Never operate an alternator on an open circuit.
With no battery or electrical load in the circuit,
alternators are capable of building high voltage (50 to
over 110 volts) which may damage diodes and
endanger anyone who touches the alternator output
Alternator maintenance is minimized by the use of
prelubricated bearings and longer lasting brushes. If a
problem exists in the charging circuit, check for a
complete field circuit by placing a large screwdriver on
the alternator rear-bearing surface. If the field circuit is
complete, there will be a strong magnetic pull on the
blade of the screwdriver, which indicates that the field
is energized. If there is no field current, the alternator
will not charge because it is excited by battery voltage.
Should you suspect troubles within the charging
system after checking the wiring connections and
battery, connect a voltmeter across the battery
terminals. If the voltage reading, with the engine speed
increased, is within the manufacturers recommended
specification, the charging system is functioning
properly. Should the alternator tests fail, the alternator
should be removed for repairs or replacement. Do
NOT forget, you must ALWAYS disconnect the
cables from the battery first.
To determine what component(s) has caused the
problem, you will be required to disassemble and test
ROTOR TESTING.To test the rotor for
grounds, shorts, and opens, perform the following:
To check for grounds, connect a test lamp or
ohmmeter from one of the slip rings to the rotor
shaft (fig. 2-29). A low ohmmeter reading or the
Figure 2-29.Testing rotor for grounds.
lighting of the test lamp indicates that the rotor
winding is grounded.
To check the rotor for shorts and opens, connect
the ohmmeter to both slip rings, as shown in
figure 2-30. An ohmmeter reading below the
manufacturers specified resistance value
indicates a short. A reading above the specified
resistance value indicates an open. If a test lamp
does not light when connected to both slip rings,
the winding is open.
STATOR TESTING.The stator winding can
be tested for opens and grounds after it has been
disconnected from the alternator end frame.
If the ohmmeter reading is low or the test lamp
lights when connected between each pair of stator
leads (fig. 2-31), the stator winding is electrically
A high ohmmeter reading or failure of the test lamp
to light when connected from any one of the leads to
Figure 2-30.Testing the rotor for opens and shorts.
Figure 2-31 .-Testing a stator for opens.