Cummins/Onan — Testing and Replacing Rotating Rectifier Diodes
Repair & overhaul · Rotating rectifier assembly
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan DFHA, Cummins/Onan DFHB, Cummins/Onan DFHC, Cummins/Onan DFHD, Cummins/Onan QST30 engine
Before you start
- Layers of dust can cause diodes to overheat and fail; brush dust off regularly
Procedure
- Disconnect the diode pigtails from the terminal posts.
- Measure resistance between each diode pigtail and the plate it is mounted on.
- Reverse the meter probes and repeat the tests.
- Replace the diode if resistance is high or low in both directions.
- Apply heat-sink compound under the head of the replacement diode (not on threads).
- Torque the diodes to 36 to 42 in-lbs (4 to 4.8 N·m) and pigtail terminals to 24 in-lbs (2.7 N·m).
Tools required
- Ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Diodes 36-42 in-lbs (4-4.8 N·m)
- Pigtail terminals 24 in-lbs (2.7 N·m)
Full service-parts list: Cummins/Onan DFHA →, Cummins/Onan DFHB →, Cummins/Onan DFHC →, Cummins/Onan DFHD →
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.