Figure 4-20.-Open diode pattern.
though both output current and voltage regulation
appear to be acceptable. As a general rule, a shorted
diode affects the output more than an open diode does.
It not only reduces the output, but it also opposes the
next pulse by allowing the current to flow back through
the winding containing the shorted diode.
As you can see from the screen pattern in figure
4-21, there is no interruption in the rectification of the
diodes. However, there is a high and low peak every
sixth pulse, indicating that the output of one diode is low
and that it may be deteriorating (high resistance). This
pattern may also occur due to a shorted winding since
the number of windings determines the amount of output
as well as the condition (resistance) of the diodes.
Depending on the location of the short, shorted
windings and shorted diodes produce similar screen
patterns because the defect is the same. Compare figures
4-19 and 4-22. The alternator test screen patterns shown
arc for diagnosis only; therefore, the alternator must be
Figure 4-22.-Shorted winding pattern.
removed to locate the defective internal component.
Now, it is a matter of verifying the problem with simple
ohmmeter tests or by replacing defective components.
TROUBLESHOOTING THE CRANKING
SYSTEM USING THE BATTERY
To determine whether a battery is fit for service, you
can perform a cranking system test with a battery starter
tester, model BST, as shown in figure 4-23. This tester,
made by Sun Electric Corporation, is designed to test
only batteries and starting systems of vehicles using 6-,
12-, 24-, or 32-volt systems.
Figure 4-21.-Poor diode pattern.
Figure 4-23.-Cranking voltage test.