Cummins/Onan — Testing the Generator Windings
Testing & diagnosis
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan MDGDA, Cummins/Onan MDGDB
Before you start
- Accidental starting of the generator set while working on it can cause severe injury or death. Disconnect the starting battery cables (negative first).
- Always remove the negative (-) cable first and reconnect it last.
Procedure
- Disconnect the starting battery cables (negative first) to prevent accidental starting.
- Disconnect the applicable winding leads from their terminals.
- Measure insulation resistance between a lead and the laminations (replace if less than 1 megohm).
- Measure winding resistance with a Wheatstone bridge or ohmmeter having at least 0.001 ohm precision.
- Compare winding resistance to Table 5-1 and replace the component if not as specified.
Tools required
- AC/DC multimeter Amazon
- Ohmmeter Amazon
- Wheatstone bridge (0.001 ohm precision) Amazon
- Frequency meter Amazon
- Load test panel Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Insulation resistance must be at least 1 megohm (1,000,000 ohms)
- Winding resistance per Table 5-1 (plus or minus 10 percent at 68 F / 20 C)
Full service-parts list: Cummins/Onan MDGDA →, Cummins/Onan MDGDB →
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.