Onan MJA Series Service Manual page 100

98
REVOLVING FIELD GENERATOR
SWITCH
FIG 4.1 - FLASHING THE FIELD. SEE SECT 4. 3.l.1
If
output voltage won't build up after pushing the reset button, flash the
field. To flash the field connect a voltmeter across the ac output. Then
run the plant and touch the leads of a 12 volt battery in series with an 8 ­
ID ohm current limiting resistor to the exciter-to-brush leads ... pos­
itive (+) to F1 and negative (-}to F2. If the resistor weren't used, bat­
tery current could destroy the Magneciter rectifiers. Watch the voltme­
ter. If voltage builds up to normal, the trouble was due. to lost residual
in the field. If the voltmeter indicates a low voltage, the Magneciter is
probably defective (See Sect 4.3.3 and the Magneciter trouble-shooting
chart for repair).
If
there is no voltage output with the battery connect­
ed to F:l and F2, trouble is in the alternator (See Sect 4.3.2 for testing
and repair)".
4.3.1. 2 Under-Voltage Condition. - Either the alternator or Magneciter
could be defective. But, the defect is probably in the Magneciter so
check
it
first, using Sect 4.3.3 and the Magneciter trouble-shooting
chart.
4.3.1.3 Over-Voltage or Fluctuating Voltage. - If the engine is opera ­
ting at the correct speed, trouble is most likely in the Magneciter . See
Sect 4.3. 3and the Magneciter trouble-shooting chart.
4.3.2 ALTERNATOR TESTING. - Most alternator testing can be per­
formed without disassembling the generator.
4. 3.2. 1 Rotor Continuity Tests. - Remove the brushes so none touches
the collector rings.
1. Using an ohmmeter, test for grounding between each slip ring and
the rotor shaft.