GMC T Series 1994 Repair Manual page 577

STARTER MOTOR TESTS
Before disassembling the starter motor for repair, the
following test should be made.
N O T I C E : N e v e r o p e r a t e
m o r e t h a n 3 0 s e c o n d s
p a u s i n g to a l l o w i t to c o o l f o r a t l e a s t tw o
m in u t e s . O v e r h e a tin g , c a u s e d b y t o o m u c h
c r a n k in g , w ill d a m a g e th e s t a r t e r m o to r .
NO-LOAD TEST
Connect a voltmeter from the motor terminal to the
motor frame and an RPM indicator to measure arma­
ture speed (figure 2). Connect the motor and an amme­
ter in series with a fully charged battery of the specified
voltage, and a switch in the open position to the switch
terminal. Close the switch and compare the RPM, cur­
rent, and voltage readings with the specifications at the
end of this section. It is not necessary to obtain the
exact voltage specified as a good reading can be made
by understanding that if the voltage is slightly higher the
rpm will be slightly higher, with the current remaining
basically unchanged. However, if the exact voltage is
desired, a carbon pile connected across the battery can
be used to reduce the voltage to the specified value.
Compare the test results as follows:
1. Rated current draw and no-load speed indicates
normal condition of the starter motor.
DIAGNOSIS
t h e s t a r t e r m o t o r
a t a t i m e w it h o u t
BATTERY
CARBON
PILE
R.P.M.
INDICATOR
A
2. Low free speed and high current draw indicates:
• Too much friction. Tight, dirty or worn bear­
ings, bent armature shaft or loose pole shoes
allowing the armature to drag.
• Shorted armature. This can be further checked
on a growler after disassembly.
• Grounded armature or fields. Check further
after disassembly.
3. Failure to operate with high current draw indicates:
• A direct ground in the terminal or fields.
• Seized bearings. This should have been noted
by turning the armature by hand.
4. Failure to operate with no current draw indicates:
• Open field circuit. This can be checked after
disassembly by inspecting internal connections
and tracing the circuit with a test lamp.
• Open armature coils. Inspect the commutator
for badly burned bars after disassembly.
• Broken brush springs, worn brushes. High
insulation between the commutator bars or oth­
er causes which would prevent good contact
between the brushes and commutator.
5. Low no-load speed and low current draw indicates
a higher internal resistance due to poor connec­
tions, defective leads, dirty commutator and causes
listed under step 4.
6. High free speed and high current draw indicates
shorted fields. If shorted fields are suspected,
replace the field coil assem bly and check for
improved performance.
AMMETER
VOLTMETER
J
batj e
lsfc
I
MJijg
L,
4
CRANKING 3
MOTOR
2906r46 92