Cummins/Onan MDKUB troubleshooting: 15 fault codes and symptom-cause-remedy entries from the manufacturer's own service documentation.
Cummins/Onan MDKUB — full specs, parts & maintenance.
All Cummins/Onan fault codes →
| Symptom | Cause | Remedy |
|---|---|---|
| Dizziness | Get out into the fresh air immediately. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired. | |
| Muscular Twitching | Get out into the fresh air immediately. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired. | |
| Vomiting | Get out into the fresh air immediately. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired. | |
| Weakness and Sleepiness | Get out into the fresh air immediately. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired. | |
| Inability to Think Coherently | Get out into the fresh air immediately. If symptoms persist, seek medical attention. Shut down the unit and do not operate until it has been inspected and repaired. | |
| Low Oil Pressure | Oil level is low. | Add oil to bring the level up to full mark. Inspect engine exterior for leaks and repair as necessary. |
| High Coolant Temperature | Check coolant level in the exhaust manifold after allowing the engine to cool down. Ensure pump belt is OK and has proper tension. The raw water flow at the exhaust outlet should be about 3 gal./min. (11.4 liter/min.). Also check cooling system cleanliness. | |
| High Exhaust Temperature | The high exhaust temperature switch closes on temperature rise above 230° F (110° C). It closes to shut down the set if raw water flow is lost. It opens again when the temperature reaches about 190° F (88° C). | |
| NO AC OUTPUT VOLTAGE AT RATED ENGINE RPM | Load circuit breakers closed, circuit open between brush block and voltage regulator, brushes stuck in holder or not making good contact with slip rings, opens or grounds in rotor, stator, incorrect field voltage, incorrect F1 and F2 leads installed on terminals | Locate cause of overload and correct as required, reset breaker or replace if bad, check continuity of circuit breakers and replace if necessary, check for open circuit between brush block and voltage regulator, release brushes if jammed in holder, clean slip rings if dirty, test continuity of rotor, stator, replace component if defective, test field voltage, check voltage regulator, examine brush block connections, connect brush leads to correct terminals |
| UNSTABLE OUTPUT VOLTAGE, ENGINE SPEED STABLE | Broken wires or loose connections on voltage regulator assembly, brushes not OK | Check wiring harness from regulator assembly to end bell, repair as required, inspect brushes, repair wiring or replace as required, replace the voltage regulator |
| OUTPUT VOLTAGE TOO HIGH OR TOO LOW | Set AC output overloaded | Remove part of the set load |
| UNBALANCED GENERATOR OUTPUT VOLTAGE | Load at generator terminals not removed, unbalanced output voltage still present | Remove load at generator terminals |
| Is battery dead? | Check CB8 circuit breaker. If okay, jumper battery cable B+ connection to K9 starter so-lenoid terminal. Does engine crank? With S10(S12) in Start position, is battery voltage present between K9 terminal S and ground? If K9 does not energize, test per Checkout [D] and replace if bad. Is battery voltage present at K9 solenoid terminal with S10(S12) in Start position? Check battery per Checkout [A] and recharge or replace. Check battery charger operation per Checkout [C]. Check battery cables for clean tight connections (ref. Checkout [B]). Check K9 starter so-lenoid and motor: if bad, repair or replace. Check B+ wiring to S10(S12), and between S10(S12) and K9. Check NC contacts of K16 per Checkout [E]. Replace if bad. | |
| Is engine getting fuel? Exhaust smoke should be blue-white and fuel flow steady from fuel return line. | Incorrect fuel? See Operator’s Manual recommendations. | Check fuel system: test fuel pump per Checkout [H], test fuel tank level, shut-off valves, fuel lines and connections, fuel filters, and injection pump. Okay? Does K12 fuel solenoid energize when S10(S12) is in Start position? Check B+ circuit through S10(S12). Test S11 per Checkout [G]. Test K12 per Checkout [F]. Check switch S10(S12) per Checkout [G]. Check heater relay per Checkout [E]. Check heater wiring. |
| Has control circuit breaker CB8 opened? | Check B+ wiring for shorts to ground causing excessive current draw. If none, check control circuit breaker CB8 for improper wiring. Check fuel solenoid adjustment. Does set stop when start switch is released? Does run relay K13 energize and contacts close when generator set comes up to operating speed? Check NC contacts of K15 shutdown relay. Check diode CR12. Check for AC voltage build-up during cranking. Check engine monitors for proper operation and wiring. See engine troubleshooting data in engine service manual. |
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Documented causes: Oil level is low.. See the table for the matching fixes.
Documented causes: Load circuit breakers closed, circuit open between brush block and voltage regulator, brushes stuck in holder or not making good contact with slip rings, opens or grounds in rotor, stator, incorrect field voltage, incorrect F1 and F2 leads installed on terminals. See the table for the matching fixes.
Documented causes: Broken wires or loose connections on voltage regulator assembly, brushes not OK. See the table for the matching fixes.