Cummins/Onan — Testing the Armature for an Open Circuit
Testing & diagnosis · Armature AC and DC windings
From a Cummins/Onan owner's manual covering: 4UF-3N41, 4UF-3N42, 7UF-3N41, 7UF-3N42, 7UF-3N61. Check it applies to your exact model before use.
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Procedure
- For AC winding: place one test prod on one slip ring and the other on a second ring; the test lamp should light.
- If there are three or four rings, test between each two in the same manner; install a new armature if open.
- For DC winding: place the armature in an armature growler connected to alternating current.
- Rub a smooth steel blade across the segments of the commutator.
- A spark should occur as the blade contacts two adjacent segments; if no spark, the winding between those segments is open circuited.
- Continue rotating and testing until the commutator segments have been tested completely around.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Source: Cummins/Onan owner's manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.