Cummins/Onan — Testing Rotating Rectifier Diodes
Testing & diagnosis · Rotating rectifier assembly
From a Cummins/Onan service manual covering: 4BT3.9-GC1, 4BT3.9-GC2, 6BT5.9-GC1, 6BT5.9-GC2, 6CT8.3-GS/GC1, 6CTA8.3-GS/GC1, 6CTA8.3-GS/GC2. Check it applies to your exact model before use.
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Before you start
- Keep assemblies clean to avoid overheating.
Procedure
- Disconnect all leads from assembly.
- Connect meter lead to positive (X) stud and other to CR1, CR2, CR3 in turn; record resistance.
- Connect meter lead to negative (XX) stud and other to CR4, CR5, CR6 in turn; record resistance.
- Reverse meter leads and record resistance again.
- All resistance readings should be high in one test and low in the other.
- If any reading is high or low in both, replace rectifier assembly.
Tools required
- ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 24 in-lbs (2.7 N-m) for nuts of X and XX studs and CR1 and CR6
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.