Cummins/Onan — Testing Diodes
Testing & diagnosis · Exciter rectifier bridge (rotating rectifier assembly)
From a Cummins/Onan service manual — check it applies to your exact model before use.
Applies to: Cummins/Onan DFAA, Cummins/Onan DFAB, Cummins/Onan DFAC, Cummins/Onan DFBF, Cummins/Onan DFCB, Cummins/Onan DFCC, Cummins/Onan DFCE, Cummins/Onan DFE
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.
- Using an ohmmeter, measure resistance between pigtail and mounting plate.
- Reverse meter test probes and repeat the tests.
- Resistance should be high in one direction and low in the other.
- If high or low in both directions, replace the diode.
Tools required
- Ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Full service-parts list: Cummins/Onan DFBF →, Cummins/Onan DFCC →, Cummins/Onan DFCE →, Cummins/Onan DFE →
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.