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Armature open-circuit test
Test to be conducted on bench
An open circuit armature may sometimes be detected by examining the commutator for evidence of burning. The burned area on the commutator is caused by an arc formed every time the commutator segment, connected to the open circuit winding, passes under the brush.
Armature field earthed circuit test
Test to be conducted on bench
This test will determine if the winding insulation has failed, permitting a conductor to touch the frame or armature case.
To determine if the armature windings are earthed, make the connections in Fig. 26. If the voltmeter indicates any voltage, the windings are earthed.
Earthed field windings can be detected by making the connections as Illustrated in Fig. 27. If the voltmeter indicates any voltage, the field windings are earthed.
Starter cranking circuit test
Excessive resistance in the starter circuit can be deter-mined from the results of this test; make the test connections as illustrated In Fig. 28. Crank the engine with ignition OFF. This is accomplished by disconnecting and earthing the high tension lead from the Ignition coil and by connecting a “jumper” lead from the battery terminal of the starter relay to the ignition switch terminal of the relay.
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The voltage drop in the circuit will be indicated by the voltmeter (zero to 1 volt range). Maximum permissible voltage drop is to be as follows
(a) With the voltmeter negative lead connected to the starter terminal, and the positive lead connected to the battery positive terminal, as shown in Fig. 28, the reading should be 0.5 volts.
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