Cummins/Onan — Testing Diodes (Rotating Rectifier Assembly)
Testing & diagnosis
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan DGBB, Cummins/Onan DGBC, Cummins/Onan DGCA, Cummins/Onan DGCB, Cummins/Onan DGDA, Cummins/Onan DGDB, Cummins/Onan DGEA, Cummins/Onan DGFA, Cummins/Onan DGFB, Cummins/Onan 4B3.9 engine
Before you start
- Brush dust off regularly; layers of dust can cause diodes to overheat and fail
Procedure
- Disconnect the diode pigtails from the terminal posts.
- Using an ohmmeter, measure resistance between each diode pigtail and the plate on which it is mounted.
- Reverse the meter test probes and repeat the tests.
- Resistance should be high in one direction and low in the other.
- If resistance is high or low in both directions, replace the diode.
- Torque diodes to 36 to 42 in-lbs (4 to 4.8 Nm) and pigtail terminals to 24 in-lbs (2.7 Nm).
Tools required
- Ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Diode torque 36 to 42 in-lbs (4 to 4.8 Nm)
- Pigtail terminal torque 24 in-lbs (2.7 Nm)
Full service-parts list: Cummins/Onan DGBB →, Cummins/Onan DGBC →, Cummins/Onan DGCA →, Cummins/Onan DGCB →, Cummins/Onan DGDA →, Cummins/Onan DGDB →, Cummins/Onan DGEA →, Cummins/Onan DGFA →, Cummins/Onan DGFB →
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.