Cummins/Onan — Testing the Alternator
Testing & diagnosis · Alternator windings
From a Cummins/Onan service manual covering: C13 N6H, C13N6H, C17 N6H, C17N6H, C20 N6, C20N6H, RS13A. Check it applies to your exact model before use.
Applies to: Cummins/Onan C13 N6H, Cummins/Onan C17 N6H, Cummins/Onan C20 N6, Cummins/Onan RS13A
Before you start
- Disconnect the AVR before doing any insulation resistance test. Discharge the windings by shorting to earth through an earthing rod for at least 5 minutes.
Procedure
- Test the rotor for grounded, open and shorted windings using an ohmmeter, testing at the brush block terminals first.
- For rotor ground resistance, set the ohmmeter to the highest scale or use a megger; touch one probe to the rotor shaft and the other to a slip ring.
- For rotor winding resistance, connect each meter test lead to a separate slip ring and compare to the resistance table.
- Test for a short between the main windings and the quad windings.
- For stator ground resistance, touch one test probe to the stack and, in turn, the other to each stator lead.
- Measure stator winding resistance with a meter having precision down to 0.001 Ohms.
- Measure each winding to the stator laminations to check for bad insulation.
Tools required
- Ohmmeter Amazon
- Megger or insulation resistance meter Amazon
- Wheatstone bridge (0.001 Ohms precision) Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Rotor ground: > 1 megohm on megger or infinity on ohmmeter
- Rotor winding F1-F2: 13 kW = 26 +/-10% Ohms; 17/20 kW = 30.5 +/-10% Ohms
- Stator L1-N: 13 kW = 0.112 +/-10% Ohms; 17/20 kW = 0.042 +/-10% Ohms
- Q1-Q2: 13 kW = 2.56 +/-10% Ohms; 17/20 kW = 1.45 +/-10% Ohms
Full service-parts list: Cummins/Onan C20 N6 →
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.