nuts.
The diode trio is identified in Fig. 6. To check the
4. Place tape over bearing and shaft to protect from
diode trio, remove it from the end frame assembly by
dirt. Use pressure-sensitive tape and not friction
detaching the nuts and attaching screw. Connect an
tape, which would leave a gummy deposit.
ohmmeter having a 11/2 volt cell and using the lowest
range scale to the single connector and to one of the
5. Inspect all leads for burned connections or opens,
three connectors, Fig. 8. Observe the reading. Then
and brushes for excessive wear. Inspect springs for
reverse the ohmmeter leads to the same two connectors.
distortion or discoloration. Replace as required.
If both readings are the same, replace the diode trio. A
Clean brushes with a soft, dry cloth if they are to be
good diode trio will give one high and one low reading.
reused. During servicing and reassembly hold
Repeat this same test between the single connector and
brushes and springs in holder with a pin or toothpick
inserted through end frame hole.