armature core iron. Touch the other probe to each
commutator segment in turn. If the armature is
grounded, the bulb in the base of the growler will light.
In contacting armature surfaces with the test probes, do
not touch the bearing or the brush surfaces of the
commutator. The arc would burn or pit the smooth
finish. Replace the armature if it is grounded.
In testing individual armature coils for open
circuits, use the test probes, as shown in figure 2-20.
Place them on the riser part of adjacent commutator
Figure 2-20.Testing an armature for open circuits.
bars, not on the brush surfaces. If the test lamp does not
light, there is a break some where in the coil. Repeat
this test on every pair of adjacent bars. Do this by
walking the probes from bar to bar. Should you find an
open coil, the fault may be at the commutator
connectors where it is possible to make repairs with a
little solder. If a coil is open-circuited internally, the
armature should be discarded.
The alternator (fig. 2-21) has replaced the dc
generator because of its improved efficiency. It is
smaller, lighter, and more dependable than the dc
generator. The alternator also produces more output
during idle which makes it ideal for late model
The alternator has a spinning magnetic field. The
output windings (stator) are stationary. As the
magnetic field rotates, it induces current in the output
Knowledge of the construction of an alternator is
required before you can understand the proper
operation, testing procedures, and repair procedures
applicable to an alternator.
Figure 2-21.Typical alternator.