Cummins/Onan — Testing Rotating Rectifiers (Diodes)
Testing & diagnosis · Rotating rectifiers CR1-CR6
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Excessive heat on these rectifiers (diodes) will destroy them.
- Excessive dust or dirt on diodes and other components will cause overheating and eventually failure. Keep these assemblies clean!
Procedure
- Disconnect one diode at a time by removing diode from heatsink.
- Use proper size wrenches to hold diode body while removing nut.
- Push diode free of heatsink mounting hole.
- Use ohmmeter: connect one lead to flag of diode and other lead to stud. Observe reading. Reverse leads and observe reading. Good diode has much higher reading in one direction than the other.
- If both readings are high, or both readings are low, diode is defective and must be replaced with a new identical part.
- To replace: Unsolder leadwires of defective diodes from flag terminals.
- Insert new diode into heatsink mounting hole. Using nut and washer provided, secure diode to heatsink.
- Torque diodes on rotating exciter assembly to 24 in-lb (2.7 N·m).
- Solder leadwires to new diode flag terminals. Use a 40 watt soldering iron. Hold a needlenose pliers between diode and soldering point to prevent destructive heating.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 24 in-lb (2.7 N·m)
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.