Cummins/Onan — Testing Rotating Rectifiers
Testing & diagnosis · Rotating rectifier assembly
From a Cummins/Onan service manual covering: MDL3, MDL4, MDL6. Check it applies to your exact model before use.
Applies to: Cummins/Onan MDL3, Cummins/Onan MDL4, Cummins/Onan MDL6
Procedure
- Disconnect all leads from assembly to be tested.
- Connect one test lead to F1+ stud and other lead to CR1, CR2, and CR3 in turn; record resistance of each rectifier.
- Connect one lead to F2- stud and other lead to CR4, CR5, and CR6 in turn; record resistance.
- Reverse ohmmeter leads and record resistance of each rectifier again.
- All resistance readings should be high in one test and low in the other; if high or low in both, rectifier assembly is defective.
- Replace defective rectifier assembly with new identical part.
Tools required
- Accurate ohmmeter Amazon
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Use 23 to 26 inch lbs (2.6 to 2.9 N·m) torque when replacing nuts of F1+ and F2-, CR1, CR2, CR3, CR4, CR5, and CR6.
Full service-parts list: Cummins/Onan MDL3 →, Cummins/Onan MDL4 →, Cummins/Onan MDL6 →
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.