TM 10-3950-206-12
change from high voltage indicates that the defect is in
rheostat resistance eliminated, is the current draw of the
the brush or rotor circuit.
rotor (field) circuit.
(g) The rotor (field) circuit may be checked
(f) Generally, this is approximately 2.0 to
in more detail a' outlined in Test No. 4.
2.5 amperes at battery voltage (12.0-12.6v). If the
(7) Test No. 3, open isolation diode test.
ammeter tends to vary when the rotor is slightly turned,
(a) Conditions: Ignition switch on. Engine
the slip rings and brushes re- quire cleaning, refer to
running at fast idle.
Direct Support Maintenance.
(b) This test will determine if the isolation
(g) If normal rotor current can not be
diode is conducting alternator energy, from the rectifier
obtained, the alternator will require disassembly and
diode assembly, to the output stud. If the isolation diode
replacement of defective parts, refer to Direct Support
is open, alternator energy would not travel beyond the
Maintenance.
rectifier diodes. A high voltage may develop in the
(h) If a satisfactory ammeter reading is
alternator that could damage the voltage regulator.
noted, proceed with Test No. 5.
(c) With the engine running at fast idle,
(9) Test No. 5, voltage regulator operating
connect voltmeter, negative lead to ground, positive lead
voltage.
to the auxiliary terminal, (fig. 4-8C). Note voltmeter
(a) Conditions: Ignition switch on. Engine
reading at this point.
running at fast idle.
(d) Move voltmeter positive lead to the
(b) This test will determine operating level
alternator output terminal. Note voltmeter read- ing.
of the voltage regulator. Voltage tests are most accurate
(e) The voltage indicated at the output
when the alternator is charging a fully charged battery, as
(positive) terminal should be 0.8 to 1.2 volts lower than
current output is at minimum. Under this condition,
voltage noted at the auxiliary terminal. If voltmeter
charging voltage will rise to regulator limiting maximum.
reading at the output (positive) terminal is battery voltage
(c) A 1/4 ohm 25 watt resistor may be
(12.0-12.6 volts) the isolation diode is "open".
placed in series with the alternator. output lead to
(f) Report an open diode to Direct Sup-
reduce current to 8 to 10 amperes for a battery that is
port Maintenance for replacement
less than fully charged. The connections for the 1/4 ohm
(8) Test No. 4, field current test.
resistor and test ammeter is shown in figure 4-8E.
(a) Conditions: Ignition switch off. Engine
(d) Connect
voltmeter
negative
to
not running.
ground, positive to output terminal.
(b) This test will determine if the electrical
(e) Start engine, run for a few minutes to
circuit (field winding) of the alternator rotor is in good
stabilize component temperature. Voltage indicated, is
condition.
the level established by the voltage regulator. Usually
(c) High field current may produce
13.9 volts to 14.7 volts depending on regulator ambient
uncontrolled alternator output and probable damage to
temperature.
the voltage regulator. Low field current will result in low
(f) High voltage may be due to a poor
alternator output.
ground connection for the voltage regulator circuit board.
(d) Disconnect the field lead from the
Since the circuit board is insulated from the regulator
alternator brush connecting terminal.
Switch test
housing, one regulator lead, usually black, serves as the
ammeter to the low current scale (0 to 10 amperes). Set
ground connection. The ground lead must be a clean
control knob of field rheostat in maximum resistance
and tight connection at the alternator ground.
position. The field rheostat will prevent damage to the
(g) If ground connection is not faulty, the
test ammeter if the rotor winding is shorted or grounded.
regulator is probably inoperative and will require
Place the field rheostat and the test ammeter in a series
replacement (para 4-22).
circuit between the alternator output (positive) and field
(h) A low charging rate is usually due to a
terminals, as shown in figure 4-8D.
faulty regulator. Replace regulator if necessary.
(e) Slowly reduce field rheostat resistance,
(i) Remove the 1/4 ohm resistor from the
noting test ammeter reading. If entire field rheostat
alternator positive output lead immediately after this test.
resistance can be eliminated with the ammeter indicating
(10) Test No. 6, alternator output and system
less than 3.5 amperes, the field winding is not shorted or
test.
grounded. The test ammeter reading, with the field
4-15
