Cummins/Onan 5.0JB troubleshooting: 19 fault codes and symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan 5.0JB — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Overheating | Dirty cooling surfaces, operating without the engine air housing, and incorrect installation. | |
| No Voltage Buildup | Remove the exciter cover and with the plant tuning, operate the residual reset button on the Magneciter. If output voltage won’t build up after pushing the reset button, flash the field. | |
| Over-Voltage or Fluctuating Voltage | If the engine is operating at the correct speed, see Magneciter Troubleshooting Chart. | |
| Generator will not build up voltage. | Circuit breaker in '‘off’' or ‘tripped’? | Reset and close breaker. |
| Open in circuit breaker. | Stop plant and check breaker continuity. | |
| No AC power to Magneciter. | Check AC voltage at E1-E2 with the plant operating, should be 5 percent of the rated voltage. If not, check continuity from EyEa back to generator. | |
| Partial loss of residual in rotor. | With plant operating, jumper from E2 to heat sink of field rectifier Z until voltage begins to build up. Then remove. | |
| Pair of field rectifiers open (either W and Z or X and Y). | Test rectifiers and replace if defective. | |
| Both field rectifiers X and Y shorted. | Test rectifiers and replace if defective. | |
| Output voltage slow to build up. | Either field rectifier X or Y shorted. | Test rectifiers and replace if defective. |
| Output voltage slow to build up and higher than rated voltage after build up. | Open circuit in one or more control rectifier. | Test rectifiers and replace if defective. Check soldered connections to rectifiers. |
| Output voltage slow to build up and 10 to 20 percent above rated voltage after build up. | Open circuit in one field rectifier. | Test rectifiers and replace if defective. |
| Output voltage builds up normally but less than rated voltage at no load and low voltage at full load. | Shorted winding in control reactor. | Test control reactor and replace if defective. |
| Output voltage builds up normally with slightly less than rated voltage at no load and 20 percent above rated voltage at full load. | Compound winding S1-S2 in- stalled backward or has open circuit through one of compound windings through reactors A and B and test for continuity. | |
| Output voltage builds up normally but 25 percent above rated voltage after build up: No regulation possible. | Open circuit in control rectifier bridge. | |
| Output voltage builds up normally but is 125 to 150 percent above rated voltage after build up. | Shorted turn in gate winding Gi-G2 of reactor A or B. | Test reactors A and B for shorted turns and replace if defective. |
| Output voltage builds up normally but 150 to 200 percent above rated voltage after build up. | Control winding C1-C2 or reactor A or B polarized incorrectly. | Check circuit connections of both reactors A and B. |
| Output voltage builds up normally but 150 to 200 percent above rated voltage after build up. No regulation possible. | Shorted turn in control winding C1-C2 or reactor A or B. | Check circuit connections of both reactors A and B. |
| Generator voltage fluctuating while engine running at constant speed. | Check resistance and reset. (4) |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Dirty cooling surfaces, operating without the engine air housing, and incorrect installation.. See the table for the matching fixes.
Documented causes: Circuit breaker in '‘off’' or ‘tripped’?. See the table for the matching fixes.
Documented causes: Either field rectifier X or Y shorted.. See the table for the matching fixes.