Cummins/Onan — Testing Main Rotor Winding
Testing & diagnosis
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Be sure to remove both rotor leads so the rotating rectifiers are isolated. Failure to do this will damage the rectifiers.
Procedure
- Testing For Open or Shorted Winding: Use a digital ohmmeter.
- Disconnect main rotor field leads which connect to rotating rectifier assemblies at F1+ and F2-.
- Check resistance across F1+ and F2- leads. Resistance should be within ± 10% of values at 77°F (25°C): JB 2.76 to 2.82 ohms; JCA & JCB 2.50 to 2.55 ohms. If low, shorted turns; if high, open. Replace
- Testing For Grounds: Use a Megger or insulation resistance meter which applies 500 VDC or more.
- Disconnect main rotor field leads. Connect test leads between one of the two leads and rotor shaft. Meter should read 1 megohm (1,000,000 ohms) or greater.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- JB: 2.76 to 2.82 ohms at 77°F (25°C)
- JCA & JCB: 2.50 to 2.55 ohms at 77°F (25°C)
- 1 megohm (1,000,000 ohms) or greater
Source: Cummins/Onan service 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.