The Cummins/Onan 6BT5.9 is a 78 kW (104 kVA) prime generator set powered by a Cummins/Onan 6BT5.9-D(M) engine. It holds 121 L (32.0 gal) of coolant, with a maximum ambient rating of 95 °C. The set weighs about 426 kg (939 lb).
Compiled from this model's family documentation — may include parts for closely-related variants. Confirm the exact part for your serial number.
Common questions
What engine does the Cummins/Onan 6BT5.9 use?
The Cummins/Onan 6BT5.9 is powered by the Cummins/Onan 6BT5.9-D(M) engine.
Troubleshooting
15
Fault codes
Code
Meaning
Remedy
RUN
Run
FAULT 1
Fault 1
FAULT 2
Fault 2
K13
K13 - HET
K4 - Fault 2
K4 - Fault 2
K5 - Fault 1
K5 - Fault 1
K17
K17 - PHET
K19
K19 - PLOP
ES-1859-1
Overvoltage Monitor
Shut down the output device and trip an optional circuit breaker
ES-1852
Overload Detector
Shut down the output device
Symptoms & remedies
Symptom
Cause
Remedy
Voltage regulator may require adjustment or is faulty
Voltage, the voltage regulator may require adjustment or is faulty
Refer to Tests and Adjustments section for adjustments, replace if faulty.
Shutdown was due to over/under frequency
If shutdown was due to over/under frequency, the engine governor may require adjustment or is faulty
Refer to Tests and Adjustments section for adjustments, replace if faulty.
No fault lamp lit
Possible defective ECM Check fuses F4 and F2 of ECM board A11.
Perform restart and check for B+ voltage in at TB1-9 and voltage out at TB1-10 to fuel solenoid. If there is voitage out at TB1-10, check fuel supply solenoid, shutoff valves, etc. If there is no voltage out at TB1-10, ECM board A11 is defective. Replace.
AC output voltage too high or low
Is engine running at correct rpm? No; Are generator output connections correct and secure? Yes; Does adjustment of Voltage Adjust control R21 (if equipped) result in correct output voltage? No; Check Voltage Regulator Adjustment. Check ok? No; Check condition of rotating diodes.
Visually inspect for loose connections, faulty diodes, etc. Check ok? Yes; Do not replace Voitage Regulator Replace Voltage Regulator VR21. ACAUTION VR21 until external trouble has been corrected to avoid possible damage to new regulator board.
Exciter field breaker trips (if equipped)
Does AC output voltage build up to 150% or more of rated voltage before breaker trips? No; Check diodes in rotor assembly. Replace if bad.
Check Voltage Regulator adjustments and connections. Check generator stator leads for proper, secure connection. Refer to appropriate electrical schematic. Check exciter rotor winding. Replace if bad. Check generator rotor field winding. Replace if bad. Check generator stator windings. Replace if bad. Do not replace Voltage Regulator Replace Voltage Regulator VR21. ACAUTION VR27 until external trouble has been corrected to avoid possible damage to new regulator board.
Fault codes and symptom→remedy steps extracted from this model’s service/owner documentation; verify against the official manual before servicing.
Service, wear & replacement parts referenced for this model and its engine — confirm the exact part for your serial number. See the full parts catalog to search and cross-reference.