Onan MJA Series Service Manual page 101

99
",
2. Test for short or open circuit in the rotor winding, by measuring
the winding resistance between the slip rings.
It
should measure
behveen 6 and 6. 5 ohms for the MJB and between 2. 5 and 3 ohms
for the
Mic
(at
70
0
F). If an accurate ohmmeter isn't available,
check the rotor for open circuit or grounding with a dc test lamp
(Fig. 4.J).
Replace the rotor if it is grounded,
or has an open circuit or short.
4.3.2.2 Stator Continuity Tests .
Disconnect the generator output
leads in the control box. Use the
wiring diagrams in Sect 10 to de­
termine the output lead coding.
1.
Using either the test lamp or
an ohmmeter, check each
TEST LAMP
winding of the stator for
fOR TESTING FOR OPEN CIRCUITS AND GROUNDS.
grounding to the laminations
USE A 3 OR 4 CANDLEPOWER BULB.
or frame.
FIG 4.J - CONTINUITY TEST
REVOLVING FIELD GENERATOR
.
NOTE: Some generators have
LAMP
ground connections to
the frame. Check the
wiring diagrams.
2. Using an accurate ohmmeter, test the resistance of each stator
winding. Compare the resistances obtained. All windings of equal
output voltage should indicate about the same resistance. An un­
usually low reading indicates a short; a high reading an open cir ­
cuit.
If the ohmmeter required for this test isn't avaiiable, check for
open circuits with the test lamp.
If
any windings are shorted, open-circuited or grounded, replace the
stator assembly. Before replacing the assembly, check the leads for
broken wires or insulation and replace any defective lead.
If
this does
not correct the fault, replace the assembly.
It
isn't practical to attempt
to rewind a defective stator except in a competent rewinding shop. .
4.3.2.3 Battery Charging Winding Tests. - Remove the lead from the
battery polarity reconnection block to ammeter at the ammeter. Install
a dc voltmeter between the lead and ground. At governed engine speed,
the average dc output should be 7 to 9 volts. If the output is incorrect,
test for open circuit or grounding in the leads and windings. If leads
are defective, replace them.
If
the winding is defective, replace the
stator.