TM 10-3950-206-12
the battery to discharge through the voltage regulator to
ground.
(d) Refer to figure 4-8A, connect volt
meter, negative to ground, positive to output terminal on
the isolation diode, voltmeter will indicate battery voltage.
With the voltmeter connected at this point, check the
leads between the alternator and battery. If movement
of the leads causes the voltmeter reading to vary, check
for loose connections. Repair as needed.
(e) Move positive voltmeter lead to the
alternator auxiliary terminal. Voltmeter should indicate
zero volts. One of two faults will cause voltmeter to
indicate above zero volts, (1) the metal wire terminal on
the alternator output (positive) lead may be in contact
with the heat sink or (2) the isolation diode may be
shorted. Reposition wire terminal and retest. If diode is
defective, report to Direct Support Maintenance.
(6) Test No. 2, excitation voltage test.
(a) Conditions: Ignition switch on, engine
not running. (fig. 4-8B).
(b) This test will indicate if the excitation
current circuit, from the vehicle's ignition switch to the
Figure 4-7. Diagram of alternator terminal points.
rotor (field) winding is correct.
(c) When the current for the voltage
wire size than the original equipment. Wires and cables
regulator and the rotor winding is passed through the
must be free of corrosion with cable sup- porting clamps
field excitation resistor, a voltage drop develops across
to reduce strain on battery posts.
the resistor. This action reduces battery voltage from
(3) All leads, junctions, switches and panel
12.0-12.6 volts at the ignition switch side of the resistor,
instruments that are directly related to the charging
to 1.5-2.5 volts on the alternator side of the resistor. If
circuit must be clean and tight.
the circuit through the regulator or the rotor is not correct,
(4) The inspection of the alternator drive
and a lighter load was passed through the excitation
system should include the following:
resistor, the voltage drop would be less.
(a) Engine driving and alternator driven
(d) This circuit may be defective in two
pulleys must be capable of transmitting required energy
ways; an open circuit or a shorted circuit.
from the engine output shaft to the alternator.
(e) Connect
voltmeter
negative
to
(b) The drive belt must be in good
ground, positive to auxiliary terminal. 'Meter should
condition, free of grease or oils that may induce slip- ping
indicate 1.5-2.5 volts, or if the circuit is defective due to:
under load.
Warning:
Never
disconnect
any
Open rotor (field) circuit 6.0-7.0 volts.
alternator circuit or battery circuit cable from battery
Grounded rotor circuit, .75-1.1 volts.
or alternator with engine running.
Open in regulator's Load Circuit, 8.5-10.0 volts.
(5) Test No. 1, battery voltage and shorted
Open key switch, or open excitation resistor,
isolation diode test.
zero volts.
(a) Conditions: Ignition switch off. Engine
(f) If initial test results are uncertain, the
not running.
voltage regulator may be bypassed with a short jumper
(b) This test will determine if the battery is
between the auxiliary and field terminals. If the jumper
properly connected to the alternator. Secondly, the
provides approximate correct voltage, fault is in the
isolation diode will be tested for a shorted condition. An
regulator. No
improperly connected battery or a shorted isolation diode
can damage the alternator and/or voltage regulator.
(c) A shorted isolation diode would allow
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