TM 5-3810-307-24-1-2
Wiring: Inspect the wiring for dam-
shoe screw will cause the armature
voltage is desired, a carbon pile
age. Inspect all connections to the
to not turn freely. If the armature
connected across the battery can
cranking motor, solenoid, magnetic
does not turn freely the motor
be used to reduce the voltage to
switch, ignition switch or any other
should be replaced immediately.
the specified value. If more than
control
switch,
and
battery,
However, if the armature does
one 12-volt battery is used,
including all ground connections.
rotate freely, the motor should be
connect the carbon pile to only one
Clean and tighten all connections
given a no-load test before
of the 12-volt batteries. If the
as required. The cranking system
replacement.
specified current draw does not
cannot operate properly with
include the solenoid, deduct from
The no-load test may point to
excessive resistance in the circuit.
the ammeter reading the specified
specific defects. Also, the no-load
current draw of the solenoid hold-in
Magnetic Switch, Solenoid and
test can identify open or shorted
winding. Make disconnections only
Control Switches:
Inspect all
fields. The no-load test also can
with the switch open. Interpret the
switches
to
determine
their
be used to indicate normal
test results as follows:
condition. From the vehicle wiring
operation on a repaired motor
1. Rated current draw and no-load
diagram, determine which circuits
before installation.
speed indicates normal con-
should be energized with the
No-Load Test (Fig. 4)
dition of the cranking motor.
starting switches closed. Use a
Connect a voltmeter from the
2. Low free speed and high current
voltmeter to detect any open
motor terminal to the motor frame,
draw indicate:
circuits.
and use an r.p.m. indicator to
a. Too much friction tight,
measure
armature
speed.
Motor: If the battery, wiring and
dirty, worn bearings, bent
Connect the motor and an
switches
are
in
satisfactory
armature shaft or loose pole
ammeter in series with a fully
condition, and the engine is known
shoes allowing armature to
charged battery of the specified
to be functioning properly, remove
drag.
voltage, and a switch in the open
the motor and follow the test
b. Shorted armature.
position from the solenoid battery
procedures outlined below.
c. Grounded armature or fields.
terminal to the solenoid switch
3. Failure to operate with high cur-
A cranking motor is designed for
terminal. Close the switch and
rent draw indicates:
intermittent duty only, and should
compare the r.p.m., current, and
a. A direct ground in the
never be operated for more than
voltage
reading
with
the
terminal or fields.
30 seconds at a time. After 30
specifications in SB 1M-188 (page
b. "Frozen"
bearings
(this
seconds, the cranking must be
E-7). It is not necessary to obtain
should
have
been
stopped for at least two minutes to
the exact voltage specified in these
determined by turning the
allow the motor to cool.
bulletins,
as
an
accurate
armature by hand).
interpretation can be made by
With the cranking motor removed
4. Failure to operate with no cur-
recognizing that if the voltage is
from the engine, the armature
rent draw indicates:
slightly higher the r.p.m. will be
should be checked for freedom of
a. Open field circuit.
proportionately higher, with the
rotation by prying the pinion with a
b. Open armature coils.
current remaining essentially un-
screwdriver. Tight bearings, a bent
c. Broken brush springs, worn
changed. However, if the exact
armature shaft, or a loose pole
brushes, high insulation be-
tween the commutator bars
or other causes which would
prevent good contact be-
tween the brushes and
commutator.
5. Low no-load speed and low cur-
rent draw indicate:
a. High internal resistance due
to poor connections, de-
fective leads, dirty com-
mutator and causes listed
under Number 4.
6. High free speed and high
current draw indicate shorted
field.
Figure 4 No-load test circuit.
Change-1 E-5/(E-6 Blank)